Re: For review: pid_namespaces(7) man page

Michael Kerrisk (man-pages)
2013-03-01T10:21:04Z

On Fri, Mar 1, 2013 at 10:10 AM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> "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.

Ahhh -- I misapprehended the scenario you were describing. Got it now.
I'll add that case.

Thanks,

Michael