Received-SPF: pass (google.com: domain of linux-kernel+bounces-60396-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Date: Sat, 10 Feb 2024 13:30:33 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Christian Brauner <brauner@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Tycho Andersen <tycho@tycho.pizza>, linux-api@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect
 PIDFD_THREAD
Message-ID: <20240210123033.GA27557@redhat.com>
References: <20240209130620.GA8039@redhat.com>
 <20240209130650.GA8048@redhat.com>
 <20240209-stangen-feuerzeug-17c8662854c9@brauner>
 <20240209154305.GC3282@redhat.com>
 <20240209-radeln-untrennbar-9d4ae05aa4cc@brauner>
 <20240209155644.GD3282@redhat.com>
 <20240210-abfinden-beimessen-2dbfea59b0da@brauner>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240210-abfinden-beimessen-2dbfea59b0da@brauner>
User-Agent: Mutt/1.5.24 (2015-08-30)

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() ?

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

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?

Oleg.