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

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Date: Mon, 12 Feb 2024 12:15:52 +0100
From: Arnd Bergmann <arnd@...nel.org>
To: Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Konovalov <adech.fo@...il.com>
Cc: Arnd Bergmann <arnd@...db.de>,
	Alexander Potapenko <glider@...gle.com>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Vincenzo Frascino <vincenzo.frascino@....com>,
	Marco Elver <elver@...gle.com>,
	kasan-dev@...glegroups.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] kasan/test: avoid gcc warning for intentional overflow

From: Arnd Bergmann <arnd@...db.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@...db.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