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

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Date:	Sun, 20 Jun 2010 18:53:52 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Louis Rilling <louis.rilling@...labs.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Linux Containers <containers@...ts.osdl.org>,
	linux-kernel@...r.kernel.org, Daniel Lezcano <dlezcano@...ibm.com>
Subject: Re: [PATCH 6/6] pidns: Support unsharing the pid namespace.

Oleg Nesterov <oleg@...hat.com> writes:

> 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?

Roughly.  The caller is not in the pid namespace so shows up as pid 0.

> 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 ?

Good question.  I need to go back and look at that.

> 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.

do_wait() can't work and I missed that dependency the first time
around.  Having looked at my earlier bug report from Daniel when
I was playing with this patchset earlier it is clear that he was
triggering the proc_mnt race with such a process.

So except for ptrace I don't think the proc_mnt problem is possible
to trigger in the current code.

> I hope I missed something, this all is too subtle for me. And I
> still do not understand 4/6 which adds ns->dead.

ns->dead is just a flag to say no more processes in the pid namespace.
Which means an unshare into the pid namespace after zap_pid_ns_processes
has been called will fail().

Eric



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