Re: [PATCH] kasan: Zero initialize tag in __kasan_kmalloc
From: Andrey Konovalov
Date: Thu May 02 2019 - 12:25:22 EST
On Thu, May 2, 2019 at 5:36 PM Nathan Chancellor
> When building with -Wuninitialized and CONFIG_KASAN_SW_TAGS unset, Clang
> 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. Just zero initialize tag, 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@xxxxxxxxx>
> 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..4c5af68f2a8b 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 = 0;
Could you change this value to 0xff? This doesn't make any difference,
since set_tag() ignores the tag anyway, but is less confusing, as all
the non-tagged kernel pointers have 0xff in the top byte.
> if (gfpflags_allow_blocking(flags))
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