The call to kmalloc_obj(observed.lines) returns "char (*)[3][512]",
a pointer to the whole 2D array. But "expect" wants to be "char (*)[512]",
the decayed pointer type, as if it were observed.lines itself (though
without the "3" bounds). This produces the following build error:

../kernel/kcsan/kcsan_test.c: In function '__report_matches':
../kernel/kcsan/kcsan_test.c:171:16: error: assignment to 'char (*)[512]' from incompatible pointer type 'char (*)[3][512]'
[-Wincompatible-pointer-types]
  171 |         expect = kmalloc_obj(observed.lines);
      |                ^

Instead of changing the "expect" type to "char (*)[3][512]" and
requiring a dereference at each use (e.g. "(expect*)[0]"), just
explicitly cast the return to the desired type.

Tested with:

$ ./tools/testing/kunit/kunit.py run \
	--kconfig_add CONFIG_DEBUG_KERNEL=y \
	--kconfig_add CONFIG_KCSAN=y \
	--kconfig_add CONFIG_KCSAN_KUNIT_TEST=y \
	--arch=x86_64 --qemu_args '-smp 2' kcsan

Reported-by: Nathan Chancellor <nathan@kernel.org>
Fixes: 69050f8d6d07 ("treewide: Replace kmalloc with kmalloc_obj for non-scalar types")
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: <kasan-dev@googlegroups.com>
---
 kernel/kcsan/kcsan_test.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/kcsan/kcsan_test.c b/kernel/kcsan/kcsan_test.c
index 79e655ea4ca1..ae758150ccb9 100644
--- a/kernel/kcsan/kcsan_test.c
+++ b/kernel/kcsan/kcsan_test.c
@@ -168,7 +168,7 @@ static bool __report_matches(const struct expect_report *r)
 	if (!report_available())
 		return false;
 
-	expect = kmalloc_obj(observed.lines);
+	expect = (typeof(expect))kmalloc_obj(observed.lines);
 	if (WARN_ON(!expect))
 		return false;
 
-- 
2.34.1

On Wed, 25 Feb 2026 at 00:24, Kees Cook <kees@kernel.org> wrote:
>
> The call to kmalloc_obj(observed.lines) returns "char (*)[3][512]",
> a pointer to the whole 2D array. But "expect" wants to be "char (*)[512]",
> the decayed pointer type, as if it were observed.lines itself (though
> without the "3" bounds). This produces the following build error:
>
> ../kernel/kcsan/kcsan_test.c: In function '__report_matches':
> ../kernel/kcsan/kcsan_test.c:171:16: error: assignment to 'char (*)[512]' from incompatible pointer type 'char (*)[3][512]'
> [-Wincompatible-pointer-types]
>   171 |         expect = kmalloc_obj(observed.lines);
>       |                ^
>
> Instead of changing the "expect" type to "char (*)[3][512]" and
> requiring a dereference at each use (e.g. "(expect*)[0]"), just
> explicitly cast the return to the desired type.
>
> Tested with:
>
> $ ./tools/testing/kunit/kunit.py run \
>         --kconfig_add CONFIG_DEBUG_KERNEL=y \
>         --kconfig_add CONFIG_KCSAN=y \
>         --kconfig_add CONFIG_KCSAN_KUNIT_TEST=y \
>         --arch=x86_64 --qemu_args '-smp 2' kcsan
>
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Fixes: 69050f8d6d07 ("treewide: Replace kmalloc with kmalloc_obj for non-scalar types")
> Signed-off-by: Kees Cook <kees@kernel.org>

Reviewed-by: Marco Elver <elver@google.com>

I'm assuming you'll take it through your tree.

Thanks!

> ---
> Cc: Marco Elver <elver@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: <kasan-dev@googlegroups.com>
> ---
>  kernel/kcsan/kcsan_test.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/kcsan/kcsan_test.c b/kernel/kcsan/kcsan_test.c
> index 79e655ea4ca1..ae758150ccb9 100644
> --- a/kernel/kcsan/kcsan_test.c
> +++ b/kernel/kcsan/kcsan_test.c
> @@ -168,7 +168,7 @@ static bool __report_matches(const struct expect_report *r)
>         if (!report_available())
>                 return false;
>
> -       expect = kmalloc_obj(observed.lines);
> +       expect = (typeof(expect))kmalloc_obj(observed.lines);
>         if (WARN_ON(!expect))
>                 return false;
>
> --
> 2.34.1
>