Re: [PATCH 6/6] pidns: Support unsharing the pid namespace.

[Oleg Nesterov <oleg@redhat.com>]

On 06/20, Oleg Nesterov wrote:
>
> On 06/20, Eric W. Biederman wrote:
> >
> > Unsharing of the pid namespace unlike unsharing of other namespaces
> > does not take affect immediately.  Instead it affects the children
> > created with fork and clone.
>
> Cough. It is too late to me to even try to understand the changelog.
>
> Instead I tried to quickly read the patch. Most probably I missed
> somthing, but still I'd like to ask the quiestion.
>
> So. If I understand correctly, the patch is simple:
>
> 	- unshare(CLONE_NEWPID) changes current->proxy->pid_ns,
> 	  but do not change current->pids[] and thus it doesn't
> 	  change task_active_pid_ns().
>
> 	- since copy_process() uses ->proxy->pid_ns for alloc_pid()
> 	  the new children will fall into the new ns.
>
> IOW, the caller becomes the "swapper" for the new namespace.
>
> Correct?
>
> If yes, I'm afraid nobody except you will understand this magic ;)
>
> But what if the task T does unshare(CLONE_NEWPID) and then, say,
> pthread_create() ? Unless I missed something, the new thread won't
> be able to see T ?

and, in this case the exiting sub-namespace init also kills its
parent?

> OK, suppose it does fork() after unshare(), then another fork().
> In this case the second child lives in the same namespace with
> init created by the 1st fork, but it is not descendant ? This means
> in particular that if the new init exits, zap_pid_ns_processes()->
> do_wait() can't work.
>
> I hope I missed something, this all is too subtle for me. And I
> still do not understand 4/6 which adds ns->dead.

And, forgot to mention. With this change proc_flush_task()->mntput()
becomes even more wrong.

Oleg.