Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect PIDFD_THREAD

[Christian Brauner <brauner@kernel.org>]

On Sat, Feb 10, 2024 at 01:30:33PM +0100, Oleg Nesterov wrote:
> Christian,
> 
> Thanks again! the last 2 commits in vfs.pidfd look good to me.
> 
> As for this patch, I am not sure I understand your concerns, and I
> have another concern, please see below.
> 
> For the moment, please forget about PIDFD_THREAD.
> 
> On 02/10, Christian Brauner wrote:
> >
> > (1) kill(-1234) => kill process group with id 1234
> > (2) kill(0)     => kill process group of @current
> >
> > which implementation wise is indicated by
> >
> > __kill_pgrp_info(..., pid ? find_vpid(-pid) ? task_pgrp(current))
> >
> > We're obviously not going to implement (2) as that doesn't really make a
> > sense for pidfd_send_signal().
> 
> Sure,
> 
> > But (1) is also wrong for pidfd_send_signal(). If we'd ever implement
> > (1) it should be via pidfd_open(1234, PIDFD_PROCESS_GROUP).
> 
> Why do you think we need another flag for open() ?

We don't need one. We could if we wanted to was my point. But let's
ignore that for now.

> 
> To me it looks fine if we allow to send the signal to pgrp if
> flags & PIDFD_SIGNAL_PROCESS_GROUP.

Yes, that's what I want too, I just wonder about the semantics.

> 
> And pidfd_send_signal() can just do
> 
> 	if (PIDFD_SIGNAL_THREAD_GROUP)
> 		ret = __kill_pgrp_info(sig, kinfo, pid);
> 	else
> 		ret = kill_pid_info_type(...);
> 		
> (yes, yes, this needs tasklist, just a pseudo code to simpliy)
> 
> Now lets recall about PIDFD_THREAD.
> 
> If the target task is a group leader - there is no difference.
> 
> If it is not a leader - then __kill_pgrp_info() will always return
> -ESRCH, do_each_pid_task(PIDTYPE_PGID) won't find any task.
> 
> And personally I think this is all we need.
> 
> ------------------------------------------------------------------------------
> But if you want to make PIDFD_SIGNAL_THREAD_GROUP work even if the
> target task is not a leader, then yes, we need something like
> 
> 	task_pgrp(pid_task(pid, PIDTYPE_PID))
> 
> like you did in the new kill_pgrp_info() helper in this patch.
> 
> I won't argue, but do you think this makes a lot of sense?

The question is what is more useful for userspace when they do:
pidfd_send_signal(1234, PIDFD_SEND_PROCESS_GROUP)?

(1) They either mean to signal a process group that is headed by 1234.
(2) Or they want to signal a process group of which 1234 is a member or
    the leader.

From a usability perspective (1) is a lot more restrictive because it
requires @pidfd to refer to a process group leader. Whereas (2) doesn't
require userspace to hold a @pidfd to a process group leader. It is
enough to just hold a @pidfd. In other words, (2) has wider scope.

And intuitively that is what I had thought is more useful. But by that
logic PIDFD_SEND_THREAD_GROUP would have to signal to the thread-group
that @pidfd is in regardless of whether @pidfd is actually a
thread-group leader.

Which is also what you're pointing out, afaict.