linux-kernel - Re: For review: pid_namespaces(7) man page

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Date:	Fri, 01 Mar 2013 01:10:23 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	mtk.manpages@...il.com
Cc:	Linux Containers <containers@...ts.linux-foundation.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>
Subject: Re: For review: pid_namespaces(7) man page

"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:

> Hi Eric,
>
> On Thu, Feb 28, 2013 at 4:24 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>
> [...]
>
>>> ==========
>>> PID_NAMESPACES(7)      Linux Programmer's Manual     PID_NAMESPACES(7)
>>>
>>> NAME
>>>        pid_namespaces - overview of Linux PID namespaces
>>>
>>> DESCRIPTION
> [...]
>
>>>    The namespace init process
>>>        The first process created in a new namespace (i.e., the process
>>>        created using clone(2) with the CLONE_NEWPID flag, or the first
>>>        child created by a process after a call to unshare(2) using the
>>>        CLONE_NEWPID flag) has the PID 1, and is the "init" process for
>>>        the namespace (see init(1)).  Children that are orphaned within
>>>        the namespace will be reparented to this  process  rather  than
>>>        init(1).
>>>
>>>        If the "init" process of a PID namespace terminates, the kernel
>>>        terminates all of the processes in the namespace via a  SIGKILL
>>>        signal.   This  behavior  reflects  the  fact  that  the "init"
>>>        process is essential for the correct operation of a PID  names‐
>>>        pace.   In this case, a subsequent fork(2) into this PID names‐
>>>        pace (e.g., from a process that has done a  setns(2)  into  the
>>>        namespace    using    an    open    file   descriptor   for   a
>>>        /proc/[pid]/ns/pid file corresponding to a process that was  in
>>>        the  namespace) will fail with the error ENOMEM; it is not pos‐
>>>        sible to create a new processes in a PID namespace whose "init"
>>>        process has terminated.
>>
>> It may be useful to mention unshare in the case of fork(2) failing just
>> because that is such an easy mistake to make.
>>
>> unshare(CLONE_NEWPID);
>> pid = fork();
>> waitpid(pid,...);
>> fork() -> ENOMEM
>
> I'm lost. Why does that sequence fail? The child of fork() becomes PID
> 1 in the new PID namespace.

Correct.
Then we wait for the child of the fork to exit();
Then we fork again into the new pid namespace.
The second fork fails because init has exited.

Eric
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