Re: [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd polling

[Christian Brauner]

On Fri, Jul 19, 2019 at 06:14:05PM +0200, Oleg Nesterov wrote:
> it seems that I missed something else...
> 
> On 07/17, Joel Fernandes (Google) wrote:
> >
> > @@ -1156,10 +1157,11 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
> >  		ptrace_unlink(p);
> >  
> >  		/* If parent wants a zombie, don't release it now */
> > -		state = EXIT_ZOMBIE;
> > +		p->exit_state = EXIT_ZOMBIE;
> >  		if (do_notify_parent(p, p->exit_signal))
> > -			state = EXIT_DEAD;
> > -		p->exit_state = state;
> > +			p->exit_state = EXIT_DEAD;
> > +
> > +		state = p->exit_state;
> >  		write_unlock_irq(&tasklist_lock);
> 
> why do you think we also need to change wait_task_zombie() ?
> 
> pidfd_poll() only needs the exit_state != 0 check, we know that it
> is not zero at this point. Why do we need to change exit_state before
> do_notify_parent() ?

Oh, because of?:

	/*
	 * Move the task's state to DEAD/TRACE, only one thread can do this.
	 */
	state = (ptrace_reparented(p) && thread_group_leader(p)) ?
		EXIT_TRACE : EXIT_DEAD;
	if (cmpxchg(&p->exit_state, EXIT_ZOMBIE, state) != EXIT_ZOMBIE)
		return 0;

So exit_state will definitely be set in this scenario. Good point.

Christian