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

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Date:   Thu, 23 May 2019 11:53:02 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2] kasan: Initialize tag to 0xff in __kasan_kmalloc



On 5/2/19 7:30 PM, Nathan Chancellor wrote:
> 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>

Fixes: 7f94ffbc4c6a ("kasan: add hooks implementation for tag-based mode")
Cc: <stable@...r.kernel.org>
Reviewed-by: Andrey Ryabinin <aryabinin@...tuozzo.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();
>