linux-kernel - Re: [PATCH v2] pidfd: implement PIDFD_THREAD flag for pidfd

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Date: Tue, 30 Jan 2024 19:27:50 -0700
From: Tycho Andersen <tycho@...ho.pizza>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Christian Brauner <brauner@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] pidfd: implement PIDFD_THREAD flag for pidfd_open()

On Tue, Jan 30, 2024 at 12:34:09PM +0100, Oleg Nesterov wrote:
> Damn. Self-NACK.
> 
> I forgot (we all ;) about mt-exec, and there are 2 problems.
> 
> 1. The "if (!thread_group_leader(tsk))" block in de_thread() needs
>    do_notify_pidfd() too, the execing non-leader thread looses its
>    old pid, pidfd_poll(PIDFD_THREAD, pid-of-execing-sub-thread)
>    should succeed. Must be fixed, I think.

I think the `test_non_tgl_exec` from my tests exercises the scenario
you're describing, and it works.

> 2. pidfd_poll(PIDFD_THREAD, pid-of-group-leader) should not succeed
>    when its sub-thread execs, the execing thread inherits the leader's
>    pid. Perhaps pidfd_task_exited() can check sig->group_exec_task,

I didn't have an explicit test for this, but I hacked one up, and
pidfd_poll(PIDFD_THREAD, pid-of-group-leader) doesn't return after
exec.

I think it's ok? But I must be missing something.

Tycho