linux-kernel - Re: Introspecting userns relationships to other namespaces?

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Date:	Thu, 7 Jul 2016 08:36:31 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, criu@...nvz.org,
	Linux API <linux-api@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Containers <containers@...ts.linux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Tycho Andersen <tycho.andersen@...onical.com>
Subject: Re: Introspecting userns relationships to other namespaces?

Quoting Michael Kerrisk (man-pages) (mtk.manpages@...il.com):
> Hi Serge,
> 
> On 6 July 2016 at 16:13, Serge E. Hallyn <serge@...lyn.com> wrote:
> > On Wed, Jul 06, 2016 at 10:41:48AM +0200, Michael Kerrisk (man-pages) wrote:
> >> [Rats! Doing now what I should have down to start with. Looping some
> >> lists and CRIU and other possibly relevant people into this
> >> conversation]
> >>
> >> Hi Eric,
> >>
> >> On 5 July 2016 at 23:47, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> >> > "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
> >> >
> >> >> Hi Eric,
> >> >>
> >> >> I have a question. Is there any way currently to discover which
> >> >> user namespace a particular nonuser namespace is governed by?
> >> >> Maybe I am missing something, but there does not seem to be a
> >> >> way to do this. Also, can one discover which userns is the
> >> >> parent of a given userns? Again, I can't see a way to do this.
> >> >>
> >> >> The point here is introspecting so that a process might determine
> >> >> what its capabilities are when operating on some resource governed
> >> >> by a (nonuser) namespace.
> >> >
> >> > To the best of my knowledge that there is not an interface to get that
> >> > information.  It would be good to have such an interface for no other
> >> > reason than the CRIU folks are going to need it at some point.  I am a
> >> > bit surprised they have not complained yet.
> >
> > I don't think they need it.  They do in fact have what they need.  Assume
> > you have tasks T1, T2, T1_1 and T2_1;  T1 and T2 are in init_user_ns;  T1
> > spawned T1_1 in a new userns;  T2 spawned T2_1 which setns()d to T1_1's ns.
> > There's some {handwave} uid mapping, does not matter.
> >
> > At restart, it doesn't matter which task originally created the new userns.
> > criu knows T1_1 and T2_1 are in the same userns;  it creates the userns, sets
> > up the mapping, and T1_1 and T2_1 setns() to it.
> 
> I'm missing something here. How does the parental relationships
> between the user namespaces get reconstructed? Those relationships
> will govern what capabilities a process will have in various user
> namespaces.

Hm.  Probably best-effort based on the process hierarchy.  So yeah you
could probably get a tree into a state that would be wrongly recreated.
Create a new netns, bind mount it, exit;  Have another task create a
new user_ns, bind mount it, exit;  Third task setns()s first to the new
netns then to the new user_ns.  I suspect criu will recreate that
wrongly.