linux-kernel - Re: [PATCH] lib/strscpy: avoid KASAN false positive

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Date:   Tue, 18 Jul 2017 10:22:44 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dave Jones <davej@...emonkey.org.uk>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/strscpy: avoid KASAN false positive

On Tue, Jul 18, 2017 at 10:15 AM, Andrey Ryabinin
<aryabinin@...tuozzo.com> wrote:
>
> +       /*
> +        * KASAN won't be happy about word-at-a-time
> +        * optimistic reads, so let's avoid them.
> +        */
> +       if (IS_ENABLED(CONFIG_KASAN))
> +               max = 0;
> +

No, please don't.

Two reasons:

 (a) it turns out that KASAN doesn't actually warn about this when
there aren't buggy users (because we only do word-at-a-time in the
spacified-to-be-safe region anyway).

 (b) havign automated testing that then just changes semantics and
implementation of what is tested is a bad bad bad idea.

So (a) says that we shouldn't need it in the first place, and (b) says
that we should avoid KASAN changing behavior unless we absolutely
*have* to.

In fact, I think we should *never* have that kind of "KASAN changes
semantics". If there is some particular load that is known to be
problematic for KASAN, we *still* shouldn't change semantics, we
should just mark that single load as being unchecked by KASAN.

                  Linus