linux-kernel - Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

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Date: Thu, 25 Jan 2024 18:51:14 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Christian Brauner <brauner@...nel.org>
Cc: Tycho Andersen <tycho@...ho.pizza>, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, Tycho Andersen <tandersen@...flix.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group
 leaders

On 01/25, Christian Brauner wrote:
>
> > > When it is reaped is "mostly unrelated".
> >
> > Then why pidfd_poll() can't simply check !task || task->exit_state ?
> >
> > Nevermind. So, currently pidfd_poll() succeeds when the leader can be
>
> Hm, the comment right above mentions:
>
>         /*
>          * Inform pollers only when the whole thread group exits.
>          * If the thread group leader exits before all other threads in the
>          * group, then poll(2) should block, similar to the wait(2) family.
>          */
> > reaped, iow the whole thread group has exited.

Yes, but the comment doesn't contradict with what I have said?


> > But even if you are the
> > parent, you can't expect that wait(WNOHANG) must succeed, the leader
> > can be traced. I guess it is too late to change this behaviour.
>
> Hm, why is that an issue though?

Well, I didn't say this is a problem. I simply do not know how/why people
use pidfd_poll().

I mostly tried to explain why do I think that do_notify_pidfd() should
be always called from exit_notify() path, not by release_task(), even
if the task is not a leader.

> Because a program would rely on WNOHANG to hang on
> a ptraced leader? That seems esoteric imho.

To me it would be usefule, but lets not discuss this now. The "patch"
I sent doesn't change the current behaviour.

> > What if we add the new PIDFD_THREAD flag? With this flag
> >
> > 	- sys_pidfd_open() doesn't require the must be a group leader
>
> Yes.
>
> >
> > 	- pidfd_poll() succeeds when the task passes exit_notify() and
> > 	  becomes a zombie, even if it is a leader and has other threads.
>
> Iiuc, if an existing user creates a pidfd for a thread-group leader and
> then polls that pidfd they would currently only get notified if the
> thread-group is empty and the leader has exited.
>
> If we now start notifying when the thread-group leader exits but the
> thread-group isn't empty then this would be a fairly big api change

Hmm... again, this patch doesn't (shouldn't) change the current behavior.

Please note "with this flag" above. If sys_pidfd_open() was called
without PIDFD_THREAD, then sys_pidfd_open() still requires that the
target task must be a group leader, and pidfd_poll() won't succeed
until the leader exits and thread_group_empty() is true.

Oleg.