Re: [RFC PATCH] pidfd: implement PIDFD_THREAD flag for pidfd_open()

[Tycho Andersen <tycho@tycho.pizza>]

On Mon, Jan 29, 2024 at 02:41:11PM +0100, Christian Brauner wrote:
> On Mon, Jan 29, 2024 at 12:23:15PM +0100, Oleg Nesterov wrote:
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -2051,7 +2051,8 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
> >  	WARN_ON_ONCE(!tsk->ptrace &&
> >  	       (tsk->group_leader != tsk || !thread_group_empty(tsk)));
> >  	/*
> > -	 * tsk is a group leader and has no threads, wake up the pidfd waiters.
> > +	 * tsk is a group leader and has no threads, wake up the !PIDFD_THREAD
> > +	 * waiters.
> >  	 */
> >  	if (thread_group_empty(tsk))
> >  		do_notify_pidfd(tsk);
> > @@ -3926,6 +3927,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> >  		prepare_kill_siginfo(sig, &kinfo);
> >  	}
> >  
> > +	/* TODO: respect PIDFD_THREAD */
> 
> So I've been thinking about this at the end of last week. Do we need to
> give userspace a way to send a thread-group wide signal even when a
> PIDFD_THREAD pidfd is passed? Or should we just not worry about this
> right now and wait until someone needs this?

I don't need it currently, but it would have been handy for some of
the tests I wrote.

Should I fix those up and send them too on top of Oleg's v2?

Tycho