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

[Christian Brauner]

On Thu, Jan 25, 2024 at 11:36:50AM -0700, Tycho Andersen wrote:
> 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.

Yes, let's rather do the explicit PIDFD_THREAD.