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Date: Thu, 25 Jan 2024 19:30:46 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Tycho Andersen <tycho@tycho.pizza>
Cc: Christian Brauner <brauner@kernel.org>, 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: <20240125183045.GE5513@redhat.com>
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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...
>
> 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.

Oleg.