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

[Oleg Nesterov <oleg@redhat.com>]

On 01/29, Christian Brauner wrote:
>
> On Mon, Jan 29, 2024 at 12:23:15PM +0100, Oleg Nesterov wrote:
> > @@ -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 know. I am fine either way, but I think this needs a separate
patch and another discussion in any case. Anyway should be trivial,
pidfd_send_signal() has the "flags" argument.

On a related note, should copy_process(CLONE_PIDFD | CLONE_THREAD) add
PIDFD_THREAD flag "automatically" depending on CLONE_THREAD? Or do we
want another CLONE_PIDFD_THREAD flag so that PIDFD_THREAD can be used
without CLONE_THREAD? Again, I do not know, needs another discussion.

> Otherwise this looks good to me!

OK, thanks, I'll send v2 in a minute. The patch is the same, I only
updated the comments.

Oleg.