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

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Date:   Sat, 4 Mar 2023 12:51:30 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 04, 2023 at 11:19:54AM -0800, Linus Torvalds wrote:
> On Fri, Mar 3, 2023 at 9:51 PM Yury Norov <yury.norov@...il.com> wrote:
> >
> > And the following code will be broken:
> >
> > cpumask_t m1, m2;
> >
> > cpumask_setall(m1); // m1 is ffff ffff ffff ffff because it uses
> >                     // compile-time optimized nr_cpumask_bits
> >
> > for_each_cpu(cpu, m1) // 32 iterations because it relied on nr_cpu_ids
> >         cpumask_set_cpu(cpu, m2); // m2 is ffff ffff XXXX XXXX
> 
> So  honestly, it looks like you picked an example of something very
> unusual to then make everything else slower.

What about bootable sticks?
 
> Rather than commit aa47a7c215e7, we should just have fixed 'setall()'
> and 'for_each_cpu()' to use nr_cpu_ids, and then the rest would
> continue to use nr_cpumask_bits.

No, that wouldn't work for all.

> 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.

> The latter is "cpu_possible_mask", which is very different indeed (and
> often what you want is "cpu_online_mask")

Don't understand this possible vs online argument, but OK. What about this?

        if (cpumask_setall_is_fixed)
                cpumask_setall(mask);
        else {
                for_each_online_cpu(cpu)
                        cpumask_set_cpu(cpu, mask);
             }

        // You forgot to 'fix' cpumask_equal()
        BUG_ON(!cpumask_equal(cpu_online_mask, mask));

Or this:

        cpumask_clear(mask);
        for_each_cpu_not(cpu, mask)
                cpumask_set_cpu(cpu, mask);
        BUG_ON(!cpumask_full(mask));

The root of the problem is that half of cpumask API relies (relied) on
compile-time nr_cpumask_bits, and the other - on boot-time nr_cpu_ids.

So, if you consistently propagate your 'fix', you'll get rid of
nr_cpumask_bits entirely with all that associate overhead.

That's what I actually did. And even tried to minimize the
overhead the best way I can think of.

> But I'd certainly be ok with using nr_cpu_ids for setall, partly
> exactly because it's so rare. It would probably be better to remove it
> entirely, but whatever.
> 
>               Linus