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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 23 Apr 2020 19:25:53 +0200
Message-ID: <CACT4Y+b7omyQ0bBBApOs5O_m0MDZWjoBi3QV6MxG4h_14gUa2g@mail.gmail.com>
Subject: Re: [PATCH v3 3/3] kasan: initialise array in kasan_memcmp test
To:     Daniel Axtens <dja@axtens.net>
Cc:     LKML <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>
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On Thu, Apr 23, 2020 at 5:45 PM Daniel Axtens <dja@axtens.net> wrote:
>
> memcmp may bail out before accessing all the memory if the buffers
> contain differing bytes. kasan_memcmp calls memcmp with a stack array.
> Stack variables are not necessarily initialised (in the absence of a
> compiler plugin, at least). Sometimes this causes the memcpy to bail
> early thus fail to trigger kasan.
>
> Make sure the array initialised to zero in the code.
>
> No other test is dependent on the contents of an array on the stack.
>
> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> ---
>  lib/test_kasan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 939f395a5392..7700097842c8 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -638,7 +638,7 @@ static noinline void __init kasan_memcmp(void)
>  {
>         char *ptr;
>         size_t size = 24;
> -       int arr[9];
> +       int arr[9] = {};
>
>         pr_info("out-of-bounds in memcmp\n");
>         ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);

My version of this function contains the following below:

memset(arr, 0, sizeof(arr));

What am I missing?