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Date: Fri, 26 Jan 2024 10:49:02 +0100
From: Christian Brauner <brauner@kernel.org>
To: Tycho Andersen <tycho@tycho.pizza>
Cc: Oleg Nesterov <oleg@redhat.com>, linux-kernel@vger.kernel.org, 
	linux-api@vger.kernel.org, Tycho Andersen <tandersen@netflix.com>, 
	"Eric W. Biederman" <ebiederm@xmission.com>
Subject: Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group
 leaders
Message-ID: <20240126-indiz-radrennen-8554de0bc074@brauner>
References: <20240123153452.170866-2-tycho@tycho.pizza>
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 <20240125-tricksen-baugrube-3f78c487a23a@brauner>
 <20240125175113.GC5513@redhat.com>
 <ZbKigMNQM0Yklc/5@tycho.pizza>
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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.