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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 24 Jan 2018 09:52:42 -0800
Message-ID: <CA+55aFwmVfiH4m1iBUN_HUYQ4RHN5Eg_4rq_bGysK-Ho0dFoeQ@mail.gmail.com>
Subject: Re: [PATCH] lib/strscpy: remove word-at-a-time optimization.
To:     Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc:     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>,
        Dmitry Vyukov <dvyukov@google.com>,
        stable <stable@vger.kernel.org>
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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.

                    Linus