linux-kernel - Re: [PATCH 4/5] pidfd: add CLONE_WAIT

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Date:   Thu, 25 Jul 2019 12:40:06 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org, arnd@...db.de, ebiederm@...ssion.com,
        keescook@...omium.org, joel@...lfernandes.org, tglx@...utronix.de,
        tj@...nel.org, dhowells@...hat.com, jannh@...gle.com,
        luto@...nel.org, akpm@...ux-foundation.org, cyphar@...har.com,
        torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
        kernel-team@...roid.com, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 4/5] pidfd: add CLONE_WAIT_PID

On Thu, Jul 25, 2019 at 12:35:44PM +0200, Oleg Nesterov wrote:
> On 07/24, Christian Brauner wrote:
> >
> > If CLONE_WAIT_PID is set the newly created process will not be
> > considered by process wait requests that wait generically on children
> > such as:
> 
> I have to admit this feature looks a bit exotic to me...

It might look like it from the kernels perspective but from the feedback
on this when presenting on this userspace has real usecases for this.

> 
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -1019,6 +1019,9 @@ eligible_child(struct wait_opts *wo, bool ptrace, struct task_struct *p)
> >  	if (!eligible_pid(wo, p))
> >  		return 0;
> >
> > +	if ((p->flags & PF_WAIT_PID) && (wo->wo_type != PIDTYPE_PID))
> > +		return 0;
> 
> Even if ptrace == T ?
> 
> This doesn't look right. Say, strace should work even if its tracee (or
> one of the tracees) has PF_WAIT_PID.

As in
	if (!ptrace && (p->flags & PF_WAIT_PID) && (wo->wo_type != PIDTYPE_PID))
		return 0;

Sure, we can allow that.

Christian