Re: hit a KASan bug related to Perf during stress test

[Oleg Nesterov <oleg@redhat.com>]

On 10/26, Peter Zijlstra wrote:
>
> On Tue, Oct 25, 2016 at 04:41:26PM +0200, Oleg Nesterov wrote:
> > >
> > > So what serialization would close that race? __task_pid_nr_ns() only
> > > seems to use RCU nothing more.
> >
> > I do not see how can we close this race, we obviously do not want to use
> > any locking.
> >
> > That is why I tried to suggest
> >
> > 	nr = __task_pid_nr_ns(p, type, event->ns);
> > 	if (!nr && !is_idle_task(p))
> > 		nr = -1;
> > 	return nr;
> >
> > but this will report -1 if p runs in another namespace, so perhaps we
> > can do
> >
> > 	nr = __task_pid_nr_ns(p, type, event->ns);
> > 	if (!nr && p->exit_state)
> > 		// it has already called exit_notify
> > 		nr = -1;
> > 	return nr;
>
> I think I'm asking how __task_pid_nr_ns() isn't susceptible to this race
> ;-)

which race ? ;) it seems that I confused you. Lets ignore the original
problem with perf_event_pid()->task_tgid_nr_ns() which can access the
freed memory. Lets suppose it is already fixed.

Another problem, as you noted, is that task_tgid_nr_ns/task_pid_nr_ns
returns zero if the task exits and this zero can be confused with the
swapper's pid.

	return pid_alive(p) ? task_pid_nr_ns(p, event->ns) : ~0

still can return zero because pid_alive(p) == T is not stable if we can
race with the exiting task, so it can't guarantee that task_pid_nr_ns()
won't return 0.

So we can check ->exit_state or, even better, that same pid_alive() after
task_pid_nr_ns() returns 0.

	nr = task_pid_nr_ns(p);
	/* avoid -1 if it is idle thread or runs in another ns */
	if (!nr && !pid_alive(p))
		nr = -1;
	return nr;

Or I misunderstood you?

Oleg.