[PATCH 2/2] lib/tests: Add KUnit test for struct stack_trace __counted_by_ptr attribute

[Bill Wendling]

Add a custom KUnit test suite 'stacktrace_counted_by' to verify that the
__counted_by_ptr annotation on the 'entries' field of 'struct stack_trace'
behaves correctly.

The test verifies that 'max_entries' correctly limits and validates access
to 'entries' when CONFIG_ARCH_STACKWALK is not defined. If it is defined,
the test is cleanly skipped at runtime to prevent compile-time or runtime
failures due to 'struct stack_trace' being undefined on modern
architectures.

Assisted-by: Gemini Next
Signed-off-by: Bill Wendling <mo...@google.com>
---
Cc: Kees Cook <ke...@kernel.org>
Cc: "Gustavo A. R. Silva" <gusta...@kernel.org>
Cc: Andrew Morton <ak...@linux-foundation.org>
Cc: Brendan Higgins <brendan...@linux.dev>
Cc: David Gow <da...@davidgow.net>
Cc: Rae Moar <raem...@gmail.com>
Cc: Ryota Sakamoto <sakamo...@gmail.com>
Cc: Kuan-Wei Chiu <visit...@gmail.com>
Cc: Pasha Tatashin <pasha.t...@soleen.com>
Cc: Dmitry Antipov <dman...@yandex.ru>
Cc: Petr Mladek <pml...@suse.com>
Cc: Kir Chou <not...@hotmail.com>
Cc: codemender-p...@google.com
Cc: linux-...@vger.kernel.org
Cc: linux-h...@vger.kernel.org
Cc: linux-k...@vger.kernel.org
Cc: kuni...@googlegroups.com
Cc: linux-h...@vger.kernel.org
---
lib/Kconfig.debug | 10 +++++++
lib/kunit/.kunitconfig | 1 +
lib/tests/Makefile | 1 +
lib/tests/stacktrace_kunit.c | 51 ++++++++++++++++++++++++++++++++++++
4 files changed, 63 insertions(+)
create mode 100644 lib/tests/stacktrace_kunit.c

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index e97bdf3a42a8..51a6ac1a2461 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2716,6 +2716,16 @@ config BITOPS_KUNIT

If unsure, say N.

+config STACKTRACE_KUNIT_TEST
+ tristate "KUnit test for stacktrace counted_by attribute" if !KUNIT_ALL_TESTS
+ depends on KUNIT
+ default KUNIT_ALL_TESTS
+ help
+ This option enables the KUnit test for verifying the __counted_by_ptr
+ attribute on struct stack_trace.
+
+ If unsure, say N.
+
config BITFIELD_KUNIT
tristate "KUnit test bitfield functions at runtime" if !KUNIT_ALL_TESTS
depends on KUNIT
diff --git a/lib/kunit/.kunitconfig b/lib/kunit/.kunitconfig
index 9235b7d42d38..b3761b41459e 100644
--- a/lib/kunit/.kunitconfig
+++ b/lib/kunit/.kunitconfig
@@ -1,3 +1,4 @@
CONFIG_KUNIT=y
CONFIG_KUNIT_TEST=y
CONFIG_KUNIT_EXAMPLE_TEST=y
+CONFIG_STACKTRACE_KUNIT_TEST=y
diff --git a/lib/tests/Makefile b/lib/tests/Makefile
index 4ead57602eac..40875e729fc8 100644
--- a/lib/tests/Makefile
+++ b/lib/tests/Makefile
@@ -6,6 +6,7 @@
CFLAGS_bitfield_kunit.o := $(DISABLE_STRUCTLEAK_PLUGIN)
obj-$(CONFIG_BASE64_KUNIT) += base64_kunit.o
obj-$(CONFIG_BITOPS_KUNIT) += bitops_kunit.o
+obj-$(CONFIG_STACKTRACE_KUNIT_TEST) += stacktrace_kunit.o
obj-$(CONFIG_BITFIELD_KUNIT) += bitfield_kunit.o
obj-$(CONFIG_BITS_TEST) += test_bits.o
obj-$(CONFIG_SHDI3_KUNIT_TEST) += shdi3_kunit.o
diff --git a/lib/tests/stacktrace_kunit.c b/lib/tests/stacktrace_kunit.c
new file mode 100644
index 000000000000..7ec48edf84fe
--- /dev/null
+++ b/lib/tests/stacktrace_kunit.c
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit test for struct stack_trace counted_by attribute.
+ */
+
+#include <kunit/test.h>
+#include <linux/stacktrace.h>
+
+#ifndef CONFIG_ARCH_STACKWALK
+static void test_stack_trace_counted_by(struct kunit *test)
+{
+ unsigned long entries_buf[4];
+ struct stack_trace trace = {
+ .entries = entries_buf,
+ .max_entries = 4,
+ };
+
+ KUNIT_EXPECT_EQ(test, trace.max_entries, 4U);
+ KUNIT_EXPECT_PTR_EQ(test, trace.entries, (unsigned long *)entries_buf);
+
+ /* Write to the allocated elements to verify access */
+ trace.entries[0] = 0xdeadbeef;
+ trace.entries[1] = 0xbeefcafe;
+ trace.entries[2] = 0xcafebabe;
+ trace.entries[3] = 0x12345678;
+
+ KUNIT_EXPECT_EQ(test, trace.entries[0], 0xdeadbeefUL);
+ KUNIT_EXPECT_EQ(test, trace.entries[1], 0xbeefcafeUL);
+ KUNIT_EXPECT_EQ(test, trace.entries[2], 0xcafebabeUL);
+ KUNIT_EXPECT_EQ(test, trace.entries[3], 0x12345678UL);
+}
+#else
+static void test_stack_trace_counted_by(struct kunit *test)
+{
+ kunit_skip(test, "CONFIG_ARCH_STACKWALK is enabled, struct stack_trace is not defined");
+}
+#endif
+
+static struct kunit_case stacktrace_test_cases[] = {
+ KUNIT_CASE(test_stack_trace_counted_by),
+ {}
+};
+
+static struct kunit_suite stacktrace_test_suite = {
+ .name = "stacktrace_counted_by",
+ .test_cases = stacktrace_test_cases,
+};
+
+kunit_test_suite(stacktrace_test_suite);
+
+MODULE_LICENSE("GPL");
--
2.55.0.860.g4b6b3295ed-goog

[Kuan-Wei Chiu]

Hi Bill,

This only does in bounds array writes.
To test __counted_by_ptr, I thought we were supposed to intentionally
trigger an out of bounds access and see if it actually catches the
error?

Regards,
Kuan-Wei

[Thomas Weißschuh]

On Sun, Aug 23, 2026 at 12:35:33PM +0000, Bill Wendling wrote:

> Add a custom KUnit test suite 'stacktrace_counted_by' to verify that the
> __counted_by_ptr annotation on the 'entries' field of 'struct stack_trace'
> behaves correctly.

The implementation of __counted_by_ptr should be tested by a dedicated unittest
for that feature. And I am fairly sure that already exists. It should not be
tested in random subsystems.

(...)

> diff --git a/lib/tests/stacktrace_kunit.c b/lib/tests/stacktrace_kunit.c
> new file mode 100644
> index 000000000000..7ec48edf84fe
> --- /dev/null
> +++ b/lib/tests/stacktrace_kunit.c
> @@ -0,0 +1,51 @@

(...)

> +}
> +#else
> +static void test_stack_trace_counted_by(struct kunit *test)
> +{
> + kunit_skip(test, "CONFIG_ARCH_STACKWALK is enabled, struct stack_trace is not defined");

If the test requires a specific kconfig symbol, then depend on it in kconfig.
Putting all of the code behind a single, giant ifdef is pointless.

> +}
> +#endif

(...)

[David Gow]

I'm a bit confused: what is this actually testing? You're creating an
array of size 4 and writing 4 entries to it, after setting max_entries
correctly? Removing the __counted_by_ptr attribute doesn't change
anything here. Is there any circumstance where this should fail? The
only case I can think of is if __counted_by_ptr is either totally
broken, or someone randomly points it to a different value. Neither of
which seem likely.

Even if this did something more exciting (like try to access elements
beyond the end of the array), I'm not sure a KUnit test is the optimal
place for it: ultimately we're looking for a compiler warning / error,
not a runtime one, and even then, one which probably isn't of much value
given it's unlikely __counted_by_ptr will spontaneously break.

Added to that, there are a bunch of stylistic issues with the test below
anyway. These mostly seem like LLM-isms, but probably should've been
caught before this went to the list.

Unless there's something I'm missing (and, if so, it's probably worth
writing a description which better explains the "why" here), I think we
can probably drop this test entirely and stick to just patch 1.

-- David

> lib/Kconfig.debug | 10 +++++++
> lib/kunit/.kunitconfig | 1 +
> lib/tests/Makefile | 1 +
> lib/tests/stacktrace_kunit.c | 51 ++++++++++++++++++++++++++++++++++++
> 4 files changed, 63 insertions(+)
> create mode 100644 lib/tests/stacktrace_kunit.c
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index e97bdf3a42a8..51a6ac1a2461 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2716,6 +2716,16 @@ config BITOPS_KUNIT
>
> If unsure, say N.
>
> +config STACKTRACE_KUNIT_TEST
> + tristate "KUnit test for stacktrace counted_by attribute" if !KUNIT_ALL_TESTS
> + depends on KUNIT

This should also just depend on CONFIG_ARCH_STACKWALK, rather than doing
the whole skip test thing.

> + default KUNIT_ALL_TESTS
> + help
> + This option enables the KUnit test for verifying the __counted_by_ptr
> + attribute on struct stack_trace.
> +
> + If unsure, say N.
> +
> config BITFIELD_KUNIT
> tristate "KUnit test bitfield functions at runtime" if !KUNIT_ALL_TESTS
> depends on KUNIT
> diff --git a/lib/kunit/.kunitconfig b/lib/kunit/.kunitconfig
> index 9235b7d42d38..b3761b41459e 100644
> --- a/lib/kunit/.kunitconfig
> +++ b/lib/kunit/.kunitconfig
> @@ -1,3 +1,4 @@
> CONFIG_KUNIT=y
> CONFIG_KUNIT_TEST=y
> CONFIG_KUNIT_EXAMPLE_TEST=y
> +CONFIG_STACKTRACE_KUNIT_TEST=y

Please don't add this here. This is not a test of KUnit itself, which is
what the lib/kunit/.kunitconfig file is meant to configure.

For general KUnit test build, CONFIG_KUNIT_ALL_TESTS will enable it.

Is this suite intended to cover more stacktrace tests than just the
counted_by one at some point? If so, it'd be better to name it
"stacktrace". If not, these structs should be named
"stacktrace_counted_by_test_suite", etc.

[Bill Wendling]

Hi Kuan-Wei,

True. This is testing only that the existing behavior doesn't fail
with the new attribute. It looks like you're not the only one to
comment on this, though...

-bw

[Bill Wendling]

On Sun, Aug 23, 2026 at 11:41 PM Thomas Weißschuh
<thomas.w...@linutronix.de> wrote:
>
> On Sun, Aug 23, 2026 at 12:35:33PM +0000, Bill Wendling wrote:
> > Add a custom KUnit test suite 'stacktrace_counted_by' to verify that the
> > __counted_by_ptr annotation on the 'entries' field of 'struct stack_trace'
> > behaves correctly.
>
> The implementation of __counted_by_ptr should be tested by a dedicated unittest
> for that feature. And I am fairly sure that already exists. It should not be
> tested in random subsystems.
>

I'm looking at `tools/unittests` and...what in the world is it
supposed to be testing? It looks like it's testing absolutely nothing
of substance. I assume you want there to be a test that runs and
produces a stack trace triggered by an invalid access, right? I'm
unable to find any tests in `tools/unittests` that do anything like
that...maybe I'm missing something?

-bw