linux-kernel - Re: pid ns feature request

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Date:	Fri, 25 Apr 2014 13:25:27 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"Serge E. Hallyn" <serge@...onical.com>,
	Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: pid ns feature request

Andy Lutomirski <luto@...capital.net> writes:

> On Fri, Apr 25, 2014 at 12:37 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Andy Lutomirski <luto@...capital.net> writes:
>>
>>> Unless I'm missing some trick, it's currently rather painful to mount
>>> a namespace /proc.  You have to actually be in the pid namespace to
>>> mount the correct /proc instance, and you can't unmount the old /proc
>>> until you've mounted the new /proc.  This means that you have to fork
>>> into the new pid namespace before you can finish setting it up.
>>
>> Yes.  You have to be inside just about all namespaces before you can
>> finish setting them up.
>>
>> I don't know the context in which needed to be inside the pid namespace
>> is a burden.
>
> I'm trying to sandbox myself.  I unshare everything, setup up new
> mounts, pivot_root, umount the old stuff, fork, and wait around for
> the child to finish.
>
> This doesn't work: the parent can't mount the new /proc, and the child
> can't either because it's too late.
>
> The only solution I can think of without kernel changes is to fork the
> child (pid 1) before pivot_root, which makes everything more
> complicated.  I suppose I can unshare, fork immediately, have the
> child set up all the mounts, and then wake the parent, but this is an
> annoying bit of extra complexity for no obvious gain.

Or perhaps just use clone and clone flags.

What are you doing with the parent process?  What value does it serve?

>>> Would it make sense to add a mount option to procfs to request a mount
>>> for pid_ns_for_children instead of task_active_pid_ns?
>>
>> This is about the using setns and unshare?
>>
>> Adding a proc amount option that takes a pid namespace file descriptor
>> would be the general solution, and might be worth implementing.
>>
>> Getting a pid namespace file descriptors when there are no pids might be
>> a challenge.
>
> Indeed, hence my request for a specific mode to mount /proc for
> pid_ns_for_children.
>
> FWIW, I also tried forking, having the child mount /proc and exit,
> then forking again later on.  That also doesn't work -- it looks like
> you can't recreate pid 1 after it does.

Nope. Once pid 1 (init) is dead the pid namespace is dead.

Eric
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