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

[Davidlohr Bueso]

On Fri, 04 Dec 2020, Linus Torvalds wrote:

> On Fri, Dec 4, 2020 at 12:30 PM Bernd Edlinger
> <bernd.edlinger@xxxxxxxxxx> wrote:
> > >
> > >    perf_event_open  (exec_update_mutex -> ovl_i_mutex)
>
> Side note: this one looks like it should be easy to fix.
>
> Is there any real reason why exec_update_mutex is actually gotten that
> early, and held for that long in the perf event code?

afaict just to validate the whole operation early. Per 79c9ce57eb2 the
mutex will guard the check and the perf_install_in_context vs exec.

> I _think_ we could move the ptrace check to be much later, to _just_ before that
>
>         * This is the point on no return; we cannot fail hereafter.
>
> point in the perf event install chain..

Peter had the idea of doing the ptrace_may_access() check twice: first
lockless and early, then under exec_update_mutex when it mattered right
before perf_install_in_context():

https://lore.kernel.org/linux-fsdevel/20200828123720.GZ1362448@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

> I don't think it needs to be moved down even that much, I think it
> would be sufficient to move it down below the "perf_event_alloc()",
> but I didn't check very much.

Yeah we could just keep a single ptrace_may_access() check just further
down until it won't deadlock.

Thanks,
Davidlohr