linux-kernel - Re: [PATCH v3 3/3] kasan: initialise array in kasan

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Date:   Thu, 23 Apr 2020 19:25:53 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Daniel Axtens <dja@...ens.net>
Cc:     LKML <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v3 3/3] kasan: initialise array in kasan_memcmp test

On Thu, Apr 23, 2020 at 5:45 PM Daniel Axtens <dja@...ens.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@...tuozzo.com>
> Cc: Alexander Potapenko <glider@...gle.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Signed-off-by: Daniel Axtens <dja@...ens.net>
> Reviewed-by: Dmitry Vyukov <dvyukov@...gle.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?