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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 24, 2016 at 03:40:13PM +0200, Oleg Nesterov wrote:
> On 10/24, Oleg Nesterov wrote:
> >
> > -static u32 perf_event_pid(struct perf_event *event, struct task_struct *p)
> > +static u32 perf_event_xxx(struct perf_event *event, struct task_struct *p,
> > +			 	enum pid_type type)
> >  {
> > +	pid_t nr;
> >  	/*
> >  	 * only top level events have the pid namespace they were created in
> >  	 */
> >  	if (event->parent)
> >  		event = event->parent;
> >
> > -	return task_tgid_nr_ns(p, event->ns);
> > +	nr = __task_pid_nr_ns(p, type, event->ns);
> > +	if (!nr && !is_idle_task(p))
> > +		nr = -1;
> > +	return nr;
> 
> And just in case... In any case __task_pid_nr_ns() and other similar helpers
> can also return zero if "p" runs in another namespace. Say, in the parent ns.

Right, I'm tempted to not change that. Its been the behaviour for a
while and changing that will upset people.

The unhash case is different in that its actively broken so we must do
something.

> Say, perf_event_switch_output(). What do we want to report in this case, zero
> or -1 ?

As with all switch_output() cases, the user had better know wth he's
doing ;-) Doing a switch_output() on a running counter is dubious to
begin with.