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

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Message-ID: <20150314192456.GA8707@redhat.com>
Date:	Sat, 14 Mar 2015 20:24:56 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Josh Triplett <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 03/14, Josh Triplett wrote:
>
> On Sat, Mar 14, 2015 at 03:35:58PM +0100, Oleg Nesterov wrote:
> > On 03/12, Josh Triplett wrote:
> > >
> > > @@ -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;
> > > +	} else if (unlikely(tsk->ptrace)) {
> > >  		int sig = thread_group_leader(tsk) &&
> > >  				thread_group_empty(tsk) &&
> > >  				!ptrace_reparented(tsk) ?
> > > @@ -612,8 +616,10 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> > >  	}
> > >
> > >  	tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;
> > > -	if (tsk->exit_state == EXIT_DEAD)
> > > +	if (tsk->exit_state == EXIT_DEAD) {
> > >  		list_add(&tsk->ptrace_entry, &dead);
> > > +		clonefd_do_notify(tsk);
> > > +	}
> >
> > And even ignoring semantics issues, this change looks simply buggy anyway ;)
> >
> > How can we do list_add(&tsk->ptrace_entry) if it is traced by _another_ task?
> > ->ptrace_entry is used by debugger.
>
> That list_add was there before; I didn't change that.

But this doesn't matter,

> I just added a
> second line inside the EXIT_DEAD case, to call clonefd_do_notify (which
> wakes up potential callers of poll/read).

No. Please read this code before and after your patch. You also added

	if (tsk->autoreap)
		autoreap = true;

at the start. At this can trigger the _wrong_ list_add(&tsk->ptrace_entry),
when the task is traced by another thread.

The current code can only use ->ptrace_entry if it was untraced (by us).

Oleg.

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