linux-kernel - Re: pid ns feature request

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Message-ID: <CALCETrWL6UheZyepgR1WhwOO6_JverEm4yMTK_OXRZKS8gCRpw@mail.gmail.com>
Date:	Fri, 25 Apr 2014 13:55:41 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"linux-kernel@...r.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

On Fri, Apr 25, 2014 at 1:46 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Fri, Apr 25, 2014 at 1:25 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> 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?
>
> I'm not entirely sure.  I'm hacking on this thing:
>
> https://github.com/amluto/sandstorm/tree/userns
>
> which isn't really my code.  But there's an inner sandbox and an outer
> sandbox, and only the inner sandbox is in a pid namespace.

That was a semi-useless link.  This is better:

https://github.com/amluto/sandstorm/blob/userns/src/sandstorm/supervisor-main.c%2B%2B

--Andy
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