linux-kernel - Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

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Date:   Fri, 3 Mar 2023 09:54:00 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Christian Brauner <brauner@...nel.org>, serge@...lyn.com,
        paul@...l-moore.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

On Fri, Mar 3, 2023 at 9:39 AM Mateusz Guzik <mjguzik@...il.com> wrote:
>
> So the key is: memset is underperforming at least on x86-64 for
> certain sizes and the init-on-alloc thingy makes it used significantly
> more, exacerbating the problem

One reason that the kernel memset isn't as optimized as memcpy, is
simply because under normal circumstances it shouldn't be *used* that
much outside of page clearing and constant-sized structure
initialization.

Page clearing is fine, and constant-sized structure inits are also
generally fine (ie the compiler does the small ones directly).

So this is literally a problem with pointless automated memset,
introduced by that hardening option. And hardening people generally
simply don't care about performance, and the people who _do _care
about performance usually don't enable the known-expensive crazy
stuff.

Honestly, I think the INIT_ONCE stuff is actively detrimental, and
only hides issues (and in this case adds its own). So I can't but help
to say "don't do that then". I think it's literally stupid to clear
allocations by default.

I'm not opposed to improving memset, but honestly, if the argument is
based on the stupid hardening behavior, I really think *that* needs to
be fixed first.

               Linus