[CRIU] Introspecting userns relationships to other namespaces?

[Michael Kerrisk (man-pages)]

On 07/08/2016 05:26 AM, 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:
>>> 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 at 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 ...

Just to reiterate, what I'm interested in is the introspection use
case (but there's clearly several other interesting use cases here).
The idea is to be able to answer these questions

1. For each userns, what is the parent of that userns?

2. For each non-user namespace, what is the owning userns?

This enables us to understand the userns hierarchy, which
matters in terms of answering the question: what capabilities
does process X have in namespace Y?
    
Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/