[PATCH AUTOSEL 6.18-5.10] kcsan: Silence -Wmaybe-uninitialized when calling __kcsan_check_access()

[Sasha Levin]

From: Marco Elver <el...@google.com>

[ Upstream commit 07a1a6562ce29e2e0c134a57882d6e52e8758492 ]

Some subsystems enable -Wmaybe-uninitialized [1], which can trigger
false positives when KCSAN is enabled. Specifically, passing an
uninitialized variable to functions that instrument accesses (e.g.,
copy_from_user()) results in calls to __kcsan_check_access().

Because __kcsan_check_access() takes a `const volatile void *ptr`, GCC
infers that the function may only read the memory location, and thus
warns if the passed variable is uninitialized.

However, KCSAN is a dynamic analysis tool for data race detection; while
it does read the memory location to detect concurrent modifications, the
"initialized'ness" of the memory location is irrelevant for its analysis.

Use absolute_pointer() in __kcsan_check_write(), kcsan_check_write(),
and kcsan_check_atomic_write() to hide the pointer from the compiler,
preventing it from concluding that the pointer passed points to
uninitialized memory.

This fixes warnings like:

| CC fs/ntfs3/file.o
| In file included from include/asm-generic/rwonce.h:27,
| from arch/arm64/include/asm/rwonce.h:81,
| from include/linux/compiler.h:369,
| from include/linux/array_size.h:5,
| from include/linux/kernel.h:16,
| from include/linux/backing-dev.h:12,
| from fs/ntfs3/file.c:10:
| In function 'instrument_copy_from_user_before',
| inlined from '_inline_copy_from_user' at include/linux/uaccess.h:184:2,
| inlined from 'copy_from_user' at include/linux/uaccess.h:221:9,
| inlined from 'ntfs_ioctl_fitrim' at fs/ntfs3/file.c:77:6,
| inlined from 'ntfs_ioctl' at fs/ntfs3/file.c:164:10:
| include/linux/kcsan-checks.h:220:28: error: 'range' may be used uninitialized [-Werror=maybe-uninitialized]
| 220 | #define kcsan_check_access __kcsan_check_access
| | ^
| include/linux/kcsan-checks.h:311:9: note: in expansion of macro 'kcsan_check_access'
| 311 | kcsan_check_access(ptr, size, KCSAN_ACCESS_WRITE)
| | ^~~~~~~~~~~~~~~~~~
| include/linux/instrumented.h:147:9: note: in expansion of macro 'kcsan_check_write'
| 147 | kcsan_check_write(to, n);
| | ^~~~~~~~~~~~~~~~~
| include/linux/kcsan-checks.h: In function 'ntfs_ioctl':
| include/linux/kcsan-checks.h:37:6: note: by argument 1 of type 'const volatile void *' to '__kcsan_check_access' declared here
| 37 | void __kcsan_check_access(const volatile void *ptr, size_t size, int type);
| | ^~~~~~~~~~~~~~~~~~~~
| fs/ntfs3/file.c:65:29: note: 'range' declared here
| 65 | struct fstrim_range range;
| | ^~~~~

Link: https://lore.kernel.org/all/5da10cca-875b-418d...@app.fastmail.com/ [1]
Reported-by: Arnd Bergmann <ar...@arndb.de>
Reviewed-by: Arnd Bergmann <ar...@arndb.de>
Tested-by: Arnd Bergmann <ar...@arndb.de>
Signed-off-by: Marco Elver <el...@google.com>
Signed-off-by: Sasha Levin <sas...@kernel.org>
---

LLM Generated explanations, may be completely bogus:

The background git history searches finished. They show `kcsan-checks.h`
has been in this 6.18 tree since at least `ac3fd01e4c1ef` (Linux
6.18-rc7), with the buggy write-check macros present and the
`absolute_pointer()` fix still missing.

That lines up with the earlier analysis: this is a small, low-risk build
fix for KCSAN + `-Werror` configurations, and it should be backported to
6.18.y.

**YES**

include/linux/kcsan-checks.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 92f3843d9ebb8..e135dacaa90f3 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -282,7 +282,7 @@ static inline void __kcsan_disable_current(void) { }
* @size: size of access
*/
#define __kcsan_check_write(ptr, size) \
- __kcsan_check_access(ptr, size, KCSAN_ACCESS_WRITE)
+ __kcsan_check_access(absolute_pointer(ptr), size, KCSAN_ACCESS_WRITE)

/**
* __kcsan_check_read_write - check regular read-write access for races
@@ -308,7 +308,7 @@ static inline void __kcsan_disable_current(void) { }
* @size: size of access
*/
#define kcsan_check_write(ptr, size) \
- kcsan_check_access(ptr, size, KCSAN_ACCESS_WRITE)
+ kcsan_check_access(absolute_pointer(ptr), size, KCSAN_ACCESS_WRITE)

/**
* kcsan_check_read_write - check regular read-write access for races
@@ -331,7 +331,7 @@ static inline void __kcsan_disable_current(void) { }
#define kcsan_check_atomic_read(ptr, size) \
kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC)
#define kcsan_check_atomic_write(ptr, size) \
- kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
+ kcsan_check_access(absolute_pointer(ptr), size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
#define kcsan_check_atomic_read_write(ptr, size) \
kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_COMPOUND)
#endif
--
2.53.0