Re: [PATCH] kcsan: test: Adjust "expect" allocation type for kmalloc_obj

[Marco Elver]

On Mon, 23 Feb 2026 at 23:22, Kees Cook <kees@xxxxxxxxxx> wrote:
>
> Instead of depending on the implicit case between a pointer to pointers
> and pointer to arrays, use the assigned variable type for the allocation
> type so they correctly match. Solves 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);
>       |                ^
>
> 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@xxxxxxxxxx>
> Fixes: 69050f8d6d07 ("treewide: Replace kmalloc with kmalloc_obj for non-scalar types")
> Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
> ---
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: <kasan-dev@xxxxxxxxxxxxxxxx>
> ---
>  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..056fa859ad9a 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 = kmalloc_obj(*expect);

This is wrong. Instead of allocating 3x512 bytes it's now only
allocating 512 bytes, so we get OOB below with this change. 'expect'
is a pointer to a 3-dimensional array of 512-char arrays (matching
observed.lines).