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

[James Bottomley <James.Bottomley@Hansenpartnership.com>]

On July 8, 2016 1:38:19 PM PDT, Andrew Vagin <avagin@virtuozzo.com> wrote:
>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:

I think I like it.  It will give us a way to enter any extant namespace.  It will work for Eric's fs namespaces as well.  Perhaps a /process/ns/<inum>
 Directory?

James

>  $ 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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>> > https://lists.linuxfoundation.org/mailman/listinfo/containers
>> > 
>> 
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