linux-kernel - [PATCH v2] kasan: Initialize tag to 0xff in __kasan

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Message-Id: <20190502163057.6603-1-natechancellor@gmail.com>
Date:   Thu,  2 May 2019 09:30:58 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     kasan-dev@...glegroups.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH v2] kasan: Initialize tag to 0xff in __kasan_kmalloc

When building with -Wuninitialized and CONFIG_KASAN_SW_TAGS unset, Clang
warns:

mm/kasan/common.c:484:40: warning: variable 'tag' is uninitialized when
used here [-Wuninitialized]
        kasan_unpoison_shadow(set_tag(object, tag), size);
                                              ^~~

set_tag ignores tag in this configuration but clang doesn't realize it
at this point in its pipeline, as it points to arch_kasan_set_tag as
being the point where it is used, which will later be expanded to
(void *)(object) without a use of tag. Initialize tag to 0xff, as it
removes this warning and doesn't change the meaning of the code.

Link: https://github.com/ClangBuiltLinux/linux/issues/465
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
---

v1 -> v2:

* Initialize tag to 0xff at Andrey's request

 mm/kasan/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 36afcf64e016..242fdc01aaa9 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -464,7 +464,7 @@ static void *__kasan_kmalloc(struct kmem_cache *cache, const void *object,
 {
 	unsigned long redzone_start;
 	unsigned long redzone_end;
-	u8 tag;
+	u8 tag = 0xff;
 
 	if (gfpflags_allow_blocking(flags))
 		quarantine_reduce();
-- 
2.21.0