Introspecting userns relationships to other namespaces?

[W. Trevor King]

On Thu, Jul 07, 2016 at 11:54:54PM -0700, Andrew Vagin wrote:
> On Thu, Jul 07, 2016 at 10:26:50PM -0700, W. Trevor King wrote:
> > On Thu, Jul 07, 2016 at 08:26:47PM -0700, James Bottomley wrote:
> > > On Thu, 2016-07-07 at 20:00 -0700, Andrew Vagin wrote:
> > > > On Thu, Jul 07, 2016 at 07:16:18PM -0700, Andrew Vagin wrote:
> > > > > I think we can show all required information in fdinfo. We open
> > > > > a namespaces file (/proc/pid/ns/N) and then read
> > > > > /proc/pid/fdinfo/X for it.
> > > > 
> > > > Here is a proof-of-concept patch.
> > > > …
> > > > In [2]: fd = os.open("/proc/self/ns/pid", os.O_RDONLY)
> > > > 
> > > > In [3]: print open("/proc/self/fdinfo/%d" % fd).read()
> > > > pos:	0
> > > > flags:	0100000
> > > > mnt_id:	2
> > > > userns: 4026531837
> > > > 
> > > > In [4]: print "/proc/self/ns/user -> %s" %
> > > > os.readlink("/proc/self/ns/user")
> > > > /proc/self/ns/user -> user:[4026531837]
> > > 
> > > can't you just do
> > > 
> > > readlink /proc/self/ns/user | sed 's/.*\[\(.*\)\]/\1/'
> > …
> > If you only put one level in fdinfo, you're stuck if one of the
> > namespaces involved has neither bind mounts nor a PID to give you
> > handle on it [1].  And if you want to put that whole ancestor tree in
> > fdinfo, you have to come up with some way to handle the two-parent
> > branching.
> 
> I think it's a bad idea to draw a tree in fdinfo. Why do we want to know
> this hierarchy? Probably we will want to access these namespaces (setns),
> in this case we need to have a way to open them.
> 
> Maybe we need to extend functionality of the nsfs filesystem
> (somethink like /proc/PID for namespaces)?

A similar idea came up during the PID-translation brainstorming [1],
but I'm not sure if anything ever came of that.  Once you're dealing
with a separate pseudo-filesystem, it seems easier to decouple it from
proc and just make a mountable namespace-hierarchy filesystem (like we
have mountable cgroup hierarchy filesystems).  That also gets you an
opt-in playground while the details of the nsfs filesystem view are
worked out.  Are you imagining something like:

  $ tree .
  .
  ├── mnt{inum}
  │   └── user -> ../user{inum}
  ├── pid{inum}
  │   ├── pid{inum}
  │   │   └── user -> ../../user{inum}/user{inum}
  │   └── user -> ../user{inum}
  └── user{inum}
      └── user{inum}

Cheers,
Trevor

[1]: http://thread.gmane.org/gmane.linux.kernel.containers/28105/focus=28164
     Subject: RE: [RFC]Pid conversion between pid namespace
     Date: Fri, 25 Jul 2014 10:01:45 +0000
     Message-ID: <5871495633F38949900D2BF2DC04883E56C7A2 at G08CNEXMBPEKD02.g08.fujitsu.local>

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