Re: [PATCH v2 0/7] CLONE_FD: Task exit notification via file descriptor

From: josh@joshtriplett.org
To: Thiago Macieira <thiago.macieira@intel.com>
Cc: Kees Cook <keescook@chromium.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Oleg Nesterov <oleg@redhat.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Rik van Riel <riel@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>
Subject: Re: [PATCH v2 0/7] CLONE_FD: Task exit notification via file descriptor
Date: Mon, 16 Mar 2015 16:29:49 -0700	[thread overview]
Message-ID: <20150316232949.GB31751@cloud> (raw)
In-Reply-To: <2381173.VxaIO6vGG3@tjmaciei-mobl4>

On Mon, Mar 16, 2015 at 03:14:14PM -0700, Thiago Macieira wrote:
> On Monday 16 March 2015 14:44:20 Kees Cook wrote:
> > >               O_CLOEXEC
> > >                      Set  the  close-on-exec  flag on the new file
> > >descriptor. See the description of the O_CLOEXEC flag in open(2)  for
> > >reasons why this may be useful.
> > 
> > This begs the question: what happens when all CLONE_FD fds for a
> > process are closed? Will the parent get SIGCHLD instead, will it
> > auto-reap, or will it be un-wait-able (I assume not this...)
> 
> Depends on CLONE_AUTOREAP. If it's on, then no one gets SIGCHLD, no one can 
> wait() on it and the process autoreaps itself.

Minor nit: CLONE_AUTOREAP makes the process autoreap and nobody can wait
on it, but if you pass SIGCHLD or some other exit signal to clone then
you'll still get that signal.

> If it's no active, then the old rules apply: parent gets SIGCHILD and can 
> wait(). If the parent exited first, then the child gets reparented to init, 
> which can do the wait().

Right.

> A child without CLONE_AUTOREAP should be wait()able. If it gets wait()ed 
> before the clonefd is read, the clonefd() will return a 0 read. If it gets 
> read before wait, then wait() reaps another child or returns -ECHILD. That's 
> no different than two threads doing simultaneous wait() on the same child.

Hrm?  That isn't the semantics we implemented; you'll *always* get an
exit notification via the clonefd if you have it open, with or without
autoreap and whether or not a wait has occurred yet.  And reading from
the clonefd does not serve as a wait; if you don't pass CLONE_AUTOREAP,
you'll still need to wait on the process.

- Josh Triplett