linux-kernel - Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd

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Date:	Sat, 14 Mar 2015 15:14:14 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	josh@...htriplett.org
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>, Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thiago Macieira <thiago.macieira@...el.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task
	exit notification via fd

On 03/13, josh@...htriplett.org wrote:
>
> On Fri, Mar 13, 2015 at 05:21:13PM +0100, Oleg Nesterov wrote:
> >
> > Again, I simply do not know what this code does at all. But I bet the usage
> > of EXIT_DEAD is wrong ;)
> >
> > OK, OK, I can be wrong. But I simply do not see what protects this task_struct
> > if it is EXIT_DEAD (in fact even if it is EXIT_ZOMBIE).
>
> If by "what protects" you mean "what keeps it alive", the file
> descriptor holds a reference to the task_struct by calling
> get_task_struct when created and put_task_struct when released.

OK, so I was wrong. Although I still have a gut feeling that the usage
of EXIT_DEAD can't be right. Because it was always wrong outside of core
"exit" code ;) Nevermind, I didn't read this series yet, forget.

> > > @@ -598,7 +600,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> > >  	if (group_dead)
> > >  		kill_orphaned_pgrp(tsk->group_leader, NULL);
> > >
> > > -	if (unlikely(tsk->ptrace)) {
> > > +	if (tsk->autoreap) {
> > > +		autoreap = true;
> >
> > Debuggers won't be happy. A ptraced task should not autoreap itself.
>
> A process launching a new process with CLONE_FD is explicitly requesting
> that the process be automatically reaped without any other process
> having to wait on it.  The task needs to not become a zombie, because
> otherwise, it'll show up in waitpid(-1, ...)

This is clear.

But please note that this task can be traced/debugged by unrelated process,
not its real_parent/creator. Say, the system admin does "strace -p". This
simply breaks the current API.

Again, again, I didn't read this series yet. But the proper solution (afaics)
should move this "autoreap" check in release_task/__ptrace_detach(). If the
task is traced. Debugger should check ->autoreap and skip another
do_notify_parent().

Speaking of autoreap... If ->exit_signal is zero, then the exiting child
doesn't send the notification to its parent, still it doesn't autoreap
itself. To me this looks strange, and in fact it seems to me that this
is only by mistake. I am wondering if we can treat ->exit_signal == 0
as "autoreap" too. As usual, most probably the answer is "no, because it
is too late to change the historical behaviour". But this is off-topic.

Oleg.

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