Introspecting userns relationships to other namespaces?

[Michael Kerrisk (man-pages)]

On 07/07/2016 09:17 PM, 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.

Your words "and enforce all namespaces having owning user_ns" were
what left me puzzled--it sounded to me that the implication was
that this is not "enforced" right now.

Cheers,

Michael

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