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

Andrei Vagin
2016-07-08T05:41:53Z

On Thu, Jul 7, 2016 at 8:26 PM, James Bottomley
<James.Bottomley@hansenpartnership.com> 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@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/'

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

In [5]: os.readlink("/proc/self/ns/user")
Out[5]: 'user:[4026531837]'

>
> ?
>
> 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
>
>
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