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

Andrew Vagin
2016-07-09T05:30:50Z

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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>