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

[James Bottomley <James.Bottomley-d9PhHud1JfjCXq6kfMZ53/egYHeGw8Jk@public.gmane.org>]

On Thu, 2016-07-07 at 22:41 -0700, Andrei Vagin wrote:
> On Thu, Jul 7, 2016 at 8:26 PM, James Bottomley
> <James.Bottomley-JuX6DAaQMKPCXq6kfMZ53/egYHeGw8Jk@public.gmane.org> 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:
> > > > 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-JuX6DAaQMKPCXq6kfMZ53/egYHeGw8Jk@public.gmane.org> 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/'
> 
> We can get fdinfo for any ns file. I used /proc/self/ns/pid as an
> example.
> 
> Look at another example:
> 
> [root@fc22-vm ~]# cat /proc/self/mountinfo | grep pid_ns_file
> 115 38 0:3 pid:[4026532306] /tmp/pid_ns_file rw shared:67 - nsfs nsfs
> rw
> 
> In [4]: print open("/proc/self/fdinfo/5").read()
> pos: 0
> flags: 0100000
> mnt_id: 115
> userns: 4026532305

OK, I'm missing where this is coming from specifically.  There would
have to be a show_fdinfo() somewhere that did this and I'm not finding
it in linux-next.

James