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

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Date:   Sat, 4 Mar 2023 13:01:15 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Mateusz Guzik <mjguzik@...il.com>,
        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 Sat, Mar 4, 2023 at 12:51 PM Yury Norov <yury.norov@...il.com> wrote:
>
> > That particular code sequence is arguably broken to begin with.
> > setall() should really only be used as a mask, most definitely not as
> > some kind of "all possible cpus".
>
> Sorry, don't understand this.

See the example patch I sent out.

Literally just make the rule be "we play games with cpumasks in that
they have two different 'sizes', so just make sure the bits in the
bigger and faster size are always clear".

That simple rule just means that we can then use that bigger constant
size in all cases where "upper bits zero" just don't matter.

Which is basically all of them.

Your for_each_cpu_not() example is actually a great example: it should
damn well not exist at all. I hadn't even noticed how broken it was.
Exactly like the other broken case (that I *did* notice -
cpumask_complement), it has no actual valid users. It _literally_ only
exists as a pointless test-case.

So this is *literally* what I'm talking about: you are making up silly
cases that then act as "arguments" for making all the _real_ cases
slower.

Stop it.

Silly useless cases are just that - silly and useless. They should not
be arguments for the real cases then being optimized and simplified.

Updated patch to remove 'for_each_cpu_not()' attached.

It's still completely untested. Treat this very much as a "Let's make
the common cases faster, at least for !MAXSMP".

                   Linus

View attachment "patch.diff" of type "text/x-patch" (7705 bytes)