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

[Arnd Bergmann]

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.

Link: https://lkml.kernel.org/r/20240212111609.869266-1-arnd@xxxxxxxxxx
Fixes: 3f15801cdc23 ("lib: add kasan test module")
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 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 318d9cec111aa..2d8ae4fbe63bb 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);
-- 
2.43.0