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Date:   Thu, 16 May 2019 17:06:12 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Christian Brauner <christian@brauner.io>, jannh@google.com,
        viro@zeniv.linux.org.uk, torvalds@linux-foundation.org,
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        Geert Uytterhoeven <geert@linux-m68k.org>
Subject: Re: [PATCH v1 1/2] pid: add pidfd_open()
Message-ID: <20190516150611.GC22564@redhat.com>
References: <20190516135944.7205-1-christian@brauner.io>
 <20190516142659.GB22564@redhat.com>
 <20190516145607.j43xyj26k6l5vmbd@yavin>
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On 05/17, Aleksa Sarai wrote:
>
> On 2019-05-16, Oleg Nesterov <oleg@redhat.com> wrote:
> > On 05/16, Christian Brauner wrote:
> > >
> > > With the introduction of pidfds through CLONE_PIDFD it is possible to
> > > created pidfds at process creation time.
> >
> > Now I am wondering why do we need CLONE_PIDFD, you can just do
> >
> > 	pid = fork();
> > 	pidfd_open(pid);
>
> While the race window would be exceptionally short, there is the
> possibility that the child will die

Yes,

> and their pid will be recycled
> before you do pidfd_open().

No.

Unless the caller's sub-thread does wait() before pidfd_open(), of course.
Or unless you do signal(SIGCHILD, SIG_IGN).

Oleg.