Re: [PATCH 3/3] exec: Transform exec_update_mutex into a rw_semaphore

[Linus Torvalds]

On Mon, Dec 7, 2020 at 1:10 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > PeterZ, is there something I'm missing?
>
> Like this?
>
>   https://lkml.kernel.org/r/20200828123720.GZ1362448@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Yes, except I think you should remove the old ptrace_may_access() check.

Because otherwise we'll just end up having KCSAN complain about the
unlocked optimistic accesses or something like that.

So do the perfmon_capable() check early - it doesn't need the
exec_update_mutex - and then just do the ptrace_may_access() one late.

I don't see any point at all in checking privileges twice, and I do
see real downsides. Not just that KCSAN issue, but also lack of
coverage (ie the second check will then effectively never be tested,
which is bad too).

               Linus