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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 24, 2016 at 01:15:27PM +0200, Oleg Nesterov wrote:
> Yes, current is still valid.
> 
> But nothing protects current->group_leader or parent/real_parent, they
> can point to the exited/freed task. We really need to nullify them in
> __unhash_process() to catch the problems like this, I wanted to do this
> many times...
> 
> So you simply can't know your tgid or even tid after release_task() calls
> __unhash_process(). Actually after exit_notify() unless the exiting task
> autoreaps itself.
> 
> How about the trivial fix below?
> 
> Oleg.
> 
> --- x/kernel/events/core.c
> +++ x/kernel/events/core.c
> @@ -1257,7 +1257,7 @@ static u32 perf_event_pid(struct perf_ev
>  	if (event->parent)
>  		event = event->parent;
>  
> -	return task_tgid_nr_ns(p, event->ns);
> +	return pid_alive(p) ? task_tgid_nr_ns(p, event->ns) : 0;
>  }
>  
>  static u32 perf_event_tid(struct perf_event *event, struct task_struct *p)

Should we do the same for perf_event_tid() and report -1 as the pid/tid
in the !alive case? -1 should be an obvious invalid pid since we limit
the pid-space to less than 32 bits.