[BUG] v6.3-rc2 regresses sched_getaffinity() for arm64

[Linus Torvalds]

On Tue, Mar 14, 2023 at 6:35 PM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> So this does look  like purely a sched_getaffinity() thing (including
> the compat handling for same).
>
> And I can see why sched_getaffinity() uses cpumask_size(): we have no
> other good helper for this.

I decided that the cleanest fix is to just keep the cpumask_size() use
as-is, and just use zalloc_cpumask_var() to make sure the cpumask is
fully initialized.

Yes, we could play games with the exact size, but there just isn't any
excuse for it. Either it's a small on-stack allocation that gets
copied to user space (in which case we really are better off just
initializing it instead of doing anything clever), or it's an explicit
allocation due to the x86-64 MAXSMP case (in which case zeroing the
allocation is the least of our problems).

And zeroing the cpumask was what other somewhat similar cases seemed
to be doing, so it's consistent.

I've pushed out my fix. It looks ObviouslyCorrect(tm), but it would be
good to get verification that it does indeed fix things for you.

Because sometimes things look a bit more obvious than they actually are ;)

              Linus