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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Wed, 24 Jan 2018 17:53:21 +0200
Message-ID: <CAHp75VdpPVKsirugMkmzo0b99RYJ9=AmVDa9WzNWzWHR4oMb=A@mail.gmail.com>
Subject: Re: [PATCH] lib/strscpy: remove word-at-a-time optimization.
To:     Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Eryu Guan <eguan@redhat.com>,
        Alexander Potapenko <glider@google.com>,
        Chris Metcalf <metcalf@alum.mit.edu>,
        David Laight <David.Laight@aculab.com>,
        Dmitry Vyukov <dvyukov@google.com>, stable@vger.kernel.org
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On Tue, Jan 9, 2018 at 6:37 PM, Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> strscpy() performs the word-at-a-time optimistic reads. So it may
> may access the memory past the end of the object, which is perfectly fine
> since strscpy() doesn't use that (past-the-end) data and makes sure the
> optimistic read won't cross a page boundary.
>
> But KASAN doesn't know anything about that so it will complain.
> There are several possible ways to address this issue, but none
> are perfect. See https://lkml.kernel.org/r/9f0a9cf6-51f7-cd1f-5dc6-6d510a7b8ec4@virtuozzo.com
>
> It seems the best solution is to simply disable word-at-a-time
> optimization. My trivial testing shows that byte-at-a-time
> could be up to x4.3 times slower than word-at-a-time.
> It may seems like a lot, but it's actually ~1.2e-10 sec per symbol vs
> ~4.8e-10 sec per symbol on modern hardware. And we don't use strscpy()
> in a performance critical paths to copy large amounts of data,
> so it shouldn't matter anyway.

What prevents you to revert the patch? After this one the all
advantages of the function becomes useless AFAIU.

-- 
With Best Regards,
Andy Shevchenko