linux-kernel - Re: [PATCH v1 1/2] pid: add pidfd

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Message-ID: <20190516150611.GC22564@redhat.com>
Date:   Thu, 16 May 2019 17:06:12 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     Christian Brauner <christian@...uner.io>, jannh@...gle.com,
        viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, arnd@...db.de,
        akpm@...ux-foundation.org, dhowells@...hat.com,
        ebiederm@...ssion.com, elena.reshetova@...el.com,
        keescook@...omium.org, luto@...capital.net, luto@...nel.org,
        tglx@...utronix.de, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kselftest@...r.kernel.org, joel@...lfernandes.org,
        dancol@...gle.com, serge@...lyn.com,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v1 1/2] pid: add pidfd_open()

On 05/17, Aleksa Sarai wrote:
>
> On 2019-05-16, Oleg Nesterov <oleg@...hat.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.