On Thu, Mar 06, 2025 at 01:17:13PM +0100, Oleg Nesterov wrote:
> On 03/06, Christian Brauner wrote:
> >
> > Back when we implemented support for PIDFD_THREAD we ended up with the
> > decision that if userspace holds:
> >
> > pidfd_leader_thread = pidfd_open(<thread-group-leader-pid>, PIDFD_THREAD)
> >
> > that exit notification is not strictly defined if a non-thread-group
> > leader thread execs:
>
> Yes, this was even documented in commit 64bef697d33b ...
Yeah, I'm aware I was just revisiting this decision.
>
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -745,8 +745,11 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> > /*
> > * sub-thread or delay_group_leader(), wake up the
> > * PIDFD_THREAD waiters.
> > + *
> > + * The thread-group leader will be taken over by the execing
> > + * task so don't cause spurious wakeups.
> > */
> > - if (!thread_group_empty(tsk))
> > + if (!thread_group_empty(tsk) && (tsk->signal->notify_count >= 0))
> > do_notify_pidfd(tsk);
> >
> > if (unlikely(tsk->ptrace)) {
>
> perhaps... but this won't help if the leader exits and that another
> thread does exec?
>
> >From the changelog
>
> Perhaps we can improve this behaviour later, pidfd_poll()
> can probably take sig->group_exec_task into account. But
> this doesn't really differ from the case when the leader
> exits before other threads (so pidfd_poll() succeeds) and
> then another thread execs and pidfd_poll() will block again.
>
> so I am not sure what can we do.
I think early thread-group leader exit is a bug in the userspace
program whereas multi-threaded exec is well-defined (if ugly).
To detect early-thread-group leader exec userspace could do:
pidfd_leader_thread = pidfd_open(<thread-group-leader-pid>, 0)
pidfd_thread = pidfd_open(<thread-group-leader-pid>, PIDFD_THREAD)
and then they need to add both file descriptors to the poll instance.
If proper multi-threaded exec happens no notification will be generated
on either file descriptor.
However, if the thread-group leader exits prematurely then a
notification will be generated on pidfd_thread allowing detection of
malformed behavior.
So the premature thread-group leader exec thing is probably something we
can ignore as a real use-case.
>
> I'll try to think more later, but I can't promise much :(
>
> Oleg.
>
