[PATCH -rcu/kcsan 00/23] kcsan: Support detecting a subset of missing memory barriers

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Marco Elver

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Oct 5, 2021, 6:59:44 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Detection of some missing memory barriers has been on the KCSAN feature
wishlist for some time: this series adds support for modeling a subset
of weak memory as defined by the LKMM, which enables detection of a
subset of data races due to missing memory barriers.

KCSAN's approach to detecting missing memory barriers is based on
modeling access reordering. Each memory access for which a watchpoint is
set up, is also selected for simulated reordering within the scope of
its function (at most 1 in-flight access).

We are limited to modeling the effects of "buffering" (delaying the
access), since the runtime cannot "prefetch" accesses. Once an access
has been selected for reordering, it is checked along every other access
until the end of the function scope. If an appropriate memory barrier is
encountered, the access will no longer be considered for reordering.

When the result of a memory operation should be ordered by a barrier,
KCSAN can then detect data races where the conflict only occurs as a
result of a missing barrier due to reordering accesses.

Some more details and an example are captured in the updated
<Documentation/dev-tools/kcsan.rst>.

Some light fuzzing with the feature also resulted in a discussion [1]
around an issue which appears to be allowed, but unlikely in practice.

[1] https://lkml.kernel.org/r/YRo58c+J...@elver.google.com


The first half of the series are core KCSAN changes, documentation
updates, and test changes. The second half adds instrumentation to
barriers, atomics, bitops, along with enabling barrier instrumentation
for some currently uninstrumented subsystems. The last two patches are
objtool changes to add the usual entries to the uaccess whitelist, but
also instruct objtool to remove memory barrier instrumentation from
noinstr code (on x86).

The series is rebased on -rcu/kcsan. The objtool patches currently
conflict with pending changes in -tip/objtool/core, which could be
separated from this series if needed.

Marco Elver (23):
kcsan: Refactor reading of instrumented memory
kcsan: Remove redundant zero-initialization of globals
kcsan: Avoid checking scoped accesses from nested contexts
kcsan: Add core support for a subset of weak memory modeling
kcsan: Add core memory barrier instrumentation functions
kcsan, kbuild: Add option for barrier instrumentation only
kcsan: Call scoped accesses reordered in reports
kcsan: Show location access was reordered to
kcsan: Document modeling of weak memory
kcsan: test: Match reordered or normal accesses
kcsan: test: Add test cases for memory barrier instrumentation
kcsan: Ignore GCC 11+ warnings about TSan runtime support
kcsan: selftest: Add test case to check memory barrier instrumentation
locking/barriers, kcsan: Add instrumentation for barriers
locking/barriers, kcsan: Support generic instrumentation
locking/atomics, kcsan: Add instrumentation for barriers
asm-generic/bitops, kcsan: Add instrumentation for barriers
x86/barriers, kcsan: Use generic instrumentation for non-smp barriers
x86/qspinlock, kcsan: Instrument barrier of pv_queued_spin_unlock()
mm, kcsan: Enable barrier instrumentation
sched, kcsan: Enable memory barrier instrumentation
objtool, kcsan: Add memory barrier instrumentation to whitelist
objtool, kcsan: Remove memory barrier instrumentation from noinstr

Documentation/dev-tools/kcsan.rst | 72 ++-
arch/x86/include/asm/barrier.h | 10 +-
arch/x86/include/asm/qspinlock.h | 1 +
include/asm-generic/barrier.h | 54 ++-
.../asm-generic/bitops/instrumented-atomic.h | 3 +
.../asm-generic/bitops/instrumented-lock.h | 3 +
include/linux/atomic/atomic-instrumented.h | 135 +++++-
include/linux/kcsan-checks.h | 51 ++-
include/linux/kcsan.h | 11 +-
include/linux/sched.h | 3 +
include/linux/spinlock.h | 2 +-
init/init_task.c | 9 +-
kernel/kcsan/Makefile | 2 +
kernel/kcsan/core.c | 326 +++++++++++---
kernel/kcsan/kcsan_test.c | 416 ++++++++++++++++--
kernel/kcsan/report.c | 51 ++-
kernel/kcsan/selftest.c | 141 ++++++
kernel/sched/Makefile | 7 +-
lib/Kconfig.kcsan | 16 +
mm/Makefile | 2 +
scripts/Makefile.kcsan | 15 +-
scripts/Makefile.lib | 5 +
scripts/atomic/gen-atomic-instrumented.sh | 41 +-
tools/objtool/check.c | 36 +-
24 files changed, 1240 insertions(+), 172 deletions(-)

--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 6:59:46 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Factor out the switch statement reading instrumented memory into a
helper read_instrumented_memory().

No functional change.

Signed-off-by: Marco Elver <el...@google.com>
---
kernel/kcsan/core.c | 51 +++++++++++++++------------------------------
1 file changed, 17 insertions(+), 34 deletions(-)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 4b84c8e7884b..6bfd3040f46b 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -325,6 +325,21 @@ static void delay_access(int type)
udelay(delay);
}

+/*
+ * Reads the instrumented memory for value change detection; value change
+ * detection is currently done for accesses up to a size of 8 bytes.
+ */
+static __always_inline u64 read_instrumented_memory(const volatile void *ptr, size_t size)
+{
+ switch (size) {
+ case 1: return READ_ONCE(*(const u8 *)ptr);
+ case 2: return READ_ONCE(*(const u16 *)ptr);
+ case 4: return READ_ONCE(*(const u32 *)ptr);
+ case 8: return READ_ONCE(*(const u64 *)ptr);
+ default: return 0; /* Ignore; we do not diff the values. */
+ }
+}
+
void kcsan_save_irqtrace(struct task_struct *task)
{
#ifdef CONFIG_TRACE_IRQFLAGS
@@ -482,23 +497,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
* Read the current value, to later check and infer a race if the data
* was modified via a non-instrumented access, e.g. from a device.
*/
- old = 0;
- switch (size) {
- case 1:
- old = READ_ONCE(*(const u8 *)ptr);
- break;
- case 2:
- old = READ_ONCE(*(const u16 *)ptr);
- break;
- case 4:
- old = READ_ONCE(*(const u32 *)ptr);
- break;
- case 8:
- old = READ_ONCE(*(const u64 *)ptr);
- break;
- default:
- break; /* ignore; we do not diff the values */
- }
+ old = read_instrumented_memory(ptr, size);

/*
* Delay this thread, to increase probability of observing a racy
@@ -511,23 +510,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
* racy access.
*/
access_mask = ctx->access_mask;
- new = 0;
- switch (size) {
- case 1:
- new = READ_ONCE(*(const u8 *)ptr);
- break;
- case 2:
- new = READ_ONCE(*(const u16 *)ptr);
- break;
- case 4:
- new = READ_ONCE(*(const u32 *)ptr);
- break;
- case 8:
- new = READ_ONCE(*(const u64 *)ptr);
- break;
- default:
- break; /* ignore; we do not diff the values */
- }
+ new = read_instrumented_memory(ptr, size);

diff = old ^ new;
if (access_mask)
--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 6:59:49 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
They are implicitly zero-initialized, remove explicit initialization.
It keeps the upcoming additions to kcsan_ctx consistent with the rest.

No functional change intended.

Signed-off-by: Marco Elver <el...@google.com>
---
init/init_task.c | 9 +--------
kernel/kcsan/core.c | 5 -----
2 files changed, 1 insertion(+), 13 deletions(-)

diff --git a/init/init_task.c b/init/init_task.c
index 2d024066e27b..61700365ce58 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -181,14 +181,7 @@ struct task_struct init_task
.kasan_depth = 1,
#endif
#ifdef CONFIG_KCSAN
- .kcsan_ctx = {
- .disable_count = 0,
- .atomic_next = 0,
- .atomic_nest_count = 0,
- .in_flat_atomic = false,
- .access_mask = 0,
- .scoped_accesses = {LIST_POISON1, NULL},
- },
+ .kcsan_ctx = { .scoped_accesses = {LIST_POISON1, NULL} },
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
.softirqs_enabled = 1,
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 6bfd3040f46b..e34a1710b7bc 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -44,11 +44,6 @@ bool kcsan_enabled;

/* Per-CPU kcsan_ctx for interrupts */
static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx) = {
- .disable_count = 0,
- .atomic_next = 0,
- .atomic_nest_count = 0,
- .in_flat_atomic = false,
- .access_mask = 0,
.scoped_accesses = {LIST_POISON1, NULL},
};

--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 6:59:50 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Avoid checking scoped accesses from nested contexts (such as nested
interrupts or in scheduler code) which share the same kcsan_ctx.

This is to avoid detecting false positive races of accesses in the same
thread with currently scoped accesses: consider setting up a watchpoint
for a non-scoped (normal) access that also "conflicts" with a current
scoped access. In a nested interrupt (or in the scheduler), which shares
the same kcsan_ctx, we cannot check scoped accesses set up in the parent
context -- simply ignore them in this case.

With the introduction of kcsan_ctx::disable_scoped, we can also clean up
kcsan_check_scoped_accesses()'s recursion guard, and do not need to
modify the list's prev pointer.

Signed-off-by: Marco Elver <el...@google.com>
---
include/linux/kcsan.h | 1 +
kernel/kcsan/core.c | 18 +++++++++++++++---
2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
index fc266ecb2a4d..13cef3458fed 100644
--- a/include/linux/kcsan.h
+++ b/include/linux/kcsan.h
@@ -21,6 +21,7 @@
*/
struct kcsan_ctx {
int disable_count; /* disable counter */
+ int disable_scoped; /* disable scoped access counter */
int atomic_next; /* number of following atomic ops */

/*
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index e34a1710b7bc..bd359f8ee63a 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -204,15 +204,17 @@ check_access(const volatile void *ptr, size_t size, int type, unsigned long ip);
static noinline void kcsan_check_scoped_accesses(void)
{
struct kcsan_ctx *ctx = get_ctx();
- struct list_head *prev_save = ctx->scoped_accesses.prev;
struct kcsan_scoped_access *scoped_access;

- ctx->scoped_accesses.prev = NULL; /* Avoid recursion. */
+ if (ctx->disable_scoped)
+ return;
+
+ ctx->disable_scoped++;
list_for_each_entry(scoped_access, &ctx->scoped_accesses, list) {
check_access(scoped_access->ptr, scoped_access->size,
scoped_access->type, scoped_access->ip);
}
- ctx->scoped_accesses.prev = prev_save;
+ ctx->disable_scoped--;
}

/* Rules for generic atomic accesses. Called from fast-path. */
@@ -465,6 +467,15 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
goto out;
}

+ /*
+ * Avoid races of scoped accesses from nested interrupts (or scheduler).
+ * Assume setting up a watchpoint for a non-scoped (normal) access that
+ * also conflicts with a current scoped access. In a nested interrupt,
+ * which shares the context, it would check a conflicting scoped access.
+ * To avoid, disable scoped access checking.
+ */
+ ctx->disable_scoped++;
+
/*
* Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
* runtime is entered for every memory access, and potentially useful
@@ -578,6 +589,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
if (!kcsan_interrupt_watcher)
local_irq_restore(irq_flags);
kcsan_restore_irqtrace(current);
+ ctx->disable_scoped--;
out:
user_access_restore(ua_flags);
}
--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 6:59:53 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Add support for modeling a subset of weak memory, which will enable
detection of a subset of data races due to missing memory barriers.

KCSAN's approach to detecting missing memory barriers is based on
modeling access reordering, and enabled if `CONFIG_KCSAN_WEAK_MEMORY=y`,
which depends on `CONFIG_KCSAN_STRICT=y`. The feature can be enabled or
disabled at boot and runtime via the `kcsan.weak_memory` boot parameter.

Each memory access for which a watchpoint is set up, is also selected
for simulated reordering within the scope of its function (at most 1
in-flight access).

We are limited to modeling the effects of "buffering" (delaying the
access), since the runtime cannot "prefetch" accesses (therefore no
acquire modeling). Once an access has been selected for reordering, it
is checked along every other access until the end of the function scope.
If an appropriate memory barrier is encountered, the access will no
longer be considered for reordering.

When the result of a memory operation should be ordered by a barrier,
KCSAN can then detect data races where the conflict only occurs as a
result of a missing barrier due to reordering accesses.

Suggested-by: Dmitry Vyukov <dvy...@google.com>
Signed-off-by: Marco Elver <el...@google.com>
---
include/linux/kcsan-checks.h | 10 +-
include/linux/kcsan.h | 10 +-
include/linux/sched.h | 3 +
kernel/kcsan/core.c | 222 ++++++++++++++++++++++++++++++++---
lib/Kconfig.kcsan | 16 +++
scripts/Makefile.kcsan | 9 +-
6 files changed, 251 insertions(+), 19 deletions(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 5f5965246877..a1c6a89fde71 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -99,7 +99,15 @@ void kcsan_set_access_mask(unsigned long mask);

/* Scoped access information. */
struct kcsan_scoped_access {
- struct list_head list;
+ union {
+ struct list_head list; /* scoped_accesses list */
+ /*
+ * Not an entry in scoped_accesses list; stack depth from where
+ * the access was initialized.
+ */
+ int stack_depth;
+ };
+
/* Access information. */
const volatile void *ptr;
size_t size;
diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
index 13cef3458fed..c07c71f5ba4f 100644
--- a/include/linux/kcsan.h
+++ b/include/linux/kcsan.h
@@ -49,8 +49,16 @@ struct kcsan_ctx {
*/
unsigned long access_mask;

- /* List of scoped accesses. */
+ /* List of scoped accesses; likely to be empty. */
struct list_head scoped_accesses;
+
+#ifdef CONFIG_KCSAN_WEAK_MEMORY
+ /*
+ * Scoped access for modeling access reordering to detect missing memory
+ * barriers; only keep 1 to keep fast-path complexity manageable.
+ */
+ struct kcsan_scoped_access reorder_access;
+#endif
};

/**
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e12b524426b0..c6a96f0cc984 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1343,6 +1343,9 @@ struct task_struct {
#ifdef CONFIG_TRACE_IRQFLAGS
struct irqtrace_events kcsan_save_irqtrace;
#endif
+#ifdef CONFIG_KCSAN_WEAK_MEMORY
+ int kcsan_stack_depth;
+#endif
#endif

#if IS_ENABLED(CONFIG_KUNIT)
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index bd359f8ee63a..0e180d1cd53d 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -12,6 +12,7 @@
#include <linux/delay.h>
#include <linux/export.h>
#include <linux/init.h>
+#include <linux/instrumentation.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/moduleparam.h>
@@ -20,6 +21,8 @@
#include <linux/sched.h>
#include <linux/uaccess.h>

+#include <asm/sections.h>
+
#include "encoding.h"
#include "kcsan.h"
#include "permissive.h"
@@ -29,6 +32,7 @@ unsigned int kcsan_udelay_task = CONFIG_KCSAN_UDELAY_TASK;
unsigned int kcsan_udelay_interrupt = CONFIG_KCSAN_UDELAY_INTERRUPT;
static long kcsan_skip_watch = CONFIG_KCSAN_SKIP_WATCH;
static bool kcsan_interrupt_watcher = IS_ENABLED(CONFIG_KCSAN_INTERRUPT_WATCHER);
+static bool kcsan_weak_memory = IS_ENABLED(CONFIG_KCSAN_WEAK_MEMORY);

#ifdef MODULE_PARAM_PREFIX
#undef MODULE_PARAM_PREFIX
@@ -39,6 +43,9 @@ module_param_named(udelay_task, kcsan_udelay_task, uint, 0644);
module_param_named(udelay_interrupt, kcsan_udelay_interrupt, uint, 0644);
module_param_named(skip_watch, kcsan_skip_watch, long, 0644);
module_param_named(interrupt_watcher, kcsan_interrupt_watcher, bool, 0444);
+#ifdef CONFIG_KCSAN_WEAK_MEMORY
+module_param_named(weak_memory, kcsan_weak_memory, bool, 0644);
+#endif

bool kcsan_enabled;

@@ -102,6 +109,22 @@ static DEFINE_PER_CPU(long, kcsan_skip);
/* For kcsan_prandom_u32_max(). */
static DEFINE_PER_CPU(u32, kcsan_rand_state);

+#if !defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_STACK_VALIDATION)
+/*
+ * Arch does not rely on noinstr, or objtool will remove memory barrier
+ * instrumentation, and no instrumentation of noinstr code is expected.
+ */
+#define kcsan_noinstr
+static inline bool within_noinstr(unsigned long ip) { return false; }
+#else
+#define kcsan_noinstr noinstr
+static __always_inline bool within_noinstr(unsigned long ip)
+{
+ return (unsigned long)__noinstr_text_start <= ip &&
+ ip < (unsigned long)__noinstr_text_end;
+}
+#endif
+
static __always_inline atomic_long_t *find_watchpoint(unsigned long addr,
size_t size,
bool expect_write,
@@ -351,6 +374,67 @@ void kcsan_restore_irqtrace(struct task_struct *task)
#endif
}

+static __always_inline int get_kcsan_stack_depth(void)
+{
+#ifdef CONFIG_KCSAN_WEAK_MEMORY
+ return current->kcsan_stack_depth;
+#else
+ BUILD_BUG();
+ return 0;
+#endif
+}
+
+static __always_inline void add_kcsan_stack_depth(int val)
+{
+#ifdef CONFIG_KCSAN_WEAK_MEMORY
+ current->kcsan_stack_depth += val;
+#else
+ BUILD_BUG();
+#endif
+}
+
+static __always_inline struct kcsan_scoped_access *get_reorder_access(struct kcsan_ctx *ctx)
+{
+#ifdef CONFIG_KCSAN_WEAK_MEMORY
+ return ctx->disable_scoped ? NULL : &ctx->reorder_access;
+#else
+ return NULL;
+#endif
+}
+
+static __always_inline bool
+find_reorder_access(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size,
+ int type, unsigned long ip)
+{
+ struct kcsan_scoped_access *reorder_access = get_reorder_access(ctx);
+
+ if (!reorder_access)
+ return false;
+
+ /*
+ * Note: If accesses are repeated while reorder_access is identical,
+ * never matches the new access, because !(type & KCSAN_ACCESS_SCOPED).
+ */
+ return reorder_access->ptr == ptr && reorder_access->size == size &&
+ reorder_access->type == type && reorder_access->ip == ip;
+}
+
+static inline void
+set_reorder_access(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size,
+ int type, unsigned long ip)
+{
+ struct kcsan_scoped_access *reorder_access = get_reorder_access(ctx);
+
+ if (!reorder_access || !kcsan_weak_memory)
+ return;
+
+ reorder_access->ptr = ptr;
+ reorder_access->size = size;
+ reorder_access->type = type | KCSAN_ACCESS_SCOPED;
+ reorder_access->ip = ip;
+ reorder_access->stack_depth = get_kcsan_stack_depth();
+}
+
/*
* Pull everything together: check_access() below contains the performance
* critical operations; the fast-path (including check_access) functions should
@@ -389,8 +473,10 @@ static noinline void kcsan_found_watchpoint(const volatile void *ptr,
* The access_mask check relies on value-change comparison. To avoid
* reporting a race where e.g. the writer set up the watchpoint, but the
* reader has access_mask!=0, we have to ignore the found watchpoint.
+ *
+ * reorder_access is never created from an access with access_mask set.
*/
- if (ctx->access_mask)
+ if (ctx->access_mask && !find_reorder_access(ctx, ptr, size, type, ip))
return;

/*
@@ -440,11 +526,13 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
atomic_long_t *watchpoint;
u64 old, new, diff;
- unsigned long access_mask;
enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
+ bool interrupt_watcher = kcsan_interrupt_watcher;
unsigned long ua_flags = user_access_save();
struct kcsan_ctx *ctx = get_ctx();
+ unsigned long access_mask = ctx->access_mask;
unsigned long irq_flags = 0;
+ bool is_reorder_access;

/*
* Always reset kcsan_skip counter in slow-path to avoid underflow; see
@@ -467,6 +555,17 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
goto out;
}

+ /*
+ * The local CPU cannot observe reordering of its own accesses, and
+ * therefore we need to take care of 2 cases to avoid false positives:
+ *
+ * 1. Races of the reordered access with interrupts. To avoid, if
+ * the current access is reorder_access, disable interrupts.
+ * 2. Avoid races of scoped accesses from nested interrupts (below).
+ */
+ is_reorder_access = find_reorder_access(ctx, ptr, size, type, ip);
+ if (is_reorder_access)
+ interrupt_watcher = false;
/*
* Avoid races of scoped accesses from nested interrupts (or scheduler).
* Assume setting up a watchpoint for a non-scoped (normal) access that
@@ -482,7 +581,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
* information is lost if dirtied by KCSAN.
*/
kcsan_save_irqtrace(current);
- if (!kcsan_interrupt_watcher)
+ if (!interrupt_watcher)
local_irq_save(irq_flags);

watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
@@ -503,7 +602,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
* Read the current value, to later check and infer a race if the data
* was modified via a non-instrumented access, e.g. from a device.
*/
- old = read_instrumented_memory(ptr, size);
+ old = is_reorder_access ? 0 : read_instrumented_memory(ptr, size);

/*
* Delay this thread, to increase probability of observing a racy
@@ -515,8 +614,17 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
* Re-read value, and check if it is as expected; if not, we infer a
* racy access.
*/
- access_mask = ctx->access_mask;
- new = read_instrumented_memory(ptr, size);
+ if (!is_reorder_access) {
+ new = read_instrumented_memory(ptr, size);
+ } else {
+ /*
+ * Reordered accesses cannot be used for value change detection,
+ * because the memory location may no longer be accessible and
+ * could result in a fault.
+ */
+ new = 0;
+ access_mask = 0;
+ }

diff = old ^ new;
if (access_mask)
@@ -585,11 +693,20 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
*/
remove_watchpoint(watchpoint);
atomic_long_dec(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
+
out_unlock:
- if (!kcsan_interrupt_watcher)
+ if (!interrupt_watcher)
local_irq_restore(irq_flags);
kcsan_restore_irqtrace(current);
ctx->disable_scoped--;
+
+ /*
+ * Reordered accesses cannot be used for value change detection,
+ * therefore never consider for reordering if access_mask is set.
+ * ASSERT_EXCLUSIVE are not real accesses, ignore them as well.
+ */
+ if (!access_mask && !is_assert)
+ set_reorder_access(ctx, ptr, size, type, ip);
out:
user_access_restore(ua_flags);
}
@@ -597,7 +714,6 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
static __always_inline void
check_access(const volatile void *ptr, size_t size, int type, unsigned long ip)
{
- const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
atomic_long_t *watchpoint;
long encoded_watchpoint;

@@ -608,12 +724,14 @@ check_access(const volatile void *ptr, size_t size, int type, unsigned long ip)
if (unlikely(size == 0))
return;

+again:
/*
* Avoid user_access_save in fast-path: find_watchpoint is safe without
* user_access_save, as the address that ptr points to is only used to
* check if a watchpoint exists; ptr is never dereferenced.
*/
- watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
+ watchpoint = find_watchpoint((unsigned long)ptr, size,
+ !(type & KCSAN_ACCESS_WRITE),
&encoded_watchpoint);
/*
* It is safe to check kcsan_is_enabled() after find_watchpoint in the
@@ -627,9 +745,42 @@ check_access(const volatile void *ptr, size_t size, int type, unsigned long ip)
else {
struct kcsan_ctx *ctx = get_ctx(); /* Call only once in fast-path. */

- if (unlikely(should_watch(ctx, ptr, size, type)))
+ if (unlikely(should_watch(ctx, ptr, size, type))) {
kcsan_setup_watchpoint(ptr, size, type, ip);
- else if (unlikely(ctx->scoped_accesses.prev))
+ return;
+ }
+
+ if (!(type & KCSAN_ACCESS_SCOPED)) {
+ struct kcsan_scoped_access *reorder_access = get_reorder_access(ctx);
+
+ if (reorder_access) {
+ /*
+ * reorder_access check: simulates reordering of
+ * the access after subsequent operations.
+ */
+ ptr = reorder_access->ptr;
+ type = reorder_access->type;
+ ip = reorder_access->ip;
+ /*
+ * Upon a nested interrupt, this context's
+ * reorder_access can be modified (shared ctx).
+ * We know that upon return, reorder_access is
+ * always invalidated by setting size to 0 via
+ * __tsan_func_exit(). Therefore we must read
+ * and check size after the other fields.
+ */
+ barrier();
+ size = READ_ONCE(reorder_access->size);
+ if (size)
+ goto again;
+ }
+ }
+
+ /*
+ * Always checked last, right before returning from runtime;
+ * if reorder_access is valid, checked after it was checked.
+ */
+ if (unlikely(ctx->scoped_accesses.prev))
kcsan_check_scoped_accesses();
}
}
@@ -916,19 +1067,60 @@ DEFINE_TSAN_VOLATILE_READ_WRITE(8);
DEFINE_TSAN_VOLATILE_READ_WRITE(16);

/*
- * The below are not required by KCSAN, but can still be emitted by the
- * compiler.
+ * Function entry and exit are used to determine the validty of reorder_access.
+ * Reordering of the access ends at the end of the function scope where the
+ * access happened. This is done for two reasons:
+ *
+ * 1. Artificially limits the scope where missing barriers are detected.
+ * This minimizes false positives due to uninstrumented functions that
+ * contain the required barriers but were missed.
+ *
+ * 2. Simplifies generating the stack trace of the access.
*/
void __tsan_func_entry(void *call_pc);
-void __tsan_func_entry(void *call_pc)
+kcsan_noinstr void __tsan_func_entry(void *call_pc)
{
+ if (!IS_ENABLED(CONFIG_KCSAN_WEAK_MEMORY) || within_noinstr(_RET_IP_))
+ return;
+
+ instrumentation_begin();
+ add_kcsan_stack_depth(1);
+ instrumentation_end();
}
EXPORT_SYMBOL(__tsan_func_entry);
+
void __tsan_func_exit(void);
-void __tsan_func_exit(void)
+kcsan_noinstr void __tsan_func_exit(void)
{
+ struct kcsan_scoped_access *reorder_access;
+
+ if (!IS_ENABLED(CONFIG_KCSAN_WEAK_MEMORY) || within_noinstr(_RET_IP_))
+ return;
+
+ instrumentation_begin();
+ reorder_access = get_reorder_access(get_ctx());
+ if (!reorder_access)
+ goto out;
+
+ if (get_kcsan_stack_depth() <= reorder_access->stack_depth) {
+ /*
+ * Access check to catch cases where write without a barrier
+ * (supposed release) was last access in function: because
+ * instrumentation is inserted before the real access, a data
+ * race due to the write giving up a c-s would only be caught if
+ * we do the conflicting access after.
+ */
+ check_access(reorder_access->ptr, reorder_access->size,
+ reorder_access->type, reorder_access->ip);
+ reorder_access->size = 0;
+ reorder_access->stack_depth = INT_MIN;
+ }
+out:
+ add_kcsan_stack_depth(-1);
+ instrumentation_end();
}
EXPORT_SYMBOL(__tsan_func_exit);
+
void __tsan_init(void);
void __tsan_init(void)
{
diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
index e0a93ffdef30..55b33bc11824 100644
--- a/lib/Kconfig.kcsan
+++ b/lib/Kconfig.kcsan
@@ -191,6 +191,22 @@ config KCSAN_STRICT
closely aligns with the rules defined by the Linux-kernel memory
consistency model (LKMM).

+config KCSAN_WEAK_MEMORY
+ bool "Enable weak memory modeling to detect missing memory barriers"
+ default y
+ depends on KCSAN_STRICT
+ help
+ Enable support for modeling a subset of weak memory, which allows
+ detecting a subset of data races due to missing memory barriers.
+
+ Depends on KCSAN_STRICT, because the options strenghtening certain
+ plain accesses by default (depending on !KCSAN_STRICT) reduce the
+ ability to detect any data races invoving reordered accesses, in
+ particular reordered writes.
+
+ Weak memory modeling relies on additional instrumentation and may
+ affect performance.
+
config KCSAN_REPORT_VALUE_CHANGE_ONLY
bool "Only report races where watcher observed a data value change"
default y
diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
index 37cb504c77e1..4c7f0d282e42 100644
--- a/scripts/Makefile.kcsan
+++ b/scripts/Makefile.kcsan
@@ -9,7 +9,12 @@ endif

# Keep most options here optional, to allow enabling more compilers if absence
# of some options does not break KCSAN nor causes false positive reports.
-export CFLAGS_KCSAN := -fsanitize=thread \
- $(call cc-option,$(call cc-param,tsan-instrument-func-entry-exit=0) -fno-optimize-sibling-calls) \
+kcsan-cflags := -fsanitize=thread -fno-optimize-sibling-calls \
$(call cc-option,$(call cc-param,tsan-compound-read-before-write=1),$(call cc-option,$(call cc-param,tsan-instrument-read-before-write=1))) \
$(call cc-param,tsan-distinguish-volatile=1)
+
+ifndef CONFIG_KCSAN_WEAK_MEMORY
+kcsan-cflags += $(call cc-option,$(call cc-param,tsan-instrument-func-entry-exit=0))
+endif
+
+export CFLAGS_KCSAN := $(kcsan-cflags)
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 6:59:56 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Add the core memory barrier instrumentation functions. These invalidate
the current in-flight reordered access based on the rules for the
respective barrier types and in-flight access type.

Signed-off-by: Marco Elver <el...@google.com>
---
include/linux/kcsan-checks.h | 41 ++++++++++++++++++++++++++++++++++--
kernel/kcsan/core.c | 36 +++++++++++++++++++++++++++++++
2 files changed, 75 insertions(+), 2 deletions(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index a1c6a89fde71..c9e7c39a7d7b 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -36,6 +36,26 @@
*/
void __kcsan_check_access(const volatile void *ptr, size_t size, int type);

+/**
+ * __kcsan_mb - full memory barrier instrumentation
+ */
+void __kcsan_mb(void);
+
+/**
+ * __kcsan_wmb - write memory barrier instrumentation
+ */
+void __kcsan_wmb(void);
+
+/**
+ * __kcsan_rmb - read memory barrier instrumentation
+ */
+void __kcsan_rmb(void);
+
+/**
+ * __kcsan_release - release barrier instrumentation
+ */
+void __kcsan_release(void);
+
/**
* kcsan_disable_current - disable KCSAN for the current context
*
@@ -159,6 +179,10 @@ void kcsan_end_scoped_access(struct kcsan_scoped_access *sa);
static inline void __kcsan_check_access(const volatile void *ptr, size_t size,
int type) { }

+static inline void __kcsan_mb(void) { }
+static inline void __kcsan_wmb(void) { }
+static inline void __kcsan_rmb(void) { }
+static inline void __kcsan_release(void) { }
static inline void kcsan_disable_current(void) { }
static inline void kcsan_enable_current(void) { }
static inline void kcsan_enable_current_nowarn(void) { }
@@ -191,12 +215,25 @@ static inline void kcsan_end_scoped_access(struct kcsan_scoped_access *sa) { }
*/
#define __kcsan_disable_current kcsan_disable_current
#define __kcsan_enable_current kcsan_enable_current_nowarn
-#else
+#else /* __SANITIZE_THREAD__ */
static inline void kcsan_check_access(const volatile void *ptr, size_t size,
int type) { }
static inline void __kcsan_enable_current(void) { }
static inline void __kcsan_disable_current(void) { }
-#endif
+#endif /* __SANITIZE_THREAD__ */
+
+#if defined(CONFIG_KCSAN_WEAK_MEMORY) && \
+ (defined(__SANITIZE_THREAD__) || defined(__KCSAN_INSTRUMENT_BARRIERS__))
+#define kcsan_mb __kcsan_mb
+#define kcsan_wmb __kcsan_wmb
+#define kcsan_rmb __kcsan_rmb
+#define kcsan_release __kcsan_release
+#else /* CONFIG_KCSAN_WEAK_MEMORY && (__SANITIZE_THREAD__ || __KCSAN_INSTRUMENT_BARRIERS__) */
+static inline void kcsan_mb(void) { }
+static inline void kcsan_wmb(void) { }
+static inline void kcsan_rmb(void) { }
+static inline void kcsan_release(void) { }
+#endif /* CONFIG_KCSAN_WEAK_MEMORY && (__SANITIZE_THREAD__ || __KCSAN_INSTRUMENT_BARRIERS__) */

/**
* __kcsan_check_read - check regular read access for races
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 0e180d1cd53d..47c95ccff19f 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -955,6 +955,28 @@ void __kcsan_check_access(const volatile void *ptr, size_t size, int type)
}
EXPORT_SYMBOL(__kcsan_check_access);

+#define DEFINE_MEMORY_BARRIER(name, order_before_cond) \
+ kcsan_noinstr void __kcsan_##name(void) \
+ { \
+ struct kcsan_scoped_access *sa; \
+ if (within_noinstr(_RET_IP_)) \
+ return; \
+ instrumentation_begin(); \
+ sa = get_reorder_access(get_ctx()); \
+ if (!sa) \
+ goto out; \
+ if (order_before_cond) \
+ sa->size = 0; \
+ out: \
+ instrumentation_end(); \
+ } \
+ EXPORT_SYMBOL(__kcsan_##name)
+
+DEFINE_MEMORY_BARRIER(mb, true);
+DEFINE_MEMORY_BARRIER(wmb, sa->type & (KCSAN_ACCESS_WRITE | KCSAN_ACCESS_COMPOUND));
+DEFINE_MEMORY_BARRIER(rmb, !(sa->type & KCSAN_ACCESS_WRITE) || (sa->type & KCSAN_ACCESS_COMPOUND));
+DEFINE_MEMORY_BARRIER(release, true);
+
/*
* KCSAN uses the same instrumentation that is emitted by supported compilers
* for ThreadSanitizer (TSAN).
@@ -1143,10 +1165,19 @@ EXPORT_SYMBOL(__tsan_init);
* functions, whose job is to also execute the operation itself.
*/

+static __always_inline void kcsan_atomic_release(int memorder)
+{
+ if (memorder == __ATOMIC_RELEASE ||
+ memorder == __ATOMIC_SEQ_CST ||
+ memorder == __ATOMIC_ACQ_REL)
+ __kcsan_release();
+}
+
#define DEFINE_TSAN_ATOMIC_LOAD_STORE(bits) \
u##bits __tsan_atomic##bits##_load(const u##bits *ptr, int memorder); \
u##bits __tsan_atomic##bits##_load(const u##bits *ptr, int memorder) \
{ \
+ kcsan_atomic_release(memorder); \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, KCSAN_ACCESS_ATOMIC, _RET_IP_); \
} \
@@ -1156,6 +1187,7 @@ EXPORT_SYMBOL(__tsan_init);
void __tsan_atomic##bits##_store(u##bits *ptr, u##bits v, int memorder); \
void __tsan_atomic##bits##_store(u##bits *ptr, u##bits v, int memorder) \
{ \
+ kcsan_atomic_release(memorder); \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, \
KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC, _RET_IP_); \
@@ -1168,6 +1200,7 @@ EXPORT_SYMBOL(__tsan_init);
u##bits __tsan_atomic##bits##_##op(u##bits *ptr, u##bits v, int memorder); \
u##bits __tsan_atomic##bits##_##op(u##bits *ptr, u##bits v, int memorder) \
{ \
+ kcsan_atomic_release(memorder); \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, \
KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | \
@@ -1200,6 +1233,7 @@ EXPORT_SYMBOL(__tsan_init);
int __tsan_atomic##bits##_compare_exchange_##strength(u##bits *ptr, u##bits *exp, \
u##bits val, int mo, int fail_mo) \
{ \
+ kcsan_atomic_release(mo); \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, \
KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | \
@@ -1215,6 +1249,7 @@ EXPORT_SYMBOL(__tsan_init);
u##bits __tsan_atomic##bits##_compare_exchange_val(u##bits *ptr, u##bits exp, u##bits val, \
int mo, int fail_mo) \
{ \
+ kcsan_atomic_release(mo); \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, \
KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | \
@@ -1246,6 +1281,7 @@ DEFINE_TSAN_ATOMIC_OPS(64);
void __tsan_atomic_thread_fence(int memorder);
void __tsan_atomic_thread_fence(int memorder)
{
+ kcsan_atomic_release(memorder);
__atomic_thread_fence(memorder);
}
EXPORT_SYMBOL(__tsan_atomic_thread_fence);
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 6:59:59 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Source files that disable KCSAN via KCSAN_SANITIZE := n, remove all
instrumentation, including explicit barrier instrumentation. With
instrumentation for memory barriers, in few places it is required to
enable just the explicit instrumentation for memory barriers to avoid
false positives.

Providing the Makefile variable KCSAN_INSTRUMENT_BARRIERS_obj.o or
KCSAN_INSTRUMENT_BARRIERS (for all files) set to 'y' only enables the
explicit barrier instrumentation.

Signed-off-by: Marco Elver <el...@google.com>
---
scripts/Makefile.lib | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 54582673fc1a..2118f63b2bc5 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -182,6 +182,11 @@ ifeq ($(CONFIG_KCSAN),y)
_c_flags += $(if $(patsubst n%,, \
$(KCSAN_SANITIZE_$(basetarget).o)$(KCSAN_SANITIZE)y), \
$(CFLAGS_KCSAN))
+# Some uninstrumented files provide implied barriers required to avoid false
+# positives: set KCSAN_INSTRUMENT_BARRIERS for barrier instrumentation only.
+_c_flags += $(if $(patsubst n%,, \
+ $(KCSAN_INSTRUMENT_BARRIERS_$(basetarget).o)$(KCSAN_INSTRUMENT_BARRIERS)n), \
+ -D__KCSAN_INSTRUMENT_BARRIERS__)
endif

# $(srctree)/$(src) for including checkin headers from generated source files
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:00 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
The scoping of an access simply denotes the scope in which it may be
reordered. However, in reports, it'll be less confusing to say the
access is "reordered". This is more accurate when the race occurred.

Signed-off-by: Marco Elver <el...@google.com>
---
kernel/kcsan/kcsan_test.c | 4 ++--
kernel/kcsan/report.c | 16 ++++++++--------
2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/kernel/kcsan/kcsan_test.c b/kernel/kcsan/kcsan_test.c
index 660729238588..6e3c2b8bc608 100644
--- a/kernel/kcsan/kcsan_test.c
+++ b/kernel/kcsan/kcsan_test.c
@@ -213,9 +213,9 @@ static bool report_matches(const struct expect_report *r)
const bool is_atomic = (ty & KCSAN_ACCESS_ATOMIC);
const bool is_scoped = (ty & KCSAN_ACCESS_SCOPED);
const char *const access_type_aux =
- (is_atomic && is_scoped) ? " (marked, scoped)"
+ (is_atomic && is_scoped) ? " (marked, reordered)"
: (is_atomic ? " (marked)"
- : (is_scoped ? " (scoped)" : ""));
+ : (is_scoped ? " (reordered)" : ""));

if (i == 1) {
/* Access 2 */
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index fc15077991c4..1b0e050bdf6a 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -215,9 +215,9 @@ static const char *get_access_type(int type)
if (type & KCSAN_ACCESS_ASSERT) {
if (type & KCSAN_ACCESS_SCOPED) {
if (type & KCSAN_ACCESS_WRITE)
- return "assert no accesses (scoped)";
+ return "assert no accesses (reordered)";
else
- return "assert no writes (scoped)";
+ return "assert no writes (reordered)";
} else {
if (type & KCSAN_ACCESS_WRITE)
return "assert no accesses";
@@ -240,17 +240,17 @@ static const char *get_access_type(int type)
case KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC:
return "read-write (marked)";
case KCSAN_ACCESS_SCOPED:
- return "read (scoped)";
+ return "read (reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_ATOMIC:
- return "read (marked, scoped)";
+ return "read (marked, reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE:
- return "write (scoped)";
+ return "write (reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC:
- return "write (marked, scoped)";
+ return "write (marked, reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE:
- return "read-write (scoped)";
+ return "read-write (reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC:
- return "read-write (marked, scoped)";
+ return "read-write (marked, reordered)";
default:
BUG();
}
--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 7:00:03 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Also show the location the access was reordered to. An example report:

| ==================================================================
| BUG: KCSAN: data-race in test_kernel_wrong_memorder / test_kernel_wrong_memorder
|
| read-write to 0xffffffffc01e61a8 of 8 bytes by task 2311 on cpu 5:
| test_kernel_wrong_memorder+0x57/0x90
| access_thread+0x99/0xe0
| kthread+0x2ba/0x2f0
| ret_from_fork+0x22/0x30
|
| read-write (reordered) to 0xffffffffc01e61a8 of 8 bytes by task 2310 on cpu 7:
| test_kernel_wrong_memorder+0x57/0x90
| access_thread+0x99/0xe0
| kthread+0x2ba/0x2f0
| ret_from_fork+0x22/0x30
| |
| +-> reordered to: test_kernel_wrong_memorder+0x80/0x90
|
| Reported by Kernel Concurrency Sanitizer on:
| CPU: 7 PID: 2310 Comm: access_thread Not tainted 5.14.0-rc1+ #18
| Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
| ==================================================================

Signed-off-by: Marco Elver <el...@google.com>
---
kernel/kcsan/report.c | 35 +++++++++++++++++++++++------------
1 file changed, 23 insertions(+), 12 deletions(-)

diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index 1b0e050bdf6a..67794404042a 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -308,10 +308,12 @@ static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries

/*
* Skips to the first entry that matches the function of @ip, and then replaces
- * that entry with @ip, returning the entries to skip.
+ * that entry with @ip, returning the entries to skip with @replaced containing
+ * the replaced entry.
*/
static int
-replace_stack_entry(unsigned long stack_entries[], int num_entries, unsigned long ip)
+replace_stack_entry(unsigned long stack_entries[], int num_entries, unsigned long ip,
+ unsigned long *replaced)
{
unsigned long symbolsize, offset;
unsigned long target_func;
@@ -330,6 +332,7 @@ replace_stack_entry(unsigned long stack_entries[], int num_entries, unsigned lon
func -= offset;

if (func == target_func) {
+ *replaced = stack_entries[skip];
stack_entries[skip] = ip;
return skip;
}
@@ -342,9 +345,10 @@ replace_stack_entry(unsigned long stack_entries[], int num_entries, unsigned lon
}

static int
-sanitize_stack_entries(unsigned long stack_entries[], int num_entries, unsigned long ip)
+sanitize_stack_entries(unsigned long stack_entries[], int num_entries, unsigned long ip,
+ unsigned long *replaced)
{
- return ip ? replace_stack_entry(stack_entries, num_entries, ip) :
+ return ip ? replace_stack_entry(stack_entries, num_entries, ip, replaced) :
get_stack_skipnr(stack_entries, num_entries);
}

@@ -360,6 +364,14 @@ static int sym_strcmp(void *addr1, void *addr2)
return strncmp(buf1, buf2, sizeof(buf1));
}

+static void
+print_stack_trace(unsigned long stack_entries[], int num_entries, unsigned long reordered_to)
+{
+ stack_trace_print(stack_entries, num_entries, 0);
+ if (reordered_to)
+ pr_err(" |\n +-> reordered to: %pS\n", (void *)reordered_to);
+}
+
static void print_verbose_info(struct task_struct *task)
{
if (!task)
@@ -378,10 +390,12 @@ static void print_report(enum kcsan_value_change value_change,
struct other_info *other_info,
u64 old, u64 new, u64 mask)
{
+ unsigned long reordered_to = 0;
unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
int num_stack_entries = stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1);
- int skipnr = sanitize_stack_entries(stack_entries, num_stack_entries, ai->ip);
+ int skipnr = sanitize_stack_entries(stack_entries, num_stack_entries, ai->ip, &reordered_to);
unsigned long this_frame = stack_entries[skipnr];
+ unsigned long other_reordered_to = 0;
unsigned long other_frame = 0;
int other_skipnr = 0; /* silence uninit warnings */

@@ -394,7 +408,7 @@ static void print_report(enum kcsan_value_change value_change,
if (other_info) {
other_skipnr = sanitize_stack_entries(other_info->stack_entries,
other_info->num_stack_entries,
- other_info->ai.ip);
+ other_info->ai.ip, &other_reordered_to);
other_frame = other_info->stack_entries[other_skipnr];

/* @value_change is only known for the other thread */
@@ -434,10 +448,9 @@ static void print_report(enum kcsan_value_change value_change,
other_info->ai.cpu_id);

/* Print the other thread's stack trace. */
- stack_trace_print(other_info->stack_entries + other_skipnr,
+ print_stack_trace(other_info->stack_entries + other_skipnr,
other_info->num_stack_entries - other_skipnr,
- 0);
-
+ other_reordered_to);
if (IS_ENABLED(CONFIG_KCSAN_VERBOSE))
print_verbose_info(other_info->task);

@@ -451,9 +464,7 @@ static void print_report(enum kcsan_value_change value_change,
get_thread_desc(ai->task_pid), ai->cpu_id);
}
/* Print stack trace of this thread. */
- stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr,
- 0);
-
+ print_stack_trace(stack_entries + skipnr, num_stack_entries - skipnr, reordered_to);
if (IS_ENABLED(CONFIG_KCSAN_VERBOSE))
print_verbose_info(current);

--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:07 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Due to reordering accesses with weak memory modeling, any access can now
appear as "(reordered)".

Match any permutation of accesses if CONFIG_KCSAN_WEAK_MEMORY=y, so that
we effectively match an access if it is denoted "(reordered)" or not.

Signed-off-by: Marco Elver <el...@google.com>
---
kernel/kcsan/kcsan_test.c | 92 +++++++++++++++++++++++++++------------
1 file changed, 63 insertions(+), 29 deletions(-)

diff --git a/kernel/kcsan/kcsan_test.c b/kernel/kcsan/kcsan_test.c
index 6e3c2b8bc608..ec054879201b 100644
--- a/kernel/kcsan/kcsan_test.c
+++ b/kernel/kcsan/kcsan_test.c
@@ -151,7 +151,7 @@ struct expect_report {

/* Check observed report matches information in @r. */
__no_kcsan
-static bool report_matches(const struct expect_report *r)
+static bool __report_matches(const struct expect_report *r)
{
const bool is_assert = (r->access[0].type | r->access[1].type) & KCSAN_ACCESS_ASSERT;
bool ret = false;
@@ -253,6 +253,40 @@ static bool report_matches(const struct expect_report *r)
return ret;
}

+static __always_inline const struct expect_report *
+__report_set_scoped(struct expect_report *r, int accesses)
+{
+ BUILD_BUG_ON(accesses > 3);
+
+ if (accesses & 1)
+ r->access[0].type |= KCSAN_ACCESS_SCOPED;
+ else
+ r->access[0].type &= ~KCSAN_ACCESS_SCOPED;
+
+ if (accesses & 2)
+ r->access[1].type |= KCSAN_ACCESS_SCOPED;
+ else
+ r->access[1].type &= ~KCSAN_ACCESS_SCOPED;
+
+ return r;
+}
+
+__no_kcsan
+static bool report_matches_any_reordered(struct expect_report *r)
+{
+ return __report_matches(__report_set_scoped(r, 0)) ||
+ __report_matches(__report_set_scoped(r, 1)) ||
+ __report_matches(__report_set_scoped(r, 2)) ||
+ __report_matches(__report_set_scoped(r, 3));
+}
+
+#ifdef CONFIG_KCSAN_WEAK_MEMORY
+/* Due to reordering accesses, any access may appear as "(reordered)". */
+#define report_matches report_matches_any_reordered
+#else
+#define report_matches __report_matches
+#endif
+
/* ===== Test kernels ===== */

static long test_sink;
@@ -438,13 +472,13 @@ static noinline void test_kernel_xor_1bit(void)
__no_kcsan
static void test_basic(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_write, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
},
};
- static const struct expect_report never = {
+ struct expect_report never = {
.access = {
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
@@ -469,14 +503,14 @@ static void test_basic(struct kunit *test)
__no_kcsan
static void test_concurrent_races(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
/* NULL will match any address. */
{ test_kernel_rmw_array, NULL, 0, __KCSAN_ACCESS_RW(KCSAN_ACCESS_WRITE) },
{ test_kernel_rmw_array, NULL, 0, __KCSAN_ACCESS_RW(0) },
},
};
- static const struct expect_report never = {
+ struct expect_report never = {
.access = {
{ test_kernel_rmw_array, NULL, 0, 0 },
{ test_kernel_rmw_array, NULL, 0, 0 },
@@ -498,13 +532,13 @@ static void test_concurrent_races(struct kunit *test)
__no_kcsan
static void test_novalue_change(struct kunit *test)
{
- const struct expect_report expect_rw = {
+ struct expect_report expect_rw = {
.access = {
{ test_kernel_write_nochange, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
},
};
- const struct expect_report expect_ww = {
+ struct expect_report expect_ww = {
.access = {
{ test_kernel_write_nochange, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
{ test_kernel_write_nochange, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
@@ -530,13 +564,13 @@ static void test_novalue_change(struct kunit *test)
__no_kcsan
static void test_novalue_change_exception(struct kunit *test)
{
- const struct expect_report expect_rw = {
+ struct expect_report expect_rw = {
.access = {
{ test_kernel_write_nochange_rcu, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
},
};
- const struct expect_report expect_ww = {
+ struct expect_report expect_ww = {
.access = {
{ test_kernel_write_nochange_rcu, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
{ test_kernel_write_nochange_rcu, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
@@ -556,7 +590,7 @@ static void test_novalue_change_exception(struct kunit *test)
__no_kcsan
static void test_unknown_origin(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
{ NULL },
@@ -578,7 +612,7 @@ static void test_unknown_origin(struct kunit *test)
__no_kcsan
static void test_write_write_assume_atomic(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_write, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
{ test_kernel_write, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
@@ -604,7 +638,7 @@ static void test_write_write_assume_atomic(struct kunit *test)
__no_kcsan
static void test_write_write_struct(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_write_struct, &test_struct, sizeof(test_struct), KCSAN_ACCESS_WRITE },
{ test_kernel_write_struct, &test_struct, sizeof(test_struct), KCSAN_ACCESS_WRITE },
@@ -626,7 +660,7 @@ static void test_write_write_struct(struct kunit *test)
__no_kcsan
static void test_write_write_struct_part(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_write_struct, &test_struct, sizeof(test_struct), KCSAN_ACCESS_WRITE },
{ test_kernel_write_struct_part, &test_struct.val[3], sizeof(test_struct.val[3]), KCSAN_ACCESS_WRITE },
@@ -658,7 +692,7 @@ static void test_read_atomic_write_atomic(struct kunit *test)
__no_kcsan
static void test_read_plain_atomic_write(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
{ test_kernel_write_atomic, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC },
@@ -679,7 +713,7 @@ static void test_read_plain_atomic_write(struct kunit *test)
__no_kcsan
static void test_read_plain_atomic_rmw(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
{ test_kernel_atomic_rmw, &test_var, sizeof(test_var),
@@ -701,13 +735,13 @@ static void test_read_plain_atomic_rmw(struct kunit *test)
__no_kcsan
static void test_zero_size_access(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_write_struct, &test_struct, sizeof(test_struct), KCSAN_ACCESS_WRITE },
{ test_kernel_write_struct, &test_struct, sizeof(test_struct), KCSAN_ACCESS_WRITE },
},
};
- const struct expect_report never = {
+ struct expect_report never = {
.access = {
{ test_kernel_write_struct, &test_struct, sizeof(test_struct), KCSAN_ACCESS_WRITE },
{ test_kernel_read_struct_zero_size, &test_struct.val[3], 0, 0 },
@@ -741,7 +775,7 @@ static void test_data_race(struct kunit *test)
__no_kcsan
static void test_assert_exclusive_writer(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_assert_writer, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT },
{ test_kernel_write_nochange, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
@@ -759,7 +793,7 @@ static void test_assert_exclusive_writer(struct kunit *test)
__no_kcsan
static void test_assert_exclusive_access(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_assert_access, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_WRITE },
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
@@ -777,19 +811,19 @@ static void test_assert_exclusive_access(struct kunit *test)
__no_kcsan
static void test_assert_exclusive_access_writer(struct kunit *test)
{
- const struct expect_report expect_access_writer = {
+ struct expect_report expect_access_writer = {
.access = {
{ test_kernel_assert_access, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_WRITE },
{ test_kernel_assert_writer, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT },
},
};
- const struct expect_report expect_access_access = {
+ struct expect_report expect_access_access = {
.access = {
{ test_kernel_assert_access, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_WRITE },
{ test_kernel_assert_access, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_WRITE },
},
};
- const struct expect_report never = {
+ struct expect_report never = {
.access = {
{ test_kernel_assert_writer, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT },
{ test_kernel_assert_writer, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT },
@@ -813,7 +847,7 @@ static void test_assert_exclusive_access_writer(struct kunit *test)
__no_kcsan
static void test_assert_exclusive_bits_change(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_assert_bits_change, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT },
{ test_kernel_change_bits, &test_var, sizeof(test_var),
@@ -844,13 +878,13 @@ static void test_assert_exclusive_bits_nochange(struct kunit *test)
__no_kcsan
static void test_assert_exclusive_writer_scoped(struct kunit *test)
{
- const struct expect_report expect_start = {
+ struct expect_report expect_start = {
.access = {
{ test_kernel_assert_writer_scoped, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_SCOPED },
{ test_kernel_write_nochange, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
},
};
- const struct expect_report expect_inscope = {
+ struct expect_report expect_inscope = {
.access = {
{ test_enter_scope, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_SCOPED },
{ test_kernel_write_nochange, &test_var, sizeof(test_var), KCSAN_ACCESS_WRITE },
@@ -871,16 +905,16 @@ static void test_assert_exclusive_writer_scoped(struct kunit *test)
__no_kcsan
static void test_assert_exclusive_access_scoped(struct kunit *test)
{
- const struct expect_report expect_start1 = {
+ struct expect_report expect_start1 = {
.access = {
{ test_kernel_assert_access_scoped, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_SCOPED },
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
},
};
- const struct expect_report expect_start2 = {
+ struct expect_report expect_start2 = {
.access = { expect_start1.access[0], expect_start1.access[0] },
};
- const struct expect_report expect_inscope = {
+ struct expect_report expect_inscope = {
.access = {
{ test_enter_scope, &test_var, sizeof(test_var), KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_SCOPED },
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
@@ -985,7 +1019,7 @@ static void test_atomic_builtins(struct kunit *test)
__no_kcsan
static void test_1bit_value_change(struct kunit *test)
{
- const struct expect_report expect = {
+ struct expect_report expect = {
.access = {
{ test_kernel_read, &test_var, sizeof(test_var), 0 },
{ test_kernel_xor_1bit, &test_var, sizeof(test_var), __KCSAN_ACCESS_RW(KCSAN_ACCESS_WRITE) },
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:07 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Document how KCSAN models a subset of weak memory and the subset of
missing memory barriers it can detect as a result.

Signed-off-by: Marco Elver <el...@google.com>
---
Documentation/dev-tools/kcsan.rst | 72 +++++++++++++++++++++++++------
1 file changed, 59 insertions(+), 13 deletions(-)

diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
index 7db43c7c09b8..4fc3773fead9 100644
--- a/Documentation/dev-tools/kcsan.rst
+++ b/Documentation/dev-tools/kcsan.rst
@@ -204,17 +204,17 @@ Ultimately this allows to determine the possible executions of concurrent code,
and if that code is free from data races.

KCSAN is aware of *marked atomic operations* (``READ_ONCE``, ``WRITE_ONCE``,
-``atomic_*``, etc.), but is oblivious of any ordering guarantees and simply
-assumes that memory barriers are placed correctly. In other words, KCSAN
-assumes that as long as a plain access is not observed to race with another
-conflicting access, memory operations are correctly ordered.
-
-This means that KCSAN will not report *potential* data races due to missing
-memory ordering. Developers should therefore carefully consider the required
-memory ordering requirements that remain unchecked. If, however, missing
-memory ordering (that is observable with a particular compiler and
-architecture) leads to an observable data race (e.g. entering a critical
-section erroneously), KCSAN would report the resulting data race.
+``atomic_*``, etc.), and a subset of ordering guarantees implied by memory
+barriers. With ``CONFIG_KCSAN_WEAK_MEMORY=y``, KCSAN models load or store
+buffering, and can detect missing ``smp_mb()``, ``smp_wmb()``, ``smp_rmb()``,
+``smp_store_release()``, and all ``atomic_*`` operations with equivalent
+implied barriers.
+
+Note, KCSAN will not report all data races due to missing memory ordering,
+specifically where a memory barrier would be required to prohibit subsequent
+memory operation from reordering before the barrier. Developers should
+therefore carefully consider the required memory ordering requirements that
+remain unchecked.

Race Detection Beyond Data Races
--------------------------------
@@ -268,6 +268,52 @@ marked operations, if all accesses to a variable that is accessed concurrently
are properly marked, KCSAN will never trigger a watchpoint and therefore never
report the accesses.

+Modeling Weak Memory
+~~~~~~~~~~~~~~~~~~~~
+
+KCSAN's approach to detecting data races due to missing memory barriers is
+based on modeling access reordering (with ``CONFIG_KCSAN_WEAK_MEMORY=y``).
+Each plain memory access for which a watchpoint is set up, is also selected for
+simulated reordering within the scope of its function (at most 1 in-flight
+access).
+
+Once an access has been selected for reordering, it is checked along every
+other access until the end of the function scope. If an appropriate memory
+barrier is encountered, the access will no longer be considered for simulated
+reordering.
+
+When the result of a memory operation should be ordered by a barrier, KCSAN can
+then detect data races where the conflict only occurs as a result of a missing
+barrier. Consider the example::
+
+ int x, flag;
+ void T1(void)
+ {
+ x = 1; // data race!
+ WRITE_ONCE(flag, 1); // correct: smp_store_release(&flag, 1)
+ }
+ void T2(void)
+ {
+ while (!READ_ONCE(flag)); // correct: smp_load_acquire(&flag)
+ ... = x; // data race!
+ }
+
+When weak memory modeling is enabled, KCSAN can consider ``x`` in ``T1`` for
+simulated reordering. After the write of ``flag``, ``x`` is again checked for
+concurrent accesses: because ``T2`` is able to proceed after the write of
+``flag``, a data race is detected. With the correct barriers in place, ``x``
+would not be considered for reordering after the proper release of ``flag``,
+and no data race would be detected.
+
+Deliberate trade-offs in complexity but also practical limitations mean only a
+subset of data races due to missing memory barriers can be detected. Recall
+that watchpoints are only set up for plain accesses, and the only access type
+for which KCSAN simulates reordering. This means reordering of marked accesses
+is not modeled. Furthermore, with the currently available compiler support, the
+implementation is limited to modeling the effects of "buffering" (delaying
+accesses), since the runtime cannot "prefetch" accesses. One implication of
+this is that acquire operations do not require barrier instrumentation.
+
Key Properties
~~~~~~~~~~~~~~

@@ -290,8 +336,8 @@ Key Properties
4. **Detects Racy Writes from Devices:** Due to checking data values upon
setting up watchpoints, racy writes from devices can also be detected.

-5. **Memory Ordering:** KCSAN is *not* explicitly aware of the LKMM's ordering
- rules; this may result in missed data races (false negatives).
+5. **Memory Ordering:** KCSAN is aware of only a subset of LKMM ordering rules;
+ this may result in missed data races (false negatives).

6. **Analysis Accuracy:** For observed executions, due to using a sampling
strategy, the analysis is *unsound* (false negatives possible), but aims to
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:10 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Adds test cases to check that memory barriers are instrumented
correctly, and detection of missing memory barriers is working as
intended if CONFIG_KCSAN_STRICT=y.

Signed-off-by: Marco Elver <el...@google.com>
---
kernel/kcsan/kcsan_test.c | 320 ++++++++++++++++++++++++++++++++++++++
1 file changed, 320 insertions(+)

diff --git a/kernel/kcsan/kcsan_test.c b/kernel/kcsan/kcsan_test.c
index ec054879201b..469f3044a5b3 100644
--- a/kernel/kcsan/kcsan_test.c
+++ b/kernel/kcsan/kcsan_test.c
@@ -16,9 +16,12 @@
#define pr_fmt(fmt) "kcsan_test: " fmt

#include <kunit/test.h>
+#include <linux/atomic.h>
+#include <linux/bitops.h>
#include <linux/jiffies.h>
#include <linux/kcsan-checks.h>
#include <linux/kernel.h>
+#include <linux/mutex.h>
#include <linux/sched.h>
#include <linux/seqlock.h>
#include <linux/spinlock.h>
@@ -305,6 +308,16 @@ static DEFINE_SEQLOCK(test_seqlock);
__no_kcsan
static noinline void sink_value(long v) { WRITE_ONCE(test_sink, v); }

+/*
+ * Generates a delay and some accesses that enter the runtime but do not produce
+ * data races.
+ */
+static noinline void test_delay(int iter)
+{
+ while (iter--)
+ sink_value(READ_ONCE(test_sink));
+}
+
static noinline void test_kernel_read(void) { sink_value(test_var); }

static noinline void test_kernel_write(void)
@@ -466,8 +479,220 @@ static noinline void test_kernel_xor_1bit(void)
kcsan_nestable_atomic_end();
}

+#define TEST_KERNEL_LOCKED(name, acquire, release) \
+ static noinline void test_kernel_##name(void) \
+ { \
+ long *flag = &test_struct.val[0]; \
+ long v = 0; \
+ if (!(acquire)) \
+ return; \
+ while (v++ < 100) { \
+ test_var++; \
+ barrier(); \
+ } \
+ release; \
+ test_delay(10); \
+ }
+
+TEST_KERNEL_LOCKED(with_memorder,
+ cmpxchg_acquire(flag, 0, 1) == 0,
+ smp_store_release(flag, 0));
+TEST_KERNEL_LOCKED(wrong_memorder,
+ cmpxchg_relaxed(flag, 0, 1) == 0,
+ WRITE_ONCE(*flag, 0));
+TEST_KERNEL_LOCKED(atomic_builtin_with_memorder,
+ __atomic_compare_exchange_n(flag, &v, 1, 0, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED),
+ __atomic_store_n(flag, 0, __ATOMIC_RELEASE));
+TEST_KERNEL_LOCKED(atomic_builtin_wrong_memorder,
+ __atomic_compare_exchange_n(flag, &v, 1, 0, __ATOMIC_RELAXED, __ATOMIC_RELAXED),
+ __atomic_store_n(flag, 0, __ATOMIC_RELAXED));
+
/* ===== Test cases ===== */

+/*
+ * Tests that various barriers have the expected effect on internal state. Not
+ * exhaustive on atomic_t operations. Unlike the selftest, also checks for
+ * too-strict barrier instrumentation; these can be tolerated, because it does
+ * not cause false positives, but at least we should be aware of such cases.
+ */
+__no_kcsan
+static void test_barrier_nothreads(struct kunit *test)
+{
+#ifdef CONFIG_KCSAN_WEAK_MEMORY
+ struct kcsan_scoped_access *reorder_access = &current->kcsan_ctx.reorder_access;
+#else
+ struct kcsan_scoped_access *reorder_access = NULL;
+#endif
+ arch_spinlock_t arch_spinlock = __ARCH_SPIN_LOCK_UNLOCKED;
+ DEFINE_SPINLOCK(spinlock);
+ DEFINE_MUTEX(mutex);
+ atomic_t dummy;
+
+ KCSAN_TEST_REQUIRES(test, reorder_access != NULL);
+ KCSAN_TEST_REQUIRES(test, IS_ENABLED(CONFIG_SMP));
+
+#define __KCSAN_EXPECT_BARRIER(access_type, barrier, order_before, name) \
+ do { \
+ reorder_access->type = (access_type) | KCSAN_ACCESS_SCOPED; \
+ reorder_access->size = sizeof(test_var); \
+ barrier; \
+ KUNIT_EXPECT_EQ_MSG(test, reorder_access->size, \
+ order_before ? 0 : sizeof(test_var), \
+ "improperly instrumented type=(" #access_type "): " name); \
+ } while (0)
+#define KCSAN_EXPECT_READ_BARRIER(b, o) __KCSAN_EXPECT_BARRIER(0, b, o, #b)
+#define KCSAN_EXPECT_WRITE_BARRIER(b, o) __KCSAN_EXPECT_BARRIER(KCSAN_ACCESS_WRITE, b, o, #b)
+#define KCSAN_EXPECT_RW_BARRIER(b, o) __KCSAN_EXPECT_BARRIER(KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE, b, o, #b)
+
+ /* Force creating a valid entry in reorder_access first. */
+ test_var = 0;
+ while (test_var++ < 1000000 && reorder_access->size != sizeof(test_var))
+ __kcsan_check_read(&test_var, sizeof(test_var));
+ KUNIT_ASSERT_EQ(test, reorder_access->size, sizeof(test_var));
+
+ kcsan_nestable_atomic_begin(); /* No watchpoints in called functions. */
+
+ KCSAN_EXPECT_READ_BARRIER(mb(), true);
+ KCSAN_EXPECT_READ_BARRIER(wmb(), false);
+ KCSAN_EXPECT_READ_BARRIER(rmb(), true);
+ KCSAN_EXPECT_READ_BARRIER(smp_mb(), true);
+ KCSAN_EXPECT_READ_BARRIER(smp_wmb(), false);
+ KCSAN_EXPECT_READ_BARRIER(smp_rmb(), true);
+ KCSAN_EXPECT_READ_BARRIER(dma_wmb(), false);
+ KCSAN_EXPECT_READ_BARRIER(dma_rmb(), true);
+ KCSAN_EXPECT_READ_BARRIER(smp_mb__before_atomic(), true);
+ KCSAN_EXPECT_READ_BARRIER(smp_mb__after_atomic(), true);
+ KCSAN_EXPECT_READ_BARRIER(smp_mb__after_spinlock(), true);
+ KCSAN_EXPECT_READ_BARRIER(smp_store_mb(test_var, 0), true);
+ KCSAN_EXPECT_READ_BARRIER(smp_load_acquire(&test_var), false);
+ KCSAN_EXPECT_READ_BARRIER(smp_store_release(&test_var, 0), true);
+ KCSAN_EXPECT_READ_BARRIER(xchg(&test_var, 0), true);
+ KCSAN_EXPECT_READ_BARRIER(xchg_release(&test_var, 0), true);
+ KCSAN_EXPECT_READ_BARRIER(xchg_relaxed(&test_var, 0), false);
+ KCSAN_EXPECT_READ_BARRIER(cmpxchg(&test_var, 0, 0), true);
+ KCSAN_EXPECT_READ_BARRIER(cmpxchg_release(&test_var, 0, 0), true);
+ KCSAN_EXPECT_READ_BARRIER(cmpxchg_relaxed(&test_var, 0, 0), false);
+ KCSAN_EXPECT_READ_BARRIER(atomic_read(&dummy), false);
+ KCSAN_EXPECT_READ_BARRIER(atomic_read_acquire(&dummy), false);
+ KCSAN_EXPECT_READ_BARRIER(atomic_set(&dummy, 0), false);
+ KCSAN_EXPECT_READ_BARRIER(atomic_set_release(&dummy, 0), true);
+ KCSAN_EXPECT_READ_BARRIER(atomic_add(1, &dummy), false);
+ KCSAN_EXPECT_READ_BARRIER(atomic_add_return(1, &dummy), true);
+ KCSAN_EXPECT_READ_BARRIER(atomic_add_return_acquire(1, &dummy), false);
+ KCSAN_EXPECT_READ_BARRIER(atomic_add_return_release(1, &dummy), true);
+ KCSAN_EXPECT_READ_BARRIER(atomic_add_return_relaxed(1, &dummy), false);
+ KCSAN_EXPECT_READ_BARRIER(atomic_fetch_add(1, &dummy), true);
+ KCSAN_EXPECT_READ_BARRIER(atomic_fetch_add_acquire(1, &dummy), false);
+ KCSAN_EXPECT_READ_BARRIER(atomic_fetch_add_release(1, &dummy), true);
+ KCSAN_EXPECT_READ_BARRIER(atomic_fetch_add_relaxed(1, &dummy), false);
+ KCSAN_EXPECT_READ_BARRIER(test_and_set_bit(0, &test_var), true);
+ KCSAN_EXPECT_READ_BARRIER(test_and_clear_bit(0, &test_var), true);
+ KCSAN_EXPECT_READ_BARRIER(test_and_change_bit(0, &test_var), true);
+ KCSAN_EXPECT_READ_BARRIER(clear_bit_unlock(0, &test_var), true);
+ KCSAN_EXPECT_READ_BARRIER(__clear_bit_unlock(0, &test_var), true);
+ KCSAN_EXPECT_READ_BARRIER(clear_bit_unlock_is_negative_byte(0, &test_var), true);
+ KCSAN_EXPECT_READ_BARRIER(arch_spin_lock(&arch_spinlock), false);
+ KCSAN_EXPECT_READ_BARRIER(arch_spin_unlock(&arch_spinlock), true);
+ KCSAN_EXPECT_READ_BARRIER(spin_lock(&spinlock), false);
+ KCSAN_EXPECT_READ_BARRIER(spin_unlock(&spinlock), true);
+ KCSAN_EXPECT_READ_BARRIER(mutex_lock(&mutex), false);
+ KCSAN_EXPECT_READ_BARRIER(mutex_unlock(&mutex), true);
+
+ KCSAN_EXPECT_WRITE_BARRIER(mb(), true);
+ KCSAN_EXPECT_WRITE_BARRIER(wmb(), true);
+ KCSAN_EXPECT_WRITE_BARRIER(rmb(), false);
+ KCSAN_EXPECT_WRITE_BARRIER(smp_mb(), true);
+ KCSAN_EXPECT_WRITE_BARRIER(smp_wmb(), true);
+ KCSAN_EXPECT_WRITE_BARRIER(smp_rmb(), false);
+ KCSAN_EXPECT_WRITE_BARRIER(dma_wmb(), true);
+ KCSAN_EXPECT_WRITE_BARRIER(dma_rmb(), false);
+ KCSAN_EXPECT_WRITE_BARRIER(smp_mb__before_atomic(), true);
+ KCSAN_EXPECT_WRITE_BARRIER(smp_mb__after_atomic(), true);
+ KCSAN_EXPECT_WRITE_BARRIER(smp_mb__after_spinlock(), true);
+ KCSAN_EXPECT_WRITE_BARRIER(smp_store_mb(test_var, 0), true);
+ KCSAN_EXPECT_WRITE_BARRIER(smp_load_acquire(&test_var), false);
+ KCSAN_EXPECT_WRITE_BARRIER(smp_store_release(&test_var, 0), true);
+ KCSAN_EXPECT_WRITE_BARRIER(xchg(&test_var, 0), true);
+ KCSAN_EXPECT_WRITE_BARRIER(xchg_release(&test_var, 0), true);
+ KCSAN_EXPECT_WRITE_BARRIER(xchg_relaxed(&test_var, 0), false);
+ KCSAN_EXPECT_WRITE_BARRIER(cmpxchg(&test_var, 0, 0), true);
+ KCSAN_EXPECT_WRITE_BARRIER(cmpxchg_release(&test_var, 0, 0), true);
+ KCSAN_EXPECT_WRITE_BARRIER(cmpxchg_relaxed(&test_var, 0, 0), false);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_read(&dummy), false);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_read_acquire(&dummy), false);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_set(&dummy, 0), false);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_set_release(&dummy, 0), true);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_add(1, &dummy), false);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_add_return(1, &dummy), true);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_add_return_acquire(1, &dummy), false);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_add_return_release(1, &dummy), true);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_add_return_relaxed(1, &dummy), false);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_fetch_add(1, &dummy), true);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_fetch_add_acquire(1, &dummy), false);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_fetch_add_release(1, &dummy), true);
+ KCSAN_EXPECT_WRITE_BARRIER(atomic_fetch_add_relaxed(1, &dummy), false);
+ KCSAN_EXPECT_WRITE_BARRIER(test_and_set_bit(0, &test_var), true);
+ KCSAN_EXPECT_WRITE_BARRIER(test_and_clear_bit(0, &test_var), true);
+ KCSAN_EXPECT_WRITE_BARRIER(test_and_change_bit(0, &test_var), true);
+ KCSAN_EXPECT_WRITE_BARRIER(clear_bit_unlock(0, &test_var), true);
+ KCSAN_EXPECT_WRITE_BARRIER(__clear_bit_unlock(0, &test_var), true);
+ KCSAN_EXPECT_WRITE_BARRIER(clear_bit_unlock_is_negative_byte(0, &test_var), true);
+ KCSAN_EXPECT_WRITE_BARRIER(arch_spin_lock(&arch_spinlock), false);
+ KCSAN_EXPECT_WRITE_BARRIER(arch_spin_unlock(&arch_spinlock), true);
+ KCSAN_EXPECT_WRITE_BARRIER(spin_lock(&spinlock), false);
+ KCSAN_EXPECT_WRITE_BARRIER(spin_unlock(&spinlock), true);
+ KCSAN_EXPECT_WRITE_BARRIER(mutex_lock(&mutex), false);
+ KCSAN_EXPECT_WRITE_BARRIER(mutex_unlock(&mutex), true);
+
+ KCSAN_EXPECT_RW_BARRIER(mb(), true);
+ KCSAN_EXPECT_RW_BARRIER(wmb(), true);
+ KCSAN_EXPECT_RW_BARRIER(rmb(), true);
+ KCSAN_EXPECT_RW_BARRIER(smp_mb(), true);
+ KCSAN_EXPECT_RW_BARRIER(smp_wmb(), true);
+ KCSAN_EXPECT_RW_BARRIER(smp_rmb(), true);
+ KCSAN_EXPECT_RW_BARRIER(dma_wmb(), true);
+ KCSAN_EXPECT_RW_BARRIER(dma_rmb(), true);
+ KCSAN_EXPECT_RW_BARRIER(smp_mb__before_atomic(), true);
+ KCSAN_EXPECT_RW_BARRIER(smp_mb__after_atomic(), true);
+ KCSAN_EXPECT_RW_BARRIER(smp_mb__after_spinlock(), true);
+ KCSAN_EXPECT_RW_BARRIER(smp_store_mb(test_var, 0), true);
+ KCSAN_EXPECT_RW_BARRIER(smp_load_acquire(&test_var), false);
+ KCSAN_EXPECT_RW_BARRIER(smp_store_release(&test_var, 0), true);
+ KCSAN_EXPECT_RW_BARRIER(xchg(&test_var, 0), true);
+ KCSAN_EXPECT_RW_BARRIER(xchg_release(&test_var, 0), true);
+ KCSAN_EXPECT_RW_BARRIER(xchg_relaxed(&test_var, 0), false);
+ KCSAN_EXPECT_RW_BARRIER(cmpxchg(&test_var, 0, 0), true);
+ KCSAN_EXPECT_RW_BARRIER(cmpxchg_release(&test_var, 0, 0), true);
+ KCSAN_EXPECT_RW_BARRIER(cmpxchg_relaxed(&test_var, 0, 0), false);
+ KCSAN_EXPECT_RW_BARRIER(atomic_read(&dummy), false);
+ KCSAN_EXPECT_RW_BARRIER(atomic_read_acquire(&dummy), false);
+ KCSAN_EXPECT_RW_BARRIER(atomic_set(&dummy, 0), false);
+ KCSAN_EXPECT_RW_BARRIER(atomic_set_release(&dummy, 0), true);
+ KCSAN_EXPECT_RW_BARRIER(atomic_add(1, &dummy), false);
+ KCSAN_EXPECT_RW_BARRIER(atomic_add_return(1, &dummy), true);
+ KCSAN_EXPECT_RW_BARRIER(atomic_add_return_acquire(1, &dummy), false);
+ KCSAN_EXPECT_RW_BARRIER(atomic_add_return_release(1, &dummy), true);
+ KCSAN_EXPECT_RW_BARRIER(atomic_add_return_relaxed(1, &dummy), false);
+ KCSAN_EXPECT_RW_BARRIER(atomic_fetch_add(1, &dummy), true);
+ KCSAN_EXPECT_RW_BARRIER(atomic_fetch_add_acquire(1, &dummy), false);
+ KCSAN_EXPECT_RW_BARRIER(atomic_fetch_add_release(1, &dummy), true);
+ KCSAN_EXPECT_RW_BARRIER(atomic_fetch_add_relaxed(1, &dummy), false);
+ KCSAN_EXPECT_RW_BARRIER(test_and_set_bit(0, &test_var), true);
+ KCSAN_EXPECT_RW_BARRIER(test_and_clear_bit(0, &test_var), true);
+ KCSAN_EXPECT_RW_BARRIER(test_and_change_bit(0, &test_var), true);
+ KCSAN_EXPECT_RW_BARRIER(clear_bit_unlock(0, &test_var), true);
+ KCSAN_EXPECT_RW_BARRIER(__clear_bit_unlock(0, &test_var), true);
+ KCSAN_EXPECT_RW_BARRIER(clear_bit_unlock_is_negative_byte(0, &test_var), true);
+ KCSAN_EXPECT_RW_BARRIER(arch_spin_lock(&arch_spinlock), false);
+ KCSAN_EXPECT_RW_BARRIER(arch_spin_unlock(&arch_spinlock), true);
+ KCSAN_EXPECT_RW_BARRIER(spin_lock(&spinlock), false);
+ KCSAN_EXPECT_RW_BARRIER(spin_unlock(&spinlock), true);
+ KCSAN_EXPECT_RW_BARRIER(mutex_lock(&mutex), false);
+ KCSAN_EXPECT_RW_BARRIER(mutex_unlock(&mutex), true);
+
+ kcsan_nestable_atomic_end();
+}
+
/* Simple test with normal data race. */
__no_kcsan
static void test_basic(struct kunit *test)
@@ -1039,6 +1264,90 @@ static void test_1bit_value_change(struct kunit *test)
KUNIT_EXPECT_TRUE(test, match);
}

+__no_kcsan
+static void test_correct_barrier(struct kunit *test)
+{
+ struct expect_report expect = {
+ .access = {
+ { test_kernel_with_memorder, &test_var, sizeof(test_var), __KCSAN_ACCESS_RW(KCSAN_ACCESS_WRITE) },
+ { test_kernel_with_memorder, &test_var, sizeof(test_var), __KCSAN_ACCESS_RW(0) },
+ },
+ };
+ bool match_expect = false;
+
+ test_struct.val[0] = 0; /* init unlocked */
+ begin_test_checks(test_kernel_with_memorder, test_kernel_with_memorder);
+ do {
+ match_expect = report_matches_any_reordered(&expect);
+ } while (!end_test_checks(match_expect));
+ KUNIT_EXPECT_FALSE(test, match_expect);
+}
+
+__no_kcsan
+static void test_missing_barrier(struct kunit *test)
+{
+ struct expect_report expect = {
+ .access = {
+ { test_kernel_wrong_memorder, &test_var, sizeof(test_var), __KCSAN_ACCESS_RW(KCSAN_ACCESS_WRITE) },
+ { test_kernel_wrong_memorder, &test_var, sizeof(test_var), __KCSAN_ACCESS_RW(0) },
+ },
+ };
+ bool match_expect = false;
+
+ test_struct.val[0] = 0; /* init unlocked */
+ begin_test_checks(test_kernel_wrong_memorder, test_kernel_wrong_memorder);
+ do {
+ match_expect = report_matches_any_reordered(&expect);
+ } while (!end_test_checks(match_expect));
+ if (IS_ENABLED(CONFIG_KCSAN_WEAK_MEMORY))
+ KUNIT_EXPECT_TRUE(test, match_expect);
+ else
+ KUNIT_EXPECT_FALSE(test, match_expect);
+}
+
+__no_kcsan
+static void test_atomic_builtins_correct_barrier(struct kunit *test)
+{
+ struct expect_report expect = {
+ .access = {
+ { test_kernel_atomic_builtin_with_memorder, &test_var, sizeof(test_var), __KCSAN_ACCESS_RW(KCSAN_ACCESS_WRITE) },
+ { test_kernel_atomic_builtin_with_memorder, &test_var, sizeof(test_var), __KCSAN_ACCESS_RW(0) },
+ },
+ };
+ bool match_expect = false;
+
+ test_struct.val[0] = 0; /* init unlocked */
+ begin_test_checks(test_kernel_atomic_builtin_with_memorder,
+ test_kernel_atomic_builtin_with_memorder);
+ do {
+ match_expect = report_matches_any_reordered(&expect);
+ } while (!end_test_checks(match_expect));
+ KUNIT_EXPECT_FALSE(test, match_expect);
+}
+
+__no_kcsan
+static void test_atomic_builtins_missing_barrier(struct kunit *test)
+{
+ struct expect_report expect = {
+ .access = {
+ { test_kernel_atomic_builtin_wrong_memorder, &test_var, sizeof(test_var), __KCSAN_ACCESS_RW(KCSAN_ACCESS_WRITE) },
+ { test_kernel_atomic_builtin_wrong_memorder, &test_var, sizeof(test_var), __KCSAN_ACCESS_RW(0) },
+ },
+ };
+ bool match_expect = false;
+
+ test_struct.val[0] = 0; /* init unlocked */
+ begin_test_checks(test_kernel_atomic_builtin_wrong_memorder,
+ test_kernel_atomic_builtin_wrong_memorder);
+ do {
+ match_expect = report_matches_any_reordered(&expect);
+ } while (!end_test_checks(match_expect));
+ if (IS_ENABLED(CONFIG_KCSAN_WEAK_MEMORY))
+ KUNIT_EXPECT_TRUE(test, match_expect);
+ else
+ KUNIT_EXPECT_FALSE(test, match_expect);
+}
+
/*
* Generate thread counts for all test cases. Values generated are in interval
* [2, 5] followed by exponentially increasing thread counts from 8 to 32.
@@ -1088,6 +1397,7 @@ static const void *nthreads_gen_params(const void *prev, char *desc)

#define KCSAN_KUNIT_CASE(test_name) KUNIT_CASE_PARAM(test_name, nthreads_gen_params)
static struct kunit_case kcsan_test_cases[] = {
+ KUNIT_CASE(test_barrier_nothreads),
KCSAN_KUNIT_CASE(test_basic),
KCSAN_KUNIT_CASE(test_concurrent_races),
KCSAN_KUNIT_CASE(test_novalue_change),
@@ -1112,6 +1422,10 @@ static struct kunit_case kcsan_test_cases[] = {
KCSAN_KUNIT_CASE(test_seqlock_noreport),
KCSAN_KUNIT_CASE(test_atomic_builtins),
KCSAN_KUNIT_CASE(test_1bit_value_change),
+ KCSAN_KUNIT_CASE(test_correct_barrier),
+ KCSAN_KUNIT_CASE(test_missing_barrier),
+ KCSAN_KUNIT_CASE(test_atomic_builtins_correct_barrier),
+ KCSAN_KUNIT_CASE(test_atomic_builtins_missing_barrier),
{},
};

@@ -1176,6 +1490,9 @@ static int test_init(struct kunit *test)
observed.nlines = 0;
spin_unlock_irqrestore(&observed.lock, flags);

+ if (strstr(test->name, "nothreads"))
+ return 0;
+
if (!torture_init_begin((char *)test->name, 1))
return -EBUSY;

@@ -1218,6 +1535,9 @@ static void test_exit(struct kunit *test)
struct task_struct **stop_thread;
int i;

+ if (strstr(test->name, "nothreads"))
+ return;
+
if (torture_cleanup_begin())
return;

--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:14 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
GCC 11 has introduced a new warning option, -Wtsan [1], to warn about
unsupported operations in the TSan runtime. But KCSAN != TSan runtime,
so none of the warnings apply.

[1] https://gcc.gnu.org/onlinedocs/gcc-11.1.0/gcc/Warning-Options.html

Ignore the warnings.

Currently the warning only fires in the test for __atomic_thread_fence():

kernel/kcsan/kcsan_test.c: In function ‘test_atomic_builtins’:
kernel/kcsan/kcsan_test.c:1234:17: warning: ‘atomic_thread_fence’ is not supported with ‘-fsanitize=thread’ [-Wtsan]
1234 | __atomic_thread_fence(__ATOMIC_SEQ_CST);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

which exists to ensure the KCSAN runtime keeps supporting the builtin
instrumentation.

Signed-off-by: Marco Elver <el...@google.com>
---
scripts/Makefile.kcsan | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
index 4c7f0d282e42..19f693b68a96 100644
--- a/scripts/Makefile.kcsan
+++ b/scripts/Makefile.kcsan
@@ -13,6 +13,12 @@ kcsan-cflags := -fsanitize=thread -fno-optimize-sibling-calls \
$(call cc-option,$(call cc-param,tsan-compound-read-before-write=1),$(call cc-option,$(call cc-param,tsan-instrument-read-before-write=1))) \
$(call cc-param,tsan-distinguish-volatile=1)

+ifdef CONFIG_CC_IS_GCC
+# GCC started warning about operations unsupported by the TSan runtime. But
+# KCSAN != TSan, so just ignore these warnings.
+kcsan-cflags += -Wno-tsan
+endif
+
ifndef CONFIG_KCSAN_WEAK_MEMORY
kcsan-cflags += $(call cc-option,$(call cc-param,tsan-instrument-func-entry-exit=0))
endif
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:15 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Memory barrier instrumentation is crucial to avoid false positives. To
avoid surprises, run a simple test case in the boot-time selftest to
ensure memory barriers are still instrumented correctly.

Signed-off-by: Marco Elver <el...@google.com>
---
kernel/kcsan/Makefile | 2 +
kernel/kcsan/selftest.c | 141 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 143 insertions(+)

diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
index c2bb07f5bcc7..ff47e896de3b 100644
--- a/kernel/kcsan/Makefile
+++ b/kernel/kcsan/Makefile
@@ -11,6 +11,8 @@ CFLAGS_core.o := $(call cc-option,-fno-conserve-stack) \
-fno-stack-protector -DDISABLE_BRANCH_PROFILING

obj-y := core.o debugfs.o report.o
+
+KCSAN_INSTRUMENT_BARRIERS_selftest.o := y
obj-$(CONFIG_KCSAN_SELFTEST) += selftest.o

CFLAGS_kcsan_test.o := $(CFLAGS_KCSAN) -g -fno-omit-frame-pointer
diff --git a/kernel/kcsan/selftest.c b/kernel/kcsan/selftest.c
index b4295a3892b7..08c6b84b9ebe 100644
--- a/kernel/kcsan/selftest.c
+++ b/kernel/kcsan/selftest.c
@@ -7,10 +7,15 @@

#define pr_fmt(fmt) "kcsan: " fmt

+#include <linux/atomic.h>
+#include <linux/bitops.h>
#include <linux/init.h>
+#include <linux/kcsan-checks.h>
#include <linux/kernel.h>
#include <linux/printk.h>
#include <linux/random.h>
+#include <linux/sched.h>
+#include <linux/spinlock.h>
#include <linux/types.h>

#include "encoding.h"
@@ -103,6 +108,141 @@ static bool __init test_matching_access(void)
return true;
}

+/*
+ * Correct memory barrier instrumentation is critical to avoiding false
+ * positives: simple test to check at boot certain barriers are always properly
+ * instrumented. See kcsan_test for a more complete test.
+ */
+static bool __init test_barrier(void)
+{
+#ifdef CONFIG_KCSAN_WEAK_MEMORY
+ struct kcsan_scoped_access *reorder_access = &current->kcsan_ctx.reorder_access;
+#else
+ struct kcsan_scoped_access *reorder_access = NULL;
+#endif
+ bool ret = true;
+ arch_spinlock_t arch_spinlock = __ARCH_SPIN_LOCK_UNLOCKED;
+ DEFINE_SPINLOCK(spinlock);
+ atomic_t dummy;
+ long test_var;
+
+ if (!reorder_access || !IS_ENABLED(CONFIG_SMP))
+ return true;
+
+#define __KCSAN_CHECK_BARRIER(access_type, barrier, name) \
+ do { \
+ reorder_access->type = (access_type) | KCSAN_ACCESS_SCOPED; \
+ reorder_access->size = 1; \
+ barrier; \
+ if (reorder_access->size != 0) { \
+ pr_err("improperly instrumented type=(" #access_type "): " name "\n"); \
+ ret = false; \
+ } \
+ } while (0)
+#define KCSAN_CHECK_READ_BARRIER(b) __KCSAN_CHECK_BARRIER(0, b, #b)
+#define KCSAN_CHECK_WRITE_BARRIER(b) __KCSAN_CHECK_BARRIER(KCSAN_ACCESS_WRITE, b, #b)
+#define KCSAN_CHECK_RW_BARRIER(b) __KCSAN_CHECK_BARRIER(KCSAN_ACCESS_WRITE | KCSAN_ACCESS_COMPOUND, b, #b)
+
+ kcsan_nestable_atomic_begin(); /* No watchpoints in called functions. */
+
+ KCSAN_CHECK_READ_BARRIER(mb());
+ KCSAN_CHECK_READ_BARRIER(rmb());
+ KCSAN_CHECK_READ_BARRIER(smp_mb());
+ KCSAN_CHECK_READ_BARRIER(smp_rmb());
+ KCSAN_CHECK_READ_BARRIER(dma_rmb());
+ KCSAN_CHECK_READ_BARRIER(smp_mb__before_atomic());
+ KCSAN_CHECK_READ_BARRIER(smp_mb__after_atomic());
+ KCSAN_CHECK_READ_BARRIER(smp_mb__after_spinlock());
+ KCSAN_CHECK_READ_BARRIER(smp_store_mb(test_var, 0));
+ KCSAN_CHECK_READ_BARRIER(smp_store_release(&test_var, 0));
+ KCSAN_CHECK_READ_BARRIER(xchg(&test_var, 0));
+ KCSAN_CHECK_READ_BARRIER(xchg_release(&test_var, 0));
+ KCSAN_CHECK_READ_BARRIER(cmpxchg(&test_var, 0, 0));
+ KCSAN_CHECK_READ_BARRIER(cmpxchg_release(&test_var, 0, 0));
+ KCSAN_CHECK_READ_BARRIER(atomic_set_release(&dummy, 0));
+ KCSAN_CHECK_READ_BARRIER(atomic_add_return(1, &dummy));
+ KCSAN_CHECK_READ_BARRIER(atomic_add_return_release(1, &dummy));
+ KCSAN_CHECK_READ_BARRIER(atomic_fetch_add(1, &dummy));
+ KCSAN_CHECK_READ_BARRIER(atomic_fetch_add_release(1, &dummy));
+ KCSAN_CHECK_READ_BARRIER(test_and_set_bit(0, &test_var));
+ KCSAN_CHECK_READ_BARRIER(test_and_clear_bit(0, &test_var));
+ KCSAN_CHECK_READ_BARRIER(test_and_change_bit(0, &test_var));
+ KCSAN_CHECK_READ_BARRIER(clear_bit_unlock(0, &test_var));
+ KCSAN_CHECK_READ_BARRIER(__clear_bit_unlock(0, &test_var));
+ KCSAN_CHECK_READ_BARRIER(clear_bit_unlock_is_negative_byte(0, &test_var));
+ arch_spin_lock(&arch_spinlock);
+ KCSAN_CHECK_READ_BARRIER(arch_spin_unlock(&arch_spinlock));
+ spin_lock(&spinlock);
+ KCSAN_CHECK_READ_BARRIER(spin_unlock(&spinlock));
+
+ KCSAN_CHECK_WRITE_BARRIER(mb());
+ KCSAN_CHECK_WRITE_BARRIER(wmb());
+ KCSAN_CHECK_WRITE_BARRIER(smp_mb());
+ KCSAN_CHECK_WRITE_BARRIER(smp_wmb());
+ KCSAN_CHECK_WRITE_BARRIER(dma_wmb());
+ KCSAN_CHECK_WRITE_BARRIER(smp_mb__before_atomic());
+ KCSAN_CHECK_WRITE_BARRIER(smp_mb__after_atomic());
+ KCSAN_CHECK_WRITE_BARRIER(smp_mb__after_spinlock());
+ KCSAN_CHECK_WRITE_BARRIER(smp_store_mb(test_var, 0));
+ KCSAN_CHECK_WRITE_BARRIER(smp_store_release(&test_var, 0));
+ KCSAN_CHECK_WRITE_BARRIER(xchg(&test_var, 0));
+ KCSAN_CHECK_WRITE_BARRIER(xchg_release(&test_var, 0));
+ KCSAN_CHECK_WRITE_BARRIER(cmpxchg(&test_var, 0, 0));
+ KCSAN_CHECK_WRITE_BARRIER(cmpxchg_release(&test_var, 0, 0));
+ KCSAN_CHECK_WRITE_BARRIER(atomic_set_release(&dummy, 0));
+ KCSAN_CHECK_WRITE_BARRIER(atomic_add_return(1, &dummy));
+ KCSAN_CHECK_WRITE_BARRIER(atomic_add_return_release(1, &dummy));
+ KCSAN_CHECK_WRITE_BARRIER(atomic_fetch_add(1, &dummy));
+ KCSAN_CHECK_WRITE_BARRIER(atomic_fetch_add_release(1, &dummy));
+ KCSAN_CHECK_WRITE_BARRIER(test_and_set_bit(0, &test_var));
+ KCSAN_CHECK_WRITE_BARRIER(test_and_clear_bit(0, &test_var));
+ KCSAN_CHECK_WRITE_BARRIER(test_and_change_bit(0, &test_var));
+ KCSAN_CHECK_WRITE_BARRIER(clear_bit_unlock(0, &test_var));
+ KCSAN_CHECK_WRITE_BARRIER(__clear_bit_unlock(0, &test_var));
+ KCSAN_CHECK_WRITE_BARRIER(clear_bit_unlock_is_negative_byte(0, &test_var));
+ arch_spin_lock(&arch_spinlock);
+ KCSAN_CHECK_WRITE_BARRIER(arch_spin_unlock(&arch_spinlock));
+ spin_lock(&spinlock);
+ KCSAN_CHECK_WRITE_BARRIER(spin_unlock(&spinlock));
+
+ KCSAN_CHECK_RW_BARRIER(mb());
+ KCSAN_CHECK_RW_BARRIER(wmb());
+ KCSAN_CHECK_RW_BARRIER(rmb());
+ KCSAN_CHECK_RW_BARRIER(smp_mb());
+ KCSAN_CHECK_RW_BARRIER(smp_wmb());
+ KCSAN_CHECK_RW_BARRIER(smp_rmb());
+ KCSAN_CHECK_RW_BARRIER(dma_wmb());
+ KCSAN_CHECK_RW_BARRIER(dma_rmb());
+ KCSAN_CHECK_RW_BARRIER(smp_mb__before_atomic());
+ KCSAN_CHECK_RW_BARRIER(smp_mb__after_atomic());
+ KCSAN_CHECK_RW_BARRIER(smp_mb__after_spinlock());
+ KCSAN_CHECK_RW_BARRIER(smp_store_mb(test_var, 0));
+ KCSAN_CHECK_RW_BARRIER(smp_store_release(&test_var, 0));
+ KCSAN_CHECK_RW_BARRIER(xchg(&test_var, 0));
+ KCSAN_CHECK_RW_BARRIER(xchg_release(&test_var, 0));
+ KCSAN_CHECK_RW_BARRIER(cmpxchg(&test_var, 0, 0));
+ KCSAN_CHECK_RW_BARRIER(cmpxchg_release(&test_var, 0, 0));
+ KCSAN_CHECK_RW_BARRIER(atomic_set_release(&dummy, 0));
+ KCSAN_CHECK_RW_BARRIER(atomic_add_return(1, &dummy));
+ KCSAN_CHECK_RW_BARRIER(atomic_add_return_release(1, &dummy));
+ KCSAN_CHECK_RW_BARRIER(atomic_fetch_add(1, &dummy));
+ KCSAN_CHECK_RW_BARRIER(atomic_fetch_add_release(1, &dummy));
+ KCSAN_CHECK_RW_BARRIER(test_and_set_bit(0, &test_var));
+ KCSAN_CHECK_RW_BARRIER(test_and_clear_bit(0, &test_var));
+ KCSAN_CHECK_RW_BARRIER(test_and_change_bit(0, &test_var));
+ KCSAN_CHECK_RW_BARRIER(clear_bit_unlock(0, &test_var));
+ KCSAN_CHECK_RW_BARRIER(__clear_bit_unlock(0, &test_var));
+ KCSAN_CHECK_RW_BARRIER(clear_bit_unlock_is_negative_byte(0, &test_var));
+ arch_spin_lock(&arch_spinlock);
+ KCSAN_CHECK_RW_BARRIER(arch_spin_unlock(&arch_spinlock));
+ spin_lock(&spinlock);
+ KCSAN_CHECK_RW_BARRIER(spin_unlock(&spinlock));
+
+ kcsan_nestable_atomic_end();
+
+ return ret;
+}
+
static int __init kcsan_selftest(void)
{
int passed = 0;
@@ -120,6 +260,7 @@ static int __init kcsan_selftest(void)
RUN_TEST(test_requires);
RUN_TEST(test_encode_decode);
RUN_TEST(test_matching_access);
+ RUN_TEST(test_barrier);

pr_info("selftest: %d/%d tests passed\n", passed, total);
if (passed != total)
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:17 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Adds the required KCSAN instrumentation for barriers if CONFIG_SMP.
KCSAN supports modeling the effects of:

smp_mb()
smp_rmb()
smp_wmb()
smp_store_release()

Signed-off-by: Marco Elver <el...@google.com>
---
include/asm-generic/barrier.h | 29 +++++++++++++++--------------
include/linux/spinlock.h | 2 +-
2 files changed, 16 insertions(+), 15 deletions(-)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 640f09479bdf..27a9c9edfef6 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -14,6 +14,7 @@
#ifndef __ASSEMBLY__

#include <linux/compiler.h>
+#include <linux/kcsan-checks.h>
#include <asm/rwonce.h>

#ifndef nop
@@ -62,15 +63,15 @@
#ifdef CONFIG_SMP

#ifndef smp_mb
-#define smp_mb() __smp_mb()
+#define smp_mb() do { kcsan_mb(); __smp_mb(); } while (0)
#endif

#ifndef smp_rmb
-#define smp_rmb() __smp_rmb()
+#define smp_rmb() do { kcsan_rmb(); __smp_rmb(); } while (0)
#endif

#ifndef smp_wmb
-#define smp_wmb() __smp_wmb()
+#define smp_wmb() do { kcsan_wmb(); __smp_wmb(); } while (0)
#endif

#else /* !CONFIG_SMP */
@@ -123,19 +124,19 @@ do { \
#ifdef CONFIG_SMP

#ifndef smp_store_mb
-#define smp_store_mb(var, value) __smp_store_mb(var, value)
+#define smp_store_mb(var, value) do { kcsan_mb(); __smp_store_mb(var, value); } while (0)
#endif

#ifndef smp_mb__before_atomic
-#define smp_mb__before_atomic() __smp_mb__before_atomic()
+#define smp_mb__before_atomic() do { kcsan_mb(); __smp_mb__before_atomic(); } while (0)
#endif

#ifndef smp_mb__after_atomic
-#define smp_mb__after_atomic() __smp_mb__after_atomic()
+#define smp_mb__after_atomic() do { kcsan_mb(); __smp_mb__after_atomic(); } while (0)
#endif

#ifndef smp_store_release
-#define smp_store_release(p, v) __smp_store_release(p, v)
+#define smp_store_release(p, v) do { kcsan_release(); __smp_store_release(p, v); } while (0)
#endif

#ifndef smp_load_acquire
@@ -178,13 +179,13 @@ do { \
#endif /* CONFIG_SMP */

/* Barriers for virtual machine guests when talking to an SMP host */
-#define virt_mb() __smp_mb()
-#define virt_rmb() __smp_rmb()
-#define virt_wmb() __smp_wmb()
-#define virt_store_mb(var, value) __smp_store_mb(var, value)
-#define virt_mb__before_atomic() __smp_mb__before_atomic()
-#define virt_mb__after_atomic() __smp_mb__after_atomic()
-#define virt_store_release(p, v) __smp_store_release(p, v)
+#define virt_mb() do { kcsan_mb(); __smp_mb(); } while (0)
+#define virt_rmb() do { kcsan_rmb(); __smp_rmb(); } while (0)
+#define virt_wmb() do { kcsan_wmb(); __smp_wmb(); } while (0)
+#define virt_store_mb(var, value) do { kcsan_mb(); __smp_store_mb(var, value); } while (0)
+#define virt_mb__before_atomic() do { kcsan_mb(); __smp_mb__before_atomic(); } while (0)
+#define virt_mb__after_atomic() do { kcsan_mb(); __smp_mb__after_atomic(); } while (0)
+#define virt_store_release(p, v) do { kcsan_release(); __smp_store_release(p, v); } while (0)
#define virt_load_acquire(p) __smp_load_acquire(p)

/**
diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 45310ea1b1d7..f6d69808b929 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -172,7 +172,7 @@ do { \
* Architectures that can implement ACQUIRE better need to take care.
*/
#ifndef smp_mb__after_spinlock
-#define smp_mb__after_spinlock() do { } while (0)
+#define smp_mb__after_spinlock() kcsan_mb()
#endif

#ifdef CONFIG_DEBUG_SPINLOCK
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:19 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Thus far only smp_*() barriers had been defined by asm-generic/barrier.h
based on __smp_*() barriers, because the !SMP case is usually generic.

With the introduction of instrumentation, it also makes sense to have
asm-generic/barrier.h assist in the definition of instrumented versions
of mb(), rmb(), wmb(), dma_rmb(), and dma_wmb().

Because there is no requirement to distinguish the !SMP case, the
definition can be simpler: we can avoid also providing fallbacks for the
__ prefixed cases, and only check if `defined(__<barrier>)`, to finally
define the KCSAN-instrumented versions.

This also allows for the compiler to complain if an architecture
accidentally defines both the normal and __ prefixed variant.

Signed-off-by: Marco Elver <el...@google.com>
---
include/asm-generic/barrier.h | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 27a9c9edfef6..02c4339c8eeb 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -21,6 +21,31 @@
#define nop() asm volatile ("nop")
#endif

+/*
+ * Architectures that want generic instrumentation can define __ prefixed
+ * variants of all barriers.
+ */
+
+#ifdef __mb
+#define mb() do { kcsan_mb(); __mb(); } while (0)
+#endif
+
+#ifdef __rmb
+#define rmb() do { kcsan_rmb(); __rmb(); } while (0)
+#endif
+
+#ifdef __wmb
+#define wmb() do { kcsan_wmb(); __wmb(); } while (0)
+#endif
+
+#ifdef __dma_rmb
+#define dma_rmb() do { kcsan_rmb(); __dma_rmb(); } while (0)
+#endif
+
+#ifdef __dma_wmb
+#define dma_wmb() do { kcsan_wmb(); __dma_wmb(); } while (0)
+#endif
+
/*
* Force strict CPU ordering. And yes, this is required on UP too when we're
* talking to devices.
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:22 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Adds the required KCSAN instrumentation for barriers of atomics.

Signed-off-by: Marco Elver <el...@google.com>
---
include/linux/atomic/atomic-instrumented.h | 135 ++++++++++++++++++++-
scripts/atomic/gen-atomic-instrumented.sh | 41 +++++--
2 files changed, 166 insertions(+), 10 deletions(-)

diff --git a/include/linux/atomic/atomic-instrumented.h b/include/linux/atomic/atomic-instrumented.h
index a0f654370da3..5d69b143c28e 100644
--- a/include/linux/atomic/atomic-instrumented.h
+++ b/include/linux/atomic/atomic-instrumented.h
@@ -45,6 +45,7 @@ atomic_set(atomic_t *v, int i)
static __always_inline void
atomic_set_release(atomic_t *v, int i)
{
+ kcsan_release();
instrument_atomic_write(v, sizeof(*v));
arch_atomic_set_release(v, i);
}
@@ -59,6 +60,7 @@ atomic_add(int i, atomic_t *v)
static __always_inline int
atomic_add_return(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_add_return(i, v);
}
@@ -73,6 +75,7 @@ atomic_add_return_acquire(int i, atomic_t *v)
static __always_inline int
atomic_add_return_release(int i, atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_add_return_release(i, v);
}
@@ -87,6 +90,7 @@ atomic_add_return_relaxed(int i, atomic_t *v)
static __always_inline int
atomic_fetch_add(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_add(i, v);
}
@@ -101,6 +105,7 @@ atomic_fetch_add_acquire(int i, atomic_t *v)
static __always_inline int
atomic_fetch_add_release(int i, atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_add_release(i, v);
}
@@ -122,6 +127,7 @@ atomic_sub(int i, atomic_t *v)
static __always_inline int
atomic_sub_return(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_sub_return(i, v);
}
@@ -136,6 +142,7 @@ atomic_sub_return_acquire(int i, atomic_t *v)
static __always_inline int
atomic_sub_return_release(int i, atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_sub_return_release(i, v);
}
@@ -150,6 +157,7 @@ atomic_sub_return_relaxed(int i, atomic_t *v)
static __always_inline int
atomic_fetch_sub(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_sub(i, v);
}
@@ -164,6 +172,7 @@ atomic_fetch_sub_acquire(int i, atomic_t *v)
static __always_inline int
atomic_fetch_sub_release(int i, atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_sub_release(i, v);
}
@@ -185,6 +194,7 @@ atomic_inc(atomic_t *v)
static __always_inline int
atomic_inc_return(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_inc_return(v);
}
@@ -199,6 +209,7 @@ atomic_inc_return_acquire(atomic_t *v)
static __always_inline int
atomic_inc_return_release(atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_inc_return_release(v);
}
@@ -213,6 +224,7 @@ atomic_inc_return_relaxed(atomic_t *v)
static __always_inline int
atomic_fetch_inc(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_inc(v);
}
@@ -227,6 +239,7 @@ atomic_fetch_inc_acquire(atomic_t *v)
static __always_inline int
atomic_fetch_inc_release(atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_inc_release(v);
}
@@ -248,6 +261,7 @@ atomic_dec(atomic_t *v)
static __always_inline int
atomic_dec_return(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_dec_return(v);
}
@@ -262,6 +276,7 @@ atomic_dec_return_acquire(atomic_t *v)
static __always_inline int
atomic_dec_return_release(atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_dec_return_release(v);
}
@@ -276,6 +291,7 @@ atomic_dec_return_relaxed(atomic_t *v)
static __always_inline int
atomic_fetch_dec(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_dec(v);
}
@@ -290,6 +306,7 @@ atomic_fetch_dec_acquire(atomic_t *v)
static __always_inline int
atomic_fetch_dec_release(atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_dec_release(v);
}
@@ -311,6 +328,7 @@ atomic_and(int i, atomic_t *v)
static __always_inline int
atomic_fetch_and(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_and(i, v);
}
@@ -325,6 +343,7 @@ atomic_fetch_and_acquire(int i, atomic_t *v)
static __always_inline int
atomic_fetch_and_release(int i, atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_and_release(i, v);
}
@@ -346,6 +365,7 @@ atomic_andnot(int i, atomic_t *v)
static __always_inline int
atomic_fetch_andnot(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_andnot(i, v);
}
@@ -360,6 +380,7 @@ atomic_fetch_andnot_acquire(int i, atomic_t *v)
static __always_inline int
atomic_fetch_andnot_release(int i, atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_andnot_release(i, v);
}
@@ -381,6 +402,7 @@ atomic_or(int i, atomic_t *v)
static __always_inline int
atomic_fetch_or(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_or(i, v);
}
@@ -395,6 +417,7 @@ atomic_fetch_or_acquire(int i, atomic_t *v)
static __always_inline int
atomic_fetch_or_release(int i, atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_or_release(i, v);
}
@@ -416,6 +439,7 @@ atomic_xor(int i, atomic_t *v)
static __always_inline int
atomic_fetch_xor(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_xor(i, v);
}
@@ -430,6 +454,7 @@ atomic_fetch_xor_acquire(int i, atomic_t *v)
static __always_inline int
atomic_fetch_xor_release(int i, atomic_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_xor_release(i, v);
}
@@ -444,6 +469,7 @@ atomic_fetch_xor_relaxed(int i, atomic_t *v)
static __always_inline int
atomic_xchg(atomic_t *v, int i)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_xchg(v, i);
}
@@ -458,6 +484,7 @@ atomic_xchg_acquire(atomic_t *v, int i)
static __always_inline int
atomic_xchg_release(atomic_t *v, int i)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_xchg_release(v, i);
}
@@ -472,6 +499,7 @@ atomic_xchg_relaxed(atomic_t *v, int i)
static __always_inline int
atomic_cmpxchg(atomic_t *v, int old, int new)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_cmpxchg(v, old, new);
}
@@ -486,6 +514,7 @@ atomic_cmpxchg_acquire(atomic_t *v, int old, int new)
static __always_inline int
atomic_cmpxchg_release(atomic_t *v, int old, int new)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_cmpxchg_release(v, old, new);
}
@@ -500,6 +529,7 @@ atomic_cmpxchg_relaxed(atomic_t *v, int old, int new)
static __always_inline bool
atomic_try_cmpxchg(atomic_t *v, int *old, int new)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
instrument_atomic_read_write(old, sizeof(*old));
return arch_atomic_try_cmpxchg(v, old, new);
@@ -516,6 +546,7 @@ atomic_try_cmpxchg_acquire(atomic_t *v, int *old, int new)
static __always_inline bool
atomic_try_cmpxchg_release(atomic_t *v, int *old, int new)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
instrument_atomic_read_write(old, sizeof(*old));
return arch_atomic_try_cmpxchg_release(v, old, new);
@@ -532,6 +563,7 @@ atomic_try_cmpxchg_relaxed(atomic_t *v, int *old, int new)
static __always_inline bool
atomic_sub_and_test(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_sub_and_test(i, v);
}
@@ -539,6 +571,7 @@ atomic_sub_and_test(int i, atomic_t *v)
static __always_inline bool
atomic_dec_and_test(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_dec_and_test(v);
}
@@ -546,6 +579,7 @@ atomic_dec_and_test(atomic_t *v)
static __always_inline bool
atomic_inc_and_test(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_inc_and_test(v);
}
@@ -553,6 +587,7 @@ atomic_inc_and_test(atomic_t *v)
static __always_inline bool
atomic_add_negative(int i, atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_add_negative(i, v);
}
@@ -560,6 +595,7 @@ atomic_add_negative(int i, atomic_t *v)
static __always_inline int
atomic_fetch_add_unless(atomic_t *v, int a, int u)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_fetch_add_unless(v, a, u);
}
@@ -567,6 +603,7 @@ atomic_fetch_add_unless(atomic_t *v, int a, int u)
static __always_inline bool
atomic_add_unless(atomic_t *v, int a, int u)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_add_unless(v, a, u);
}
@@ -574,6 +611,7 @@ atomic_add_unless(atomic_t *v, int a, int u)
static __always_inline bool
atomic_inc_not_zero(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_inc_not_zero(v);
}
@@ -581,6 +619,7 @@ atomic_inc_not_zero(atomic_t *v)
static __always_inline bool
atomic_inc_unless_negative(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_inc_unless_negative(v);
}
@@ -588,6 +627,7 @@ atomic_inc_unless_negative(atomic_t *v)
static __always_inline bool
atomic_dec_unless_positive(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_dec_unless_positive(v);
}
@@ -595,6 +635,7 @@ atomic_dec_unless_positive(atomic_t *v)
static __always_inline int
atomic_dec_if_positive(atomic_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_dec_if_positive(v);
}
@@ -623,6 +664,7 @@ atomic64_set(atomic64_t *v, s64 i)
static __always_inline void
atomic64_set_release(atomic64_t *v, s64 i)
{
+ kcsan_release();
instrument_atomic_write(v, sizeof(*v));
arch_atomic64_set_release(v, i);
}
@@ -637,6 +679,7 @@ atomic64_add(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_add_return(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_add_return(i, v);
}
@@ -651,6 +694,7 @@ atomic64_add_return_acquire(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_add_return_release(s64 i, atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_add_return_release(i, v);
}
@@ -665,6 +709,7 @@ atomic64_add_return_relaxed(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_add(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_add(i, v);
}
@@ -679,6 +724,7 @@ atomic64_fetch_add_acquire(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_add_release(s64 i, atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_add_release(i, v);
}
@@ -700,6 +746,7 @@ atomic64_sub(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_sub_return(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_sub_return(i, v);
}
@@ -714,6 +761,7 @@ atomic64_sub_return_acquire(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_sub_return_release(s64 i, atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_sub_return_release(i, v);
}
@@ -728,6 +776,7 @@ atomic64_sub_return_relaxed(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_sub(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_sub(i, v);
}
@@ -742,6 +791,7 @@ atomic64_fetch_sub_acquire(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_sub_release(s64 i, atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_sub_release(i, v);
}
@@ -763,6 +813,7 @@ atomic64_inc(atomic64_t *v)
static __always_inline s64
atomic64_inc_return(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_inc_return(v);
}
@@ -777,6 +828,7 @@ atomic64_inc_return_acquire(atomic64_t *v)
static __always_inline s64
atomic64_inc_return_release(atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_inc_return_release(v);
}
@@ -791,6 +843,7 @@ atomic64_inc_return_relaxed(atomic64_t *v)
static __always_inline s64
atomic64_fetch_inc(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_inc(v);
}
@@ -805,6 +858,7 @@ atomic64_fetch_inc_acquire(atomic64_t *v)
static __always_inline s64
atomic64_fetch_inc_release(atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_inc_release(v);
}
@@ -826,6 +880,7 @@ atomic64_dec(atomic64_t *v)
static __always_inline s64
atomic64_dec_return(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_dec_return(v);
}
@@ -840,6 +895,7 @@ atomic64_dec_return_acquire(atomic64_t *v)
static __always_inline s64
atomic64_dec_return_release(atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_dec_return_release(v);
}
@@ -854,6 +910,7 @@ atomic64_dec_return_relaxed(atomic64_t *v)
static __always_inline s64
atomic64_fetch_dec(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_dec(v);
}
@@ -868,6 +925,7 @@ atomic64_fetch_dec_acquire(atomic64_t *v)
static __always_inline s64
atomic64_fetch_dec_release(atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_dec_release(v);
}
@@ -889,6 +947,7 @@ atomic64_and(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_and(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_and(i, v);
}
@@ -903,6 +962,7 @@ atomic64_fetch_and_acquire(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_and_release(s64 i, atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_and_release(i, v);
}
@@ -924,6 +984,7 @@ atomic64_andnot(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_andnot(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_andnot(i, v);
}
@@ -938,6 +999,7 @@ atomic64_fetch_andnot_acquire(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_andnot_release(s64 i, atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_andnot_release(i, v);
}
@@ -959,6 +1021,7 @@ atomic64_or(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_or(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_or(i, v);
}
@@ -973,6 +1036,7 @@ atomic64_fetch_or_acquire(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_or_release(s64 i, atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_or_release(i, v);
}
@@ -994,6 +1058,7 @@ atomic64_xor(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_xor(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_xor(i, v);
}
@@ -1008,6 +1073,7 @@ atomic64_fetch_xor_acquire(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_xor_release(s64 i, atomic64_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_xor_release(i, v);
}
@@ -1022,6 +1088,7 @@ atomic64_fetch_xor_relaxed(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_xchg(atomic64_t *v, s64 i)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_xchg(v, i);
}
@@ -1036,6 +1103,7 @@ atomic64_xchg_acquire(atomic64_t *v, s64 i)
static __always_inline s64
atomic64_xchg_release(atomic64_t *v, s64 i)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_xchg_release(v, i);
}
@@ -1050,6 +1118,7 @@ atomic64_xchg_relaxed(atomic64_t *v, s64 i)
static __always_inline s64
atomic64_cmpxchg(atomic64_t *v, s64 old, s64 new)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_cmpxchg(v, old, new);
}
@@ -1064,6 +1133,7 @@ atomic64_cmpxchg_acquire(atomic64_t *v, s64 old, s64 new)
static __always_inline s64
atomic64_cmpxchg_release(atomic64_t *v, s64 old, s64 new)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_cmpxchg_release(v, old, new);
}
@@ -1078,6 +1148,7 @@ atomic64_cmpxchg_relaxed(atomic64_t *v, s64 old, s64 new)
static __always_inline bool
atomic64_try_cmpxchg(atomic64_t *v, s64 *old, s64 new)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
instrument_atomic_read_write(old, sizeof(*old));
return arch_atomic64_try_cmpxchg(v, old, new);
@@ -1094,6 +1165,7 @@ atomic64_try_cmpxchg_acquire(atomic64_t *v, s64 *old, s64 new)
static __always_inline bool
atomic64_try_cmpxchg_release(atomic64_t *v, s64 *old, s64 new)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
instrument_atomic_read_write(old, sizeof(*old));
return arch_atomic64_try_cmpxchg_release(v, old, new);
@@ -1110,6 +1182,7 @@ atomic64_try_cmpxchg_relaxed(atomic64_t *v, s64 *old, s64 new)
static __always_inline bool
atomic64_sub_and_test(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_sub_and_test(i, v);
}
@@ -1117,6 +1190,7 @@ atomic64_sub_and_test(s64 i, atomic64_t *v)
static __always_inline bool
atomic64_dec_and_test(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_dec_and_test(v);
}
@@ -1124,6 +1198,7 @@ atomic64_dec_and_test(atomic64_t *v)
static __always_inline bool
atomic64_inc_and_test(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_inc_and_test(v);
}
@@ -1131,6 +1206,7 @@ atomic64_inc_and_test(atomic64_t *v)
static __always_inline bool
atomic64_add_negative(s64 i, atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_add_negative(i, v);
}
@@ -1138,6 +1214,7 @@ atomic64_add_negative(s64 i, atomic64_t *v)
static __always_inline s64
atomic64_fetch_add_unless(atomic64_t *v, s64 a, s64 u)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_fetch_add_unless(v, a, u);
}
@@ -1145,6 +1222,7 @@ atomic64_fetch_add_unless(atomic64_t *v, s64 a, s64 u)
static __always_inline bool
atomic64_add_unless(atomic64_t *v, s64 a, s64 u)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_add_unless(v, a, u);
}
@@ -1152,6 +1230,7 @@ atomic64_add_unless(atomic64_t *v, s64 a, s64 u)
static __always_inline bool
atomic64_inc_not_zero(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_inc_not_zero(v);
}
@@ -1159,6 +1238,7 @@ atomic64_inc_not_zero(atomic64_t *v)
static __always_inline bool
atomic64_inc_unless_negative(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_inc_unless_negative(v);
}
@@ -1166,6 +1246,7 @@ atomic64_inc_unless_negative(atomic64_t *v)
static __always_inline bool
atomic64_dec_unless_positive(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_dec_unless_positive(v);
}
@@ -1173,6 +1254,7 @@ atomic64_dec_unless_positive(atomic64_t *v)
static __always_inline s64
atomic64_dec_if_positive(atomic64_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic64_dec_if_positive(v);
}
@@ -1201,6 +1283,7 @@ atomic_long_set(atomic_long_t *v, long i)
static __always_inline void
atomic_long_set_release(atomic_long_t *v, long i)
{
+ kcsan_release();
instrument_atomic_write(v, sizeof(*v));
arch_atomic_long_set_release(v, i);
}
@@ -1215,6 +1298,7 @@ atomic_long_add(long i, atomic_long_t *v)
static __always_inline long
atomic_long_add_return(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_add_return(i, v);
}
@@ -1229,6 +1313,7 @@ atomic_long_add_return_acquire(long i, atomic_long_t *v)
static __always_inline long
atomic_long_add_return_release(long i, atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_add_return_release(i, v);
}
@@ -1243,6 +1328,7 @@ atomic_long_add_return_relaxed(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_add(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_add(i, v);
}
@@ -1257,6 +1343,7 @@ atomic_long_fetch_add_acquire(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_add_release(long i, atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_add_release(i, v);
}
@@ -1278,6 +1365,7 @@ atomic_long_sub(long i, atomic_long_t *v)
static __always_inline long
atomic_long_sub_return(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_sub_return(i, v);
}
@@ -1292,6 +1380,7 @@ atomic_long_sub_return_acquire(long i, atomic_long_t *v)
static __always_inline long
atomic_long_sub_return_release(long i, atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_sub_return_release(i, v);
}
@@ -1306,6 +1395,7 @@ atomic_long_sub_return_relaxed(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_sub(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_sub(i, v);
}
@@ -1320,6 +1410,7 @@ atomic_long_fetch_sub_acquire(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_sub_release(long i, atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_sub_release(i, v);
}
@@ -1341,6 +1432,7 @@ atomic_long_inc(atomic_long_t *v)
static __always_inline long
atomic_long_inc_return(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_inc_return(v);
}
@@ -1355,6 +1447,7 @@ atomic_long_inc_return_acquire(atomic_long_t *v)
static __always_inline long
atomic_long_inc_return_release(atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_inc_return_release(v);
}
@@ -1369,6 +1462,7 @@ atomic_long_inc_return_relaxed(atomic_long_t *v)
static __always_inline long
atomic_long_fetch_inc(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_inc(v);
}
@@ -1383,6 +1477,7 @@ atomic_long_fetch_inc_acquire(atomic_long_t *v)
static __always_inline long
atomic_long_fetch_inc_release(atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_inc_release(v);
}
@@ -1404,6 +1499,7 @@ atomic_long_dec(atomic_long_t *v)
static __always_inline long
atomic_long_dec_return(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_dec_return(v);
}
@@ -1418,6 +1514,7 @@ atomic_long_dec_return_acquire(atomic_long_t *v)
static __always_inline long
atomic_long_dec_return_release(atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_dec_return_release(v);
}
@@ -1432,6 +1529,7 @@ atomic_long_dec_return_relaxed(atomic_long_t *v)
static __always_inline long
atomic_long_fetch_dec(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_dec(v);
}
@@ -1446,6 +1544,7 @@ atomic_long_fetch_dec_acquire(atomic_long_t *v)
static __always_inline long
atomic_long_fetch_dec_release(atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_dec_release(v);
}
@@ -1467,6 +1566,7 @@ atomic_long_and(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_and(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_and(i, v);
}
@@ -1481,6 +1581,7 @@ atomic_long_fetch_and_acquire(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_and_release(long i, atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_and_release(i, v);
}
@@ -1502,6 +1603,7 @@ atomic_long_andnot(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_andnot(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_andnot(i, v);
}
@@ -1516,6 +1618,7 @@ atomic_long_fetch_andnot_acquire(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_andnot_release(long i, atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_andnot_release(i, v);
}
@@ -1537,6 +1640,7 @@ atomic_long_or(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_or(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_or(i, v);
}
@@ -1551,6 +1655,7 @@ atomic_long_fetch_or_acquire(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_or_release(long i, atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_or_release(i, v);
}
@@ -1572,6 +1677,7 @@ atomic_long_xor(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_xor(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_xor(i, v);
}
@@ -1586,6 +1692,7 @@ atomic_long_fetch_xor_acquire(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_xor_release(long i, atomic_long_t *v)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_xor_release(i, v);
}
@@ -1600,6 +1707,7 @@ atomic_long_fetch_xor_relaxed(long i, atomic_long_t *v)
static __always_inline long
atomic_long_xchg(atomic_long_t *v, long i)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_xchg(v, i);
}
@@ -1614,6 +1722,7 @@ atomic_long_xchg_acquire(atomic_long_t *v, long i)
static __always_inline long
atomic_long_xchg_release(atomic_long_t *v, long i)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_xchg_release(v, i);
}
@@ -1628,6 +1737,7 @@ atomic_long_xchg_relaxed(atomic_long_t *v, long i)
static __always_inline long
atomic_long_cmpxchg(atomic_long_t *v, long old, long new)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_cmpxchg(v, old, new);
}
@@ -1642,6 +1752,7 @@ atomic_long_cmpxchg_acquire(atomic_long_t *v, long old, long new)
static __always_inline long
atomic_long_cmpxchg_release(atomic_long_t *v, long old, long new)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_cmpxchg_release(v, old, new);
}
@@ -1656,6 +1767,7 @@ atomic_long_cmpxchg_relaxed(atomic_long_t *v, long old, long new)
static __always_inline bool
atomic_long_try_cmpxchg(atomic_long_t *v, long *old, long new)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
instrument_atomic_read_write(old, sizeof(*old));
return arch_atomic_long_try_cmpxchg(v, old, new);
@@ -1672,6 +1784,7 @@ atomic_long_try_cmpxchg_acquire(atomic_long_t *v, long *old, long new)
static __always_inline bool
atomic_long_try_cmpxchg_release(atomic_long_t *v, long *old, long new)
{
+ kcsan_release();
instrument_atomic_read_write(v, sizeof(*v));
instrument_atomic_read_write(old, sizeof(*old));
return arch_atomic_long_try_cmpxchg_release(v, old, new);
@@ -1688,6 +1801,7 @@ atomic_long_try_cmpxchg_relaxed(atomic_long_t *v, long *old, long new)
static __always_inline bool
atomic_long_sub_and_test(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_sub_and_test(i, v);
}
@@ -1695,6 +1809,7 @@ atomic_long_sub_and_test(long i, atomic_long_t *v)
static __always_inline bool
atomic_long_dec_and_test(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_dec_and_test(v);
}
@@ -1702,6 +1817,7 @@ atomic_long_dec_and_test(atomic_long_t *v)
static __always_inline bool
atomic_long_inc_and_test(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_inc_and_test(v);
}
@@ -1709,6 +1825,7 @@ atomic_long_inc_and_test(atomic_long_t *v)
static __always_inline bool
atomic_long_add_negative(long i, atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_add_negative(i, v);
}
@@ -1716,6 +1833,7 @@ atomic_long_add_negative(long i, atomic_long_t *v)
static __always_inline long
atomic_long_fetch_add_unless(atomic_long_t *v, long a, long u)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_fetch_add_unless(v, a, u);
}
@@ -1723,6 +1841,7 @@ atomic_long_fetch_add_unless(atomic_long_t *v, long a, long u)
static __always_inline bool
atomic_long_add_unless(atomic_long_t *v, long a, long u)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_add_unless(v, a, u);
}
@@ -1730,6 +1849,7 @@ atomic_long_add_unless(atomic_long_t *v, long a, long u)
static __always_inline bool
atomic_long_inc_not_zero(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_inc_not_zero(v);
}
@@ -1737,6 +1857,7 @@ atomic_long_inc_not_zero(atomic_long_t *v)
static __always_inline bool
atomic_long_inc_unless_negative(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_inc_unless_negative(v);
}
@@ -1744,6 +1865,7 @@ atomic_long_inc_unless_negative(atomic_long_t *v)
static __always_inline bool
atomic_long_dec_unless_positive(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_dec_unless_positive(v);
}
@@ -1751,6 +1873,7 @@ atomic_long_dec_unless_positive(atomic_long_t *v)
static __always_inline long
atomic_long_dec_if_positive(atomic_long_t *v)
{
+ kcsan_mb();
instrument_atomic_read_write(v, sizeof(*v));
return arch_atomic_long_dec_if_positive(v);
}
@@ -1758,6 +1881,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
#define xchg(ptr, ...) \
({ \
typeof(ptr) __ai_ptr = (ptr); \
+ kcsan_mb(); \
instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
arch_xchg(__ai_ptr, __VA_ARGS__); \
})
@@ -1772,6 +1896,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
#define xchg_release(ptr, ...) \
({ \
typeof(ptr) __ai_ptr = (ptr); \
+ kcsan_release(); \
instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
arch_xchg_release(__ai_ptr, __VA_ARGS__); \
})
@@ -1786,6 +1911,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
#define cmpxchg(ptr, ...) \
({ \
typeof(ptr) __ai_ptr = (ptr); \
+ kcsan_mb(); \
instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
arch_cmpxchg(__ai_ptr, __VA_ARGS__); \
})
@@ -1800,6 +1926,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
#define cmpxchg_release(ptr, ...) \
({ \
typeof(ptr) __ai_ptr = (ptr); \
+ kcsan_release(); \
instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
arch_cmpxchg_release(__ai_ptr, __VA_ARGS__); \
})
@@ -1814,6 +1941,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
#define cmpxchg64(ptr, ...) \
({ \
typeof(ptr) __ai_ptr = (ptr); \
+ kcsan_mb(); \
instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
arch_cmpxchg64(__ai_ptr, __VA_ARGS__); \
})
@@ -1828,6 +1956,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
#define cmpxchg64_release(ptr, ...) \
({ \
typeof(ptr) __ai_ptr = (ptr); \
+ kcsan_release(); \
instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
arch_cmpxchg64_release(__ai_ptr, __VA_ARGS__); \
})
@@ -1843,6 +1972,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
({ \
typeof(ptr) __ai_ptr = (ptr); \
typeof(oldp) __ai_oldp = (oldp); \
+ kcsan_mb(); \
instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
instrument_atomic_write(__ai_oldp, sizeof(*__ai_oldp)); \
arch_try_cmpxchg(__ai_ptr, __ai_oldp, __VA_ARGS__); \
@@ -1861,6 +1991,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
({ \
typeof(ptr) __ai_ptr = (ptr); \
typeof(oldp) __ai_oldp = (oldp); \
+ kcsan_release(); \
instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
instrument_atomic_write(__ai_oldp, sizeof(*__ai_oldp)); \
arch_try_cmpxchg_release(__ai_ptr, __ai_oldp, __VA_ARGS__); \
@@ -1892,6 +2023,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
#define sync_cmpxchg(ptr, ...) \
({ \
typeof(ptr) __ai_ptr = (ptr); \
+ kcsan_mb(); \
instrument_atomic_write(__ai_ptr, sizeof(*__ai_ptr)); \
arch_sync_cmpxchg(__ai_ptr, __VA_ARGS__); \
})
@@ -1899,6 +2031,7 @@ atomic_long_dec_if_positive(atomic_long_t *v)
#define cmpxchg_double(ptr, ...) \
({ \
typeof(ptr) __ai_ptr = (ptr); \
+ kcsan_mb(); \
instrument_atomic_write(__ai_ptr, 2 * sizeof(*__ai_ptr)); \
arch_cmpxchg_double(__ai_ptr, __VA_ARGS__); \
})
@@ -1912,4 +2045,4 @@ atomic_long_dec_if_positive(atomic_long_t *v)
})

#endif /* _LINUX_ATOMIC_INSTRUMENTED_H */
-// 2a9553f0a9d5619f19151092df5cabbbf16ce835
+// 87c974b93032afd42143613434d1a7788fa598f9
diff --git a/scripts/atomic/gen-atomic-instrumented.sh b/scripts/atomic/gen-atomic-instrumented.sh
index 035ceb4ee85c..68f902731d01 100755
--- a/scripts/atomic/gen-atomic-instrumented.sh
+++ b/scripts/atomic/gen-atomic-instrumented.sh
@@ -34,6 +34,14 @@ gen_param_check()
gen_params_checks()
{
local meta="$1"; shift
+ local order="$1"; shift
+
+ if [ "${order}" = "_release" ]; then
+ printf "\tkcsan_release();\n"
+ elif [ -z "${order}" ] && ! meta_in "$meta" "slv"; then
+ # RMW with return value is fully ordered
+ printf "\tkcsan_mb();\n"
+ fi

while [ "$#" -gt 0 ]; do
gen_param_check "$meta" "$1"
@@ -56,7 +64,7 @@ gen_proto_order_variant()

local ret="$(gen_ret_type "${meta}" "${int}")"
local params="$(gen_params "${int}" "${atomic}" "$@")"
- local checks="$(gen_params_checks "${meta}" "$@")"
+ local checks="$(gen_params_checks "${meta}" "${order}" "$@")"
local args="$(gen_args "$@")"
local retstmt="$(gen_ret_stmt "${meta}")"

@@ -75,29 +83,44 @@ EOF
gen_xchg()
{
local xchg="$1"; shift
+ local order="$1"; shift
local mult="$1"; shift

+ kcsan_barrier=""
+ if [ "${xchg%_local}" = "${xchg}" ]; then
+ case "$order" in
+ _release) kcsan_barrier="kcsan_release()" ;;
+ "") kcsan_barrier="kcsan_mb()" ;;
+ esac
+ fi
+
if [ "${xchg%${xchg#try_cmpxchg}}" = "try_cmpxchg" ] ; then

cat <<EOF
-#define ${xchg}(ptr, oldp, ...) \\
+#define ${xchg}${order}(ptr, oldp, ...) \\
({ \\
typeof(ptr) __ai_ptr = (ptr); \\
typeof(oldp) __ai_oldp = (oldp); \\
+EOF
+[ -n "$kcsan_barrier" ] && printf "\t${kcsan_barrier}; \\\\\n"
+cat <<EOF
instrument_atomic_write(__ai_ptr, ${mult}sizeof(*__ai_ptr)); \\
instrument_atomic_write(__ai_oldp, ${mult}sizeof(*__ai_oldp)); \\
- arch_${xchg}(__ai_ptr, __ai_oldp, __VA_ARGS__); \\
+ arch_${xchg}${order}(__ai_ptr, __ai_oldp, __VA_ARGS__); \\
})
EOF

else

cat <<EOF
-#define ${xchg}(ptr, ...) \\
+#define ${xchg}${order}(ptr, ...) \\
({ \\
typeof(ptr) __ai_ptr = (ptr); \\
+EOF
+[ -n "$kcsan_barrier" ] && printf "\t${kcsan_barrier}; \\\\\n"
+cat <<EOF
instrument_atomic_write(__ai_ptr, ${mult}sizeof(*__ai_ptr)); \\
- arch_${xchg}(__ai_ptr, __VA_ARGS__); \\
+ arch_${xchg}${order}(__ai_ptr, __VA_ARGS__); \\
})
EOF

@@ -145,21 +168,21 @@ done

for xchg in "xchg" "cmpxchg" "cmpxchg64" "try_cmpxchg"; do
for order in "" "_acquire" "_release" "_relaxed"; do
- gen_xchg "${xchg}${order}" ""
+ gen_xchg "${xchg}" "${order}" ""
printf "\n"
done
done

for xchg in "cmpxchg_local" "cmpxchg64_local" "sync_cmpxchg"; do
- gen_xchg "${xchg}" ""
+ gen_xchg "${xchg}" "" ""
printf "\n"
done

-gen_xchg "cmpxchg_double" "2 * "
+gen_xchg "cmpxchg_double" "" "2 * "

printf "\n\n"

-gen_xchg "cmpxchg_double_local" "2 * "
+gen_xchg "cmpxchg_double_local" "" "2 * "

cat <<EOF

--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:23 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Adds the required KCSAN instrumentation for barriers of atomic bitops.

Signed-off-by: Marco Elver <el...@google.com>
---
include/asm-generic/bitops/instrumented-atomic.h | 3 +++
include/asm-generic/bitops/instrumented-lock.h | 3 +++
2 files changed, 6 insertions(+)

diff --git a/include/asm-generic/bitops/instrumented-atomic.h b/include/asm-generic/bitops/instrumented-atomic.h
index 81915dcd4b4e..c90192b1c755 100644
--- a/include/asm-generic/bitops/instrumented-atomic.h
+++ b/include/asm-generic/bitops/instrumented-atomic.h
@@ -67,6 +67,7 @@ static inline void change_bit(long nr, volatile unsigned long *addr)
*/
static inline bool test_and_set_bit(long nr, volatile unsigned long *addr)
{
+ kcsan_mb();
instrument_atomic_read_write(addr + BIT_WORD(nr), sizeof(long));
return arch_test_and_set_bit(nr, addr);
}
@@ -80,6 +81,7 @@ static inline bool test_and_set_bit(long nr, volatile unsigned long *addr)
*/
static inline bool test_and_clear_bit(long nr, volatile unsigned long *addr)
{
+ kcsan_mb();
instrument_atomic_read_write(addr + BIT_WORD(nr), sizeof(long));
return arch_test_and_clear_bit(nr, addr);
}
@@ -93,6 +95,7 @@ static inline bool test_and_clear_bit(long nr, volatile unsigned long *addr)
*/
static inline bool test_and_change_bit(long nr, volatile unsigned long *addr)
{
+ kcsan_mb();
instrument_atomic_read_write(addr + BIT_WORD(nr), sizeof(long));
return arch_test_and_change_bit(nr, addr);
}
diff --git a/include/asm-generic/bitops/instrumented-lock.h b/include/asm-generic/bitops/instrumented-lock.h
index 75ef606f7145..eb64bd4f11f3 100644
--- a/include/asm-generic/bitops/instrumented-lock.h
+++ b/include/asm-generic/bitops/instrumented-lock.h
@@ -22,6 +22,7 @@
*/
static inline void clear_bit_unlock(long nr, volatile unsigned long *addr)
{
+ kcsan_release();
instrument_atomic_write(addr + BIT_WORD(nr), sizeof(long));
arch_clear_bit_unlock(nr, addr);
}
@@ -37,6 +38,7 @@ static inline void clear_bit_unlock(long nr, volatile unsigned long *addr)
*/
static inline void __clear_bit_unlock(long nr, volatile unsigned long *addr)
{
+ kcsan_release();
instrument_write(addr + BIT_WORD(nr), sizeof(long));
arch___clear_bit_unlock(nr, addr);
}
@@ -71,6 +73,7 @@ static inline bool test_and_set_bit_lock(long nr, volatile unsigned long *addr)
static inline bool
clear_bit_unlock_is_negative_byte(long nr, volatile unsigned long *addr)
{
+ kcsan_release();
instrument_atomic_write(addr + BIT_WORD(nr), sizeof(long));
return arch_clear_bit_unlock_is_negative_byte(nr, addr);
}
--
2.33.0.800.g4c38ced690-goog

Marco Elver

unread,
Oct 5, 2021, 7:00:26 AMOct 5
to el...@google.com, Paul E . McKenney, Alexander Potapenko, Boqun Feng, Borislav Petkov, Dmitry Vyukov, Ingo Molnar, Josh Poimboeuf, Mark Rutland, Peter Zijlstra, Thomas Gleixner, Waiman Long, Will Deacon, kasa...@googlegroups.com, linux...@vger.kernel.org, linu...@vger.kernel.org, linux-...@vger.kernel.org, linux-...@vger.kernel.org, linu...@kvack.org, x...@kernel.org
Prefix all barriers with __, now that asm-generic/barriers.h supports
defining the final instrumented version of these barriers. The change is
limited to barriers used by x86-64.

Signed-off-by: Marco Elver <el...@google.com>
---
arch/x86/include/asm/barrier.h | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 3ba772a69cc8..35389b2af88e 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -19,9 +19,9 @@
#define wmb() asm volatile(ALTERNATIVE("lock; addl $0,-4(%%esp)", "sfence", \
X86_FEATURE_XMM2) ::: "memory", "cc")
#else
-#define mb() asm volatile("mfence":::"memory")
-#define rmb() asm volatile("lfence":::"memory")
-#define wmb() asm volatile("sfence" ::: "memory")
+#define __mb() asm volatile("mfence":::"memory")
+#define __rmb() asm volatile("lfence":::"memory")
+#define __wmb() asm volatile("sfence" ::: "memory")
#endif

/**
@@ -51,8 +51,8 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
/* Prevent speculative execution past this barrier. */
#define barrier_nospec() alternative("", "lfence", X86_FEATURE_LFENCE_RDTSC)

-#define dma_rmb() barrier()
-#define dma_wmb() barrier()
+#define __dma_rmb() barrier()
+#define __dma_wmb() barrier()

#define __smp_mb() asm volatile("lock; addl $0,-4(%%" _ASM_SP ")" ::: "memory", "cc")

--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 7:00:28 AMOct 5
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If CONFIG_PARAVIRT_SPINLOCKS=y, queued_spin_unlock() is implemented
using pv_queued_spin_unlock() which is entirely inline asm based. As
such, we do not receive any KCSAN barrier instrumentation via regular
atomic operations.

Add the missing KCSAN barrier instrumentation for the
CONFIG_PARAVIRT_SPINLOCKS case.

Signed-off-by: Marco Elver <el...@google.com>
---
arch/x86/include/asm/qspinlock.h | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index d86ab942219c..d87451df480b 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -53,6 +53,7 @@ static inline void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)

static inline void queued_spin_unlock(struct qspinlock *lock)
{
+ kcsan_release();
pv_queued_spin_unlock(lock);
}

--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 7:00:30 AMOct 5
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Some memory management calls imply memory barriers that are required to
avoid false positives. For example, without the correct instrumentation,
we could observe data races of the following variant:

T0 | T1
------------------------+------------------------
|
*a = 42; ---+ |
kfree(a); | |
| | b = kmalloc(..); // b == a
<reordered> <-+ | *b = 42; // not a data race!
|

Therefore, instrument memory barriers in all allocator code currently
not being instrumented in a default build.

Signed-off-by: Marco Elver <el...@google.com>
---
mm/Makefile | 2 ++
1 file changed, 2 insertions(+)

diff --git a/mm/Makefile b/mm/Makefile
index fc60a40ce954..11e9fcd410be 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -15,6 +15,8 @@ KCSAN_SANITIZE_slab_common.o := n
KCSAN_SANITIZE_slab.o := n
KCSAN_SANITIZE_slub.o := n
KCSAN_SANITIZE_page_alloc.o := n
+# But enable explicit instrumentation for memory barriers.
+KCSAN_INSTRUMENT_BARRIERS := y

# These files are disabled because they produce non-interesting and/or
# flaky coverage that is not a function of syscall inputs. E.g. slab is out of
--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 7:00:33 AMOct 5
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There's no fundamental reason to disable KCSAN for scheduler code,
except for excessive noise and performance concerns (instrumenting
scheduler code is usually a good way to stress test KCSAN itself).

However, several core sched functions imply memory barriers that are
invisible to KCSAN without instrumentation, but are required to avoid
false positives. Therefore, unconditionally enable instrumentation of
memory barriers in scheduler code. Also update the comment to reflect
this and be a bit more brief.

Signed-off-by: Marco Elver <el...@google.com>
---
kernel/sched/Makefile | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
index 978fcfca5871..90da599f5560 100644
--- a/kernel/sched/Makefile
+++ b/kernel/sched/Makefile
@@ -7,11 +7,10 @@ endif
# that is not a function of syscall inputs. E.g. involuntary context switches.
KCOV_INSTRUMENT := n

-# There are numerous data races here, however, most of them are due to plain accesses.
-# This would make it even harder for syzbot to find reproducers, because these
-# bugs trigger without specific input. Disable by default, but should re-enable
-# eventually.
+# Disable KCSAN to avoid excessive noise and performance degradation. To avoid
+# false positives ensure barriers implied by sched functions are instrumented.
KCSAN_SANITIZE := n
+KCSAN_INSTRUMENT_BARRIERS := y

ifneq ($(CONFIG_SCHED_OMIT_FRAME_POINTER),y)
# According to Alan Modra <al...@linuxcare.com.au>, the -fno-omit-frame-pointer is
--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 7:00:36 AMOct 5
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Adds KCSAN's memory barrier instrumentation to objtool's uaccess
whitelist.

Signed-off-by: Marco Elver <el...@google.com>
---
tools/objtool/check.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index e5947fbb9e7a..7e8cd3ba5482 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -651,6 +651,10 @@ static const char *uaccess_safe_builtin[] = {
"__asan_report_store16_noabort",
/* KCSAN */
"__kcsan_check_access",
+ "__kcsan_mb",
+ "__kcsan_wmb",
+ "__kcsan_rmb",
+ "__kcsan_release",
"kcsan_found_watchpoint",
"kcsan_setup_watchpoint",
"kcsan_check_scoped_accesses",
--
2.33.0.800.g4c38ced690-goog

Marco Elver

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Oct 5, 2021, 7:00:38 AMOct 5
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Teach objtool to turn instrumentation required for memory barrier
modeling into nops in noinstr text.

The __tsan_func_entry/exit calls are still emitted by compilers even
with the __no_sanitize_thread attribute. The memory barrier
instrumentation will be inserted explicitly (without compiler help), and
thus needs to also explicitly be removed.

Signed-off-by: Marco Elver <el...@google.com>
---
tools/objtool/check.c | 32 ++++++++++++++++++++++++++------
1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 7e8cd3ba5482..7b694e639164 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -965,6 +965,31 @@ static struct symbol *find_call_destination(struct section *sec, unsigned long o
return call_dest;
}

+static bool should_remove_if_noinstr(const char *name)
+{
+ /*
+ * Many compilers cannot disable KCOV with a function attribute so they
+ * need a little help, NOP out any KCOV calls from noinstr text.
+ */
+ if (!strncmp(name, "__sanitizer_cov_", 16))
+ return true;
+
+ /*
+ * Compilers currently do not remove __tsan_func_entry/exit with the
+ * __no_sanitize_thread attribute, remove them. Memory barrier
+ * instrumentation is not emitted by the compiler, but inserted
+ * explicitly, so we need to also remove them.
+ */
+ if (!strncmp(name, "__tsan_func_", 12) ||
+ !strcmp(name, "__kcsan_mb") ||
+ !strcmp(name, "__kcsan_wmb") ||
+ !strcmp(name, "__kcsan_rmb") ||
+ !strcmp(name, "__kcsan_release"))
+ return true;
+
+ return false;
+}
+
/*
* Find the destination instructions for all calls.
*/
@@ -1031,13 +1056,8 @@ static int add_call_destinations(struct objtool_file *file)
&file->static_call_list);
}

- /*
- * Many compilers cannot disable KCOV with a function attribute
- * so they need a little help, NOP out any KCOV calls from noinstr
- * text.
- */
if (insn->sec->noinstr &&
- !strncmp(insn->call_dest->name, "__sanitizer_cov_", 16)) {
+ should_remove_if_noinstr(insn->call_dest->name)) {
if (reloc) {
reloc->type = R_NONE;
elf_write_reloc(file->elf, reloc);
--
2.33.0.800.g4c38ced690-goog

Peter Zijlstra

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Oct 5, 2021, 7:44:54 AMOct 5
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I find that very hard to read.. kcsan_atomic_release() it not in fact a
release. It might be a release if @memorder implies one.

Peter Zijlstra

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Oct 5, 2021, 7:45:43 AMOct 5
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Also, what's the atomic part signify? Is that because you're modeling
the difference in acquire/release semantics between
smp_load_{acquire,release}() and atomic*_{acquire,release}() ?

Marco Elver

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Oct 5, 2021, 7:50:53 AMOct 5
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On Tue, 5 Oct 2021 at 13:45, Peter Zijlstra <pet...@infradead.org> wrote:
> On Tue, Oct 05, 2021 at 01:41:18PM +0200, Peter Zijlstra wrote:
> > On Tue, Oct 05, 2021 at 12:58:47PM +0200, Marco Elver wrote:
> > > +static __always_inline void kcsan_atomic_release(int memorder)
> > > +{
> > > + if (memorder == __ATOMIC_RELEASE ||
> > > + memorder == __ATOMIC_SEQ_CST ||
> > > + memorder == __ATOMIC_ACQ_REL)
> > > + __kcsan_release();
> > > +}
> > > +
[...]
> > > + kcsan_atomic_release(memorder);
> > > __atomic_thread_fence(memorder);
> > > }
> > > EXPORT_SYMBOL(__tsan_atomic_thread_fence);
> >
> > I find that very hard to read.. kcsan_atomic_release() it not in fact a
> > release. It might be a release if @memorder implies one.

You're right, this name can be improved.
`kcsan_atomic_builtin_memorder(..)` is probably better

> Also, what's the atomic part signify? Is that because you're modeling
> the difference in acquire/release semantics between
> smp_load_{acquire,release}() and atomic*_{acquire,release}() ?

Sorry, just a bad name. It's about the builtins. The above suggested
name should hopefully be clearer.

Peter Zijlstra

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Oct 5, 2021, 8:03:49 AMOct 5
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On Tue, Oct 05, 2021 at 12:58:58PM +0200, Marco Elver wrote:
> @@ -59,6 +60,7 @@ atomic_add(int i, atomic_t *v)
> static __always_inline int
> atomic_add_return(int i, atomic_t *v)
> {
> + kcsan_mb();
> instrument_atomic_read_write(v, sizeof(*v));
> return arch_atomic_add_return(i, v);
> }

This and others,.. is this actually correct? Should that not be
something like:

kscan_mb();
instrument_atomic_read_write(...);
ret = arch_atomic_add_return(i, v);
kcsan_mb();
return ret;

?

Marco Elver

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Oct 5, 2021, 8:17:07 AMOct 5
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In theory, yes, but right now it's redundant.

Because right now KCSAN only models "buffering", and no "prefetching".
So there's no way that a later instruction would be reordered before
this point. And atomic accesses are never considered for reordering,
so it's also impossible that it would be reordered later.

Each kcsan_mb() is a call, so right now it makes sense to just have 1
call to be a bit more efficient.

Peter Zijlstra

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Oct 5, 2021, 8:53:04 AMOct 5
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On Tue, Oct 05, 2021 at 12:58:46PM +0200, Marco Elver wrote:
> +#if !defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_STACK_VALIDATION)
> +/*
> + * Arch does not rely on noinstr, or objtool will remove memory barrier
> + * instrumentation, and no instrumentation of noinstr code is expected.
> + */
> +#define kcsan_noinstr

I think this still wants to be at the very least:

#define kcsan_noinstr noinline notrace

without noinline it is possible LTO (or similarly daft things) will end
up inlining the calls, and since we rely on objtool to NOP out CALLs
this must not happen.

And since you want to mark these functions as uaccess_safe, there must
not be any tracing on, hence notrace.

Marco Elver

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Oct 5, 2021, 9:13:37 AMOct 5
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On Tue, 5 Oct 2021 at 14:53, Peter Zijlstra <pet...@infradead.org> wrote:
> On Tue, Oct 05, 2021 at 12:58:46PM +0200, Marco Elver wrote:
> > +#if !defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_STACK_VALIDATION)
> > +/*
> > + * Arch does not rely on noinstr, or objtool will remove memory barrier
> > + * instrumentation, and no instrumentation of noinstr code is expected.
> > + */
> > +#define kcsan_noinstr
>
> I think this still wants to be at the very least:
>
> #define kcsan_noinstr noinline notrace
>
> without noinline it is possible LTO (or similarly daft things) will end
> up inlining the calls, and since we rely on objtool to NOP out CALLs
> this must not happen.

Good point about noinline, will add.

> And since you want to mark these functions as uaccess_safe, there must
> not be any tracing on, hence notrace.

In the Makefile we've relied on:

CFLAGS_REMOVE_core.o = $(CC_FLAGS_FTRACE)

just to disable it for all code here. That should be enough, right?

Peter Zijlstra

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Oct 5, 2021, 10:20:44 AMOct 5
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I find these implicit notrace thingies terribly confusing :/ I've
reported fail to rostedt a number of times only to be (re)told about
these Makefile level thingies.

Ideally we'd script notrace on every implicit symbol or something.

Peter Zijlstra

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Oct 5, 2021, 10:37:25 AMOct 5
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On Tue, Oct 05, 2021 at 12:59:05PM +0200, Marco Elver wrote:
> Teach objtool to turn instrumentation required for memory barrier
> modeling into nops in noinstr text.
>
> The __tsan_func_entry/exit calls are still emitted by compilers even
> with the __no_sanitize_thread attribute. The memory barrier
> instrumentation will be inserted explicitly (without compiler help), and
> thus needs to also explicitly be removed.

How is arm64 and others using kernel/entry + noinstr going to fix this?

ISTR they fully rely on the compilers not emitting instrumentation,
since they don't have objtool to fix up stray issues like this.

Marco Elver

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Oct 5, 2021, 11:13:43 AMOct 5
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So this is where I'd like to hear if the approach of:

| #if !defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_STACK_VALIDATION)
| ...
| #else
| #define kcsan_noinstr noinstr
| static __always_inline bool within_noinstr(unsigned long ip)
| {
| return (unsigned long)__noinstr_text_start <= ip &&
| ip < (unsigned long)__noinstr_text_end;
| }
| #endif

and then (using the !STACK_VALIDATION definitions)

| kcsan_noinstr void instrumentation_may_appear_in_noinstr(void)
| {
| if (within_noinstr(_RET_IP_))
| return;

works for the non-x86 arches that select ARCH_WANTS_NO_INSTR.

If it doesn't I can easily just remove kcsan_noinstr/within_noinstr, and
add a "depends on !ARCH_WANTS_NO_INSTR || STACK_VALIDATION" to the
KCSAN_WEAK_MEMORY option.

Looking at a previous discussion [1], however, I was under the
impression that this would work.

[1] https://lkml.kernel.org/r/CANpmjNMAZiW-Er=2QDgGP+_3hg1LOvPYcbfGSPMv=aR6M...@mail.gmail.com

Marco Elver

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Nov 11, 2021, 5:12:26 AMNov 11
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I'll send v2 of this series after 5.16-rc1. So far I think we haven't
been able to say the above doesn't work, which means I'll assume it
works on non-x86 architectures with ARCH_WANTS_NO_INSTR until we get
evidence of the opposite.

Peter Zijlstra

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Nov 11, 2021, 6:35:08 AMNov 11
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On Thu, Nov 11, 2021 at 11:11:00AM +0100, Marco Elver wrote:
> On Tue, 5 Oct 2021 at 17:13, Marco Elver <el...@google.com> wrote:

> > So this is where I'd like to hear if the approach of:
> >
> > | #if !defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_STACK_VALIDATION)
> > | ...
> > | #else
> > | #define kcsan_noinstr noinstr
> > | static __always_inline bool within_noinstr(unsigned long ip)
> > | {
> > | return (unsigned long)__noinstr_text_start <= ip &&
> > | ip < (unsigned long)__noinstr_text_end;

Provided these turn into compile time constants this stands a fair
chance of working I suppose. Once this needs data loads things get a
*lot* more tricky.

> > | }
> > | #endif
> >
> > and then (using the !STACK_VALIDATION definitions)
> >
> > | kcsan_noinstr void instrumentation_may_appear_in_noinstr(void)
> > | {
> > | if (within_noinstr(_RET_IP_))
> > | return;
> >
> > works for the non-x86 arches that select ARCH_WANTS_NO_INSTR.
> >
> > If it doesn't I can easily just remove kcsan_noinstr/within_noinstr, and
> > add a "depends on !ARCH_WANTS_NO_INSTR || STACK_VALIDATION" to the
> > KCSAN_WEAK_MEMORY option.
> >
> > Looking at a previous discussion [1], however, I was under the
> > impression that this would work.
> >
> > [1] https://lkml.kernel.org/r/CANpmjNMAZiW-Er=2QDgGP+_3hg1LOvPYcbfGSPMv=aR6M...@mail.gmail.com
>
> I'll send v2 of this series after 5.16-rc1. So far I think we haven't
> been able to say the above doesn't work, which means I'll assume it
> works on non-x86 architectures with ARCH_WANTS_NO_INSTR until we get
> evidence of the opposite.

Fair enough.
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