Re: For review: pid_namespaces(7) man page

[ebiederm@xmission.com (Eric W. Biederman)]

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

> Hi Eric,
>
> On Thu, Feb 28, 2013 at 4:24 PM, Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.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