linux-kernel - Re: pid ns feature request

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Message-ID: <CALCETrU_SE=7QH-rDOqOsB5T4v+VEGL_3ZYRPgt6QWHB5BEGdg@mail.gmail.com>
Date:	Fri, 25 Apr 2014 13:46:23 -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 1:25 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.

I suppose what what I'm doing is a bit strange.

--Andy
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