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References: <20190502153538.2326-1-natechancellor@gmail.com>
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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Thu, 2 May 2019 18:24:47 +0200
Message-ID: <CAAeHK+xb8oV_YuVHJivW9c1R0h=AWA_-G1K28GPiZmF9LO_FAw@mail.gmail.com>
Subject: Re: [PATCH] kasan: Zero initialize tag in __kasan_kmalloc
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        clang-built-linux@googlegroups.com
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On Thu, May 2, 2019 at 5:36 PM Nathan Chancellor
<natechancellor@gmail.com> 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. 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@gmail.com>
> ---
>  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;

Hi Nathan,

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.

Thanks!

>
>         if (gfpflags_allow_blocking(flags))
>                 quarantine_reduce();
> --
> 2.21.0
>
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