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

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Date:   Fri, 3 Mar 2023 12:08:01 -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 11:37 AM Mateusz Guzik <mjguzik@...il.com> wrote:
>
> I mentioned in the previous e-mail that memset is used a lot even
> without the problematic opt and even have shown size distribution of
> what's getting passed there.

Well, I *have* been pushing Intel to try to fix memcpy and memset for
over two decades by now, but with FSRM I haven't actually seen the
huge problems any more.

It may just be that the loads I look at don't have issues (or the
machines I've done profiles on don't tend to show them as much).

Hmm. Just re-did my usual kernel profile. It may also be that
something has changed. I do see "clear_page" at the top, but yes,
memset is higher up than I remembered.

I actually used to have the reverse of your hack for this - I've had
various hacks over the year that made memcpy and memset be inlined
"rep movs/stos", which (along with inlined spinlocks) is a great way
to see the _culprit_ (without having to deal with the call chains -
which always get done the wrong way around imnsho).

            Linus