linux-kernel - Re: [PATCH] lib/string: Bring optimized memcmp from glibc

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 21 Jul 2021 13:27:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Sterba <dsterba@...e.cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nikolay Borisov <nborisov@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH] lib/string: Bring optimized memcmp from glibc

On Wed, Jul 21, 2021 at 1:13 PM David Sterba <dsterba@...e.cz> wrote:
>
> adding a memcmp_large that compares by native words or u64 could be
> the best option.

Yeah, we could just special-case that one place.

But see the patches I sent out - I think we can get the best of both worlds.

A small and simple memcmp() that is good enough and not the
_completely_ stupid thing we have now.

The second patch I sent out even gets the mutually aligned case right.

Of course, the glibc code also ended up unrolling things a bit, but
honestly, the way it did it was too disgusting for words.

And if it really turns out that the unrolling makes a big difference -
although I doubt it's meaningful with any modern core - I can add a
couple of lines to that simple patch I sent out to do that too.
Without getting the monster that is that glibc code.

Of course, my patch depends on the fact that "get_unaligned()" is
cheap on all CPU's that really matter, and that caches aren't
direct-mapped any more. The glibc code seems to be written for a world
where registers are cheap, unaligned accesses are prohibitively
expensive, and unrolling helps because L1 caches are direct-mapped and
you really want to do chunking to not get silly way conflicts.

If old-style Sparc or MIPS was our primary target, that would be one
thing. But it really isn't.

              Linus