Re: Introspecting userns relationships to other namespaces?

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:

quoted

On Thu, Jul 07, 2016 at 08:26:47PM -0700, James Bottomley wrote:

quoted

On Thu, 2016-07-07 at 20:00 -0700, Andrew Vagin wrote:

quoted

On Thu, Jul 07, 2016 at 07:16:18PM -0700, Andrew Vagin wrote:

quoted

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: [off-list ref]

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