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 11:36:50 -0700
From: Tycho Andersen <tycho@...ho.pizza>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Christian Brauner <brauner@...nel.org>, 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 Thu, Jan 25, 2024 at 07:30:46PM +0100, Oleg Nesterov wrote:
> On 01/25, Oleg Nesterov wrote:
> >
> > On 01/25, Tycho Andersen wrote:
> > >
> > > One of the things I don't like about PIDFD_THREAD is that it's hard to
> > > tell whether an arbitrary thread is a leader or not. Right now we do
> > > it by parsing /proc/pid/status, which shows all the stuff from
> > > do_task_stat() that we don't care about but which is quite expensive
> > > to compute. (Maybe there's a better way?)
> > >
> > > With PIDFD_THREAD we could could do it twice, once with the flag, get
> > > EINVAL, and then do it again. But ideally we wouldn't have to.
> >
> > Too late for me, most probably I misunderstood.
> >
> > If you want the PIDFD_THREAD behaviour, you can always use this flag
> > without any check...

Sorry, I hadn't read the patch. If it's ok to use PIDFD_THREAD on a
leader, then we can just always specify it. (We don't care about the
behavior of pidfd_poll().)

> > Could you spell?
> 
> Just in case, we can even add PIDFD_AUTO (modulo naming) which acts as
> PIDFD_THREAD if the target task is not a leader or 0 (current behaviour)
> otherwise. Trivial.

Yep, or given the above, maybe it'll work as-is, thank you.

Tycho