[PATCH] kasan/test: avoid gcc warning for intentional overflow

[PATCH] kasan/test: avoid gcc warning for intentional overflow

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

The out-of-bounds test allocates an object that is three bytes too
short in order to validate the bounds checking. Starting with gcc-14,
this causes a compile-time warning as gcc has grown smart enough to
understand the sizeof() logic:

mm/kasan/kasan_test.c: In function 'kmalloc_oob_16':
mm/kasan/kasan_test.c:443:14: error: allocation of insufficient size '13' for type 'struct <anonymous>' with size '16' [-Werror=alloc-size]
  443 |         ptr1 = kmalloc(sizeof(*ptr1) - 3, GFP_KERNEL);
      |              ^

Hide the actual computation behind a RELOC_HIDE() that ensures
the compiler misses the intentional bug.

Fixes: 3f15801cdc23 ("lib: add kasan test module")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 mm/kasan/kasan_test.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/kasan_test.c b/mm/kasan/kasan_test.c
index 318d9cec111a..2d8ae4fbe63b 100644
--- a/mm/kasan/kasan_test.c
+++ b/mm/kasan/kasan_test.c
@@ -440,7 +440,8 @@ static void kmalloc_oob_16(struct kunit *test)
 	/* This test is specifically crafted for the generic mode. */
 	KASAN_TEST_NEEDS_CONFIG_ON(test, CONFIG_KASAN_GENERIC);
 
-	ptr1 = kmalloc(sizeof(*ptr1) - 3, GFP_KERNEL);
+	/* RELOC_HIDE to prevent gcc from warning about short alloc */
+	ptr1 = RELOC_HIDE(kmalloc(sizeof(*ptr1) - 3, GFP_KERNEL), 0);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
 
 	ptr2 = kmalloc(sizeof(*ptr2), GFP_KERNEL);
-- 
2.39.2

Re: [PATCH] kasan/test: avoid gcc warning for intentional overflow

[Andrey Konovalov]

On Mon, Feb 12, 2024 at 12:16 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> The out-of-bounds test allocates an object that is three bytes too
> short in order to validate the bounds checking. Starting with gcc-14,
> this causes a compile-time warning as gcc has grown smart enough to
> understand the sizeof() logic:
>
> mm/kasan/kasan_test.c: In function 'kmalloc_oob_16':
> mm/kasan/kasan_test.c:443:14: error: allocation of insufficient size '13' for type 'struct <anonymous>' with size '16' [-Werror=alloc-size]
>   443 |         ptr1 = kmalloc(sizeof(*ptr1) - 3, GFP_KERNEL);
>       |              ^
>
> Hide the actual computation behind a RELOC_HIDE() that ensures
> the compiler misses the intentional bug.
>
> Fixes: 3f15801cdc23 ("lib: add kasan test module")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  mm/kasan/kasan_test.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kasan/kasan_test.c b/mm/kasan/kasan_test.c
> index 318d9cec111a..2d8ae4fbe63b 100644
> --- a/mm/kasan/kasan_test.c
> +++ b/mm/kasan/kasan_test.c
> @@ -440,7 +440,8 @@ static void kmalloc_oob_16(struct kunit *test)
>         /* This test is specifically crafted for the generic mode. */
>         KASAN_TEST_NEEDS_CONFIG_ON(test, CONFIG_KASAN_GENERIC);
>
> -       ptr1 = kmalloc(sizeof(*ptr1) - 3, GFP_KERNEL);
> +       /* RELOC_HIDE to prevent gcc from warning about short alloc */
> +       ptr1 = RELOC_HIDE(kmalloc(sizeof(*ptr1) - 3, GFP_KERNEL), 0);
>         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
>
>         ptr2 = kmalloc(sizeof(*ptr2), GFP_KERNEL);
> --
> 2.39.2
>

Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>