Introduce the cloning with pids functionality [LWN.net]

From:		Pavel Emelyanov <xemul@parallels.com>
To:		Andrew Morton <akpm@linux-foundation.org>
Subject:		[PATCH 0/3] Introduce the cloning with pids functionality
Date:		Thu, 10 Nov 2011 21:15:02 +0400
Message-ID:		<4EBC0696.9030103@parallels.com>
Cc:		Cyrill Gorcunov <gorcunov@openvz.org>, Glauber Costa <glommer@parallels.com>, Nathan Lynch <ntl@pobox.com>, Tejun Heo <tj@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Oleg Nesterov <oleg@redhat.com>, Serge Hallyn <serue@us.ibm.com>, Daniel Lezcano <dlezcano@fr.ibm.com>
Archive-link:		Article, Thread

When restoring a task (or a set of tasks) we need to recreate them 
with exactly the same pid(s) as they had before. Thus we need the 
ability to create a task with specified pid. The proposal is to reuse 
the already free CLONE_STOPPED clone flag, introduce the new one
called CLONE_CHILD_USEPIDS and point to the desired pids with the
child_tidptr.

There are two cnages from the previous set doing the same.

1. The child_tidptr points to an array of pids for current namespace 
   and its ancestors. When 0 is met in this array the pid number for
   the corresponding namespace is generated, rather than set.

2. For security reasons after a regular clone/fork is done in a namespace
   further cloning with predefined pid is not allowed.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
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