Re: [CRIU] Introspecting userns relationships to other namespaces?

[James Bottomley <James.Bottomley@HansenPartnership.com>]

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:
> > On Thu, Jul 07, 2016 at 12:17:35PM -0700, James Bottomley wrote:
> > > On Thu, 2016-07-07 at 20:21 +0200, Michael Kerrisk (man-pages)
> > > wrote:
> > > > On 7 July 2016 at 17:01, James Bottomley
> > > > <James.Bottomley@hansenpartnership.com> wrote:
> > > [Serge already answered the parenting issue]
> > > > > On Thu, 2016-07-07 at 08:36 -0500, Serge E. Hallyn wrote:
> > > > > > 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.
> > > > > 
> > > > > This is a bit pathological, and you have to be root to do it:
> > > > > so 
> > > > > root can set up a nesting hierarchy, bind it and destroy the
> > > > > pids 
> > > > > but I know of no current orchestration system which does
> > > > > this.
> > > > > 
> > > > > Actually, I have to back pedal a bit: the way I currently set
> > > > > up
> > > > > architecture emulation containers does precisely this: I set
> > > > > up the
> > > > > namespaces unprivileged with child mount namespaces, but then
> > > > > I ask
> > > > > root to bind the userns and kill the process that created it
> > > > > so I 
> > > > > have a permanent handle to enter the namespace by, so I
> > > > > suspect 
> > > > > that when our current orchestration systems get more
> > > > > sophisticated, 
> > > > > they might eventually want to do something like this as well.
> > > > > 
> > > > > In theory, we could get nsfs to show this information as an
> > > > > option
> > > > > (just add a show_options entry to the superblock ops), but
> > > > > the 
> > > > > problem is that although each namespace has a parent user_ns,
> > > > > there's no way to get it without digging in the namespace
> > > > > specific 
> > > > > structure.  Probably we should restructure to move it into 
> > > > > ns_common, then we could display it (and enforce all
> > > > > namespaces 
> > > > > having owning user_ns) but it would be a
> > > > 
> > > > I'm missing something here. Is it not already the case that all
> > > > namespaces have an owning user_ns?
> > > 
> > > Um, yes, I don't believe I said they don't.  The problem I
> > > thought you
> > > were having is that there's no way of seeing what it is.
> > > 
> > > nsfs is the Namespace fileystem where bound namespaces appear to
> > > a cat
> > > of /proc/self/mounts.  It can display any information that's in
> > > ns_common (the common core of namespaces) but the owning user_ns
> > > pointer currently isn't in this structure.  Every user namespace
> > > has a
> > > pointer to it, but they're all privately embedded in the
> > > individual
> > > namespace specific structures.  What I was proposing was that
> > > since
> > > every current namespace has a pointer somewhere to the owning
> > > user
> > > namespace, we could abstract this out into ns_common so it's now
> > > accessible to be displayed by nsfs, probably as a mount option.
> > 
> > James, I am not sure that I understood you correctly. We have one
> > file system for all namespace files, how we can show per-file
> > properties
> > in mount options. 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.
> 
> How it works:
> 
> In [1]: import os
> 
> 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/'

?

But what Michael was asking about was the parent user_ns of all the
other namespaces ... I don't think there's any way we can get that out
of any information in /proc/self/

James