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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 25 Jan 2018 10:08:13 +0100
Message-ID: <CACT4Y+aF0Z-QtuEDUKibQTG_Ntt+A0KzgxQQZXUad=Wm9RJsaQ@mail.gmail.com>
Subject: Re: [PATCH] lib/strscpy: remove word-at-a-time optimization.
To:     David Laight <David.Laight@aculab.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
        stable <stable@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>
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On Thu, Jan 25, 2018 at 9:42 AM, David Laight <David.Laight@aculab.com> wrote:
> From: Dmitry Vyukov [mailto:dvyukov@google.com]
>> Sent: 25 January 2018 08:33
>>
>> 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).
>
> The first byte might not have been written either.
> For example, doing a strlen() on a misaligned string you might read
> the aligned word containing the first byte and adjust the value so
> that the initial byte(s) are not zero.
> After scanning for a zero byte the length would be corrected.

Was the first byte at least kmalloc-ed? That's what KASAN checks, it
does not care about "written". KMSAN can detect uses of uninit data,
but that's another story.