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

[Andrew Vagin <avagin@virtuozzo.com>]

On Fri, Jul 08, 2016 at 07:35:33AM -0700, James Bottomley wrote:
> On Fri, 2016-07-08 at 02:44 -0500, Eric W. Biederman wrote:
> > Andrew Vagin <avagin@virtuozzo.com> writes:
> > 
> > > On Wed, Jul 06, 2016 at 10:46:33AM -0500, Eric W. Biederman wrote:
> > > > "Serge E. Hallyn" <serge@hallyn.com> writes:
> > > > 
> > > > > On Wed, Jul 06, 2016 at 10:41:48AM +0200, Michael Kerrisk (man
> > > > > -pages) wrote:
> > > > > > [Rats! Doing now what I should have down to start with.
> > > > > > Looping some
> > > > > > lists and CRIU and other possibly relevant people into this
> > > > > > conversation]
> > > > > > 
> > > > > > Hi Eric,
> > > > > > 
> > > > > > On 5 July 2016 at 23:47, Eric W. Biederman <
> > > > > > ebiederm@xmission.com> wrote:
> > > > > > > "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
> > > > > > > writes:
> > > > > > > 
> > > > > > > > Hi Eric,
> > > > > > > > 
> > > > > > > > I have a question. Is there any way currently to discover 
> > > > > > > > which user namespace a particular nonuser namespace is 
> > > > > > > > governed by? Maybe I am missing something, but there does 
> > > > > > > > not seem to be a way to do this. Also, can one discover 
> > > > > > > > which userns is the parent of a given userns? Again, I 
> > > > > > > > can't see a way to do this.
> > > > > > > > 
> > > > > > > > The point here is introspecting so that a process might 
> > > > > > > > determine what its capabilities are when operating on 
> > > > > > > > some resource governed by a (nonuser) namespace.
> > > > > > > 
> > > > > > > To the best of my knowledge that there is not an interface 
> > > > > > > to get that information.  It would be good to have such an 
> > > > > > > interface for no other reason than the CRIU folks are going 
> > > > > > > to need it at some point.  I am a bit surprised they have
> > > > > > > not complained yet.
> > > > > 
> > > > > I don't think they need it.  They do in fact have what they 
> > > > > need.  Assume you have tasks T1, T2, T1_1 and T2_1;  T1 and T2 
> > > > > are in init_user_ns;  T1 spawned T1_1 in a new userns;  T2 
> > > > > spawned T2_1 which setns()d to T1_1's ns. There's some
> > > > > {handwave} uid mapping, does not matter.
> > > > > 
> > > > > At restart, it doesn't matter which task originally created the 
> > > > > new userns. criu knows T1_1 and T2_1 are in the same userns; 
> > > > >  it creates the userns, sets up the mapping, and T1_1 and T2_1
> > > > > setns() to it.
> > > > 
> > > > Given that the simple cases are so easy it probably doesn't 
> > > > matter in that sense.
> > > > 
> > > > However we now have the case where user namespaces own pid 
> > > > namespaces, and uts namespaces, and network namespaces, and ipc 
> > > > namespaces, and filesystems.  Throw in some mount propagation and 
> > > > use of setns and things could get confusing.   It is something 
> > > > that will need to be figured out if CRIU is going to properly 
> > > > checkpoint containers containing containers containing containers 
> > > > containing containers.
> > > 
> > > It isn't a joke:). We have a few requests to support CR of 
> > > containers with Docker containers inside. And we are going to start 
> > > this task in a near future, so we would like to have interface to 
> > > get dependencies between namespaces too.
> > > 
> > > BTW: CRIU already supports nested mount namespaces, because systemd
> > > creates them for services.
> > 
> > The tricky part about this and what messes up James proposed plan is
> > that the interface needs to be something that returns a namespace 
> > file descriptor.  So we can't print something out in a simple text
> > file.
> 
> I actually described two problems: the first was how we get the
> information in the first place.  Currently the owning or parent user_ns
> is tucked inside an opaque structure.  I think we need to move that to
> ns_common where it would be the owning userns for all non-user
> namespaces and the parent for the userns.

I'm agree with this.

> 
> Once we actually have the information, we can also add a set of proc
> links, say either
> 
> /proc/<pid>/ns/X-userns
> 
> Which might be a bit messy since it doubles the number of files, or
> perhaps in a simple directory.

In this case we will need to enter into each namespace to build a full
chain of dependencies.

It's tricky, because if we enter into a child userns, we can't to enter
into a parent userns from the same process, so to get the next branch,
we will need to create a new process.

				    process A
					|
init_user_ns->child_user_ns_1->child_userns_2

fork() -> B
  B: setns(/proc/A/ns/userns-parent)
readlink(/proc/B/ns/userns)

fork() -> C
  C: setns(/proc/B/ns/userns-parent)
readlink(/proc/C/ns/userns)


> 
> > Well I suppose we could print an device number and inode number pair.
> > But then someone would still have to scour processes looking for a 
> > user namespace so that is likely less than ideal.
> 
> There's no reason any of the proposed methods so far have to be
> exclusive: nsfs.c has a lot of flexibility.


What do you think about the idea to mount nsfs and be able to look up
any alive namespace by inum:

  $ tree .
  .
  ├── mnt{inum}
  │   └── user -> ../user{inum}
  ├── pid{inum}
  │   ├── pid{inum}
  │   │   └── user -> ../../user{inum}/user{inum}
  │   └── user -> ../user{inum}
  └── user{inum}
      └── user{inum}

https://lkml.org/lkml/2016/7/8/59

I think it solves all requirements which were mentioned in this thread.

> 
> > Starting with 4.8 we are also going to need to be able to retrieve 
> > the user namespace owner of filesystems.  That will be an interesting
> > mix.
> 
> This is per mount point, isn't it? so it can't be in /proc/fs/ and it
> would have to be per local mount tree.  Yes, that is a bit nasty. 
>  Sounds like we might need to unfold mount or mountinfo into something
> that has one directory per entry?

If we will be able to look up namespaces in nsfs by inum, we can print
an userns inum in mountinfo.

> 
> James
> 
> > Eric
> > 
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