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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 25 Jan 2018 09:55:00 -0800
Message-ID: <CA+55aFwqDxBSKf3u2Uv-90vrhx4WSoQunOPU3witzCAuNhYBhg@mail.gmail.com>
Subject: Re: [PATCH] lib/strscpy: remove word-at-a-time optimization.
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andrew Morton <akpm@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>,
        stable <stable@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>
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On Thu, Jan 25, 2018 at 12:32 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Wed, Jan 24, 2018 at 6:52 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> So I'd *much* rather have some way to tell KASAN that word-at-a-time
>> is going on. Because that approach definitely makes a difference in
>> other places.
>
> The other option was to use READ_ONCE_NOCHECK().

How about just using the same accessor that we do for the dcache case.
That gives a reasonable example of the whole word-at-a-time model, and
should be good.

COMPLETELY UNTESTED patch attached. This needs to be verified.

It does limit the word-at-a-time code to the architectures that select
both HAVE_EFFICIENT_UNALIGNED_ACCESS and DCACHE_WORD_ACCESS, but that
seems a reasonable choice anyway.

               Linus

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