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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 25 Jan 2018 09:32:51 +0100
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Subject: Re: [PATCH] lib/strscpy: remove word-at-a-time optimization.
To:     Linus Torvalds <torvalds@linux-foundation.org>
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 Wed, Jan 24, 2018 at 6:52 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Wed, Jan 24, 2018 at 12:54 AM, Rasmus Villemoes
> <rasmus.villemoes@prevas.dk> wrote:
>>
>> I see something similar, but at the 30->31 transition, and the
>> branch-misses remain at 1-3% for higher values, until 42 where it drops
>> back to 0%. Anyway, I highly doubt we do a lot of string copies of
>> strings longer then 32.
>
> So I really dislike that microbenchmark, because it just has the same
> length all the time. Which is very wrong, and makes the benchmark
> pointless. A big part of this all is branch mispredicts, you shouldn't
> just hand it the pattern on a plate.
>
> Anyway, the reason I really dislike the patch is not because I think
> strscpy() is all that important, but I *do* think that the
> word-at-a-time thing is conceptually something we do care about, and I
> hate removing it just because of KASAN not understanding it.
>
> 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(). Not sure if the "read
once" part will affect codegen here, though.
But if word-at-a-time thing is conceptually something we do care
about, we could also introduce something like READ_PARTIALLY_VALID(),
which would check that at least first byte of the read is valid and
that it does not cross heap block boundary (but outside of KASAN is a
normal read).