Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd

[Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>]

On Sat, Mar 14, 2015 at 08:24:56PM +0100, Oleg Nesterov wrote:
> On 03/14, Josh Triplett wrote:
> >
> > On Sat, Mar 14, 2015 at 03:35:58PM +0100, Oleg Nesterov wrote:
> > > On 03/12, Josh Triplett wrote:
> > > >
> > > > @@ -598,7 +600,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> > > >  	if (group_dead)
> > > >  		kill_orphaned_pgrp(tsk->group_leader, NULL);
> > > >
> > > > -	if (unlikely(tsk->ptrace)) {
> > > > +	if (tsk->autoreap) {
> > > > +		autoreap = true;
> > > > +	} else if (unlikely(tsk->ptrace)) {
> > > >  		int sig = thread_group_leader(tsk) &&
> > > >  				thread_group_empty(tsk) &&
> > > >  				!ptrace_reparented(tsk) ?
> > > > @@ -612,8 +616,10 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> > > >  	}
> > > >
> > > >  	tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;
> > > > -	if (tsk->exit_state == EXIT_DEAD)
> > > > +	if (tsk->exit_state == EXIT_DEAD) {
> > > >  		list_add(&tsk->ptrace_entry, &dead);
> > > > +		clonefd_do_notify(tsk);
> > > > +	}
> > >
> > > And even ignoring semantics issues, this change looks simply buggy anyway ;)
> > >
> > > How can we do list_add(&tsk->ptrace_entry) if it is traced by _another_ task?
> > > ->ptrace_entry is used by debugger.
> >
> > That list_add was there before; I didn't change that.
> 
> But this doesn't matter,
> 
> > I just added a
> > second line inside the EXIT_DEAD case, to call clonefd_do_notify (which
> > wakes up potential callers of poll/read).
> 
> No. Please read this code before and after your patch. You also added
> 
> 	if (tsk->autoreap)
> 		autoreap = true;
> 
> at the start. At this can trigger the _wrong_ list_add(&tsk->ptrace_entry),
> when the task is traced by another thread.
> 
> The current code can only use ->ptrace_entry if it was untraced (by us).

Ugh.  I finally realized just how magic the logic is there; thanks for
catching this.  The call to forget_original_parent at the top of
exit_notify can potentially add the process to the list "dead" here,
either in exit_ptrace() or in reparent_leader() (the latter of which has
its own duplicate of part of exit_notify's logic, including
do_notify_parent and setting exit_state).  Then exit_notify can add the
task to "dead" itself under some conditions that clearly depend on the
exact nature of the existing three-way conditional above.  And finally
exit_notify loops over "dead" and releases all the tasks there.

I'll investigate this further and make sure the ptrace case gets
handled correctly.

- Josh Triplett