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SubjectRe: [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces

On 14 July 2016 at 20:20, Andrey Vagin <> wrote:
> Each namespace has an owning user namespace and now there is not way
> to discover these relationships.
> Pid and user namepaces are hierarchical. There is no way to discover
> parent-child relationships too.
> Why we may want to know relationships between namespaces?
> One use would be visualization, in order to understand the running system.

This looks interesting to me because I am interested in representing
in a graphical way the relationship between different mounts in
different mount namespaces (showing the ID, the parent-children
relationships, mount peer groups, the master-slave relationships etc),
specially for containers. The first idea was to take both
/proc/1/mountinfo and /proc/$OTHER_PID/mountinfo and I can correlate
the "shared:" and "master:" fields in the mountinfo files.

But I cannot read the /proc/$pid/mountinfo of mount namespaces when
there are no processes in those mount namespaces. For example, if
those mount namespaces stay alive only because they contain
"shared&slave" mounts between master mounts and slave mounts that I
can see in /proc/$pid/mountinfo. Fictional example:

# mntns 1, mountinfo 1 (visible via /proc/1/mountinfo)
61 0 253:1 / / rw shared:1

# mntns 2, mountinfo 2 (not visible via any /proc/$pid/mountinfo)
731 569 0:75 / / rw master:1 shared:42

# mntns 3, mountinfo 3 (not visible via any /proc/${container_pid}/mountinfo)
762 597 0:82 / / rw master:42 shared:76

As far as I understand, I cannot get a reference to the mntns2 fd
because mnt namespaces are not hierarchical, and I cannot get its
/proc/???/mountinfo because no processes live inside.

Is there a way around it? Should this use case be handled together?


> Another would be to answer the question: what capability does process X have to
> perform operations on a resource governed by namespace Y?
> One more use-case (which usually called abnormal) is checkpoint/restart.
> In CRIU we age going to dump and restore nested namespaces.
> There [1] was a discussion about which interface to choose to determing
> relationships between namespaces.
> Eric suggested to add two ioctl-s [2]:
>> Grumble, Grumble. I think this may actually a case for creating ioctls
>> for these two cases. Now that random nsfs file descriptors are bind
>> mountable the original reason for using proc files is not as pressing.
>> One ioctl for the user namespace that owns a file descriptor.
>> One ioctl for the parent namespace of a namespace file descriptor.
> Here is an implementaions of these ioctl-s.
> [1]
> [2]
> Cc: "Eric W. Biederman" <>
> Cc: James Bottomley <>
> Cc: "Michael Kerrisk (man-pages)" <>
> Cc: "W. Trevor King" <>
> Cc: Alexander Viro <>
> Cc: Serge Hallyn <>
> --
> 2.5.5
> _______________________________________________
> Containers mailing list

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