Documenting the ioctl interfaces to discover relationships between namespaces

Michael Kerrisk (man-pages) mtk.manpages at
Mon Dec 12 16:01:14 UTC 2016

On 12/11/2016 11:30 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages at> writes:
>> [was: [PATCH 0/4 v3] Add an interface to discover relationships
>> between namespaces]
> One small comment below.
>>    Introspecting namespace relationships
>>        Since Linux 4.9, two ioctl(2) operations  are  provided  to  allow
>>        introspection  of  namespace relationships (see user_namespaces(7)
>>        and pid_namespaces(7)).  The form of the calls is:
>>            ioctl(fd, request);
>>        In each case, fd refers to a /proc/[pid]/ns/* file.
>>        NS_GET_USERNS
>>               Returns a file descriptor that refers to  the  owning  user
>>               namespace for the namespace referred to by fd.
>>        NS_GET_PARENT
>>               Returns  a file descriptor that refers to the parent names‐
>>               pace of the namespace referred to by fd.  This operation is
>>               valid  only for hierarchical namespaces (i.e., PID and user
>>               namespaces).  For user namespaces, NS_GET_PARENT is synony‐
>>               mous with NS_GET_USERNS.
>>        In each case, the returned file descriptor is opened with O_RDONLY
>>        and O_CLOEXEC (close-on-exec).
>>        By applying fstat(2) to the returned file descriptor, one  obtains
>>        a  stat structure whose st_ino (inode number) field identifies the
>>        owning/parent namespace.  This inode number can  be  matched  with
>>        the  inode  number  of  another  /proc/[pid]/ns/{pid,user} file to
>>        determine whether that is the owning/parent namespace.
> Like all fstat inode comparisons to be fully accurate you need to
> compare both the st_ino and st_dev.  I reserve the right for st_dev to
> be significant when comparing namespaces.  Otherwise I might have to
> create a namespace of namespaces someday and that is ugly.
>>        Either of these ioctl(2) operations can fail  with  the  following
>>        error:
>>        EPERM  The  requested  namespace is outside of the caller's names‐
>>               pace scope.  This error can occur if, for example, the own‐
>>               ing  user  namespace is an ancestor of the caller's current
>>               user namespace.  It can also occur on  attempts  to  obtain
>>               the parent of the initial user or PID namespace.
>>        Additionally,  the  NS_GET_PARENT operation can fail with the fol‐
>>        lowing error:
>>        EINVAL fd refers to a nonhierarchical namespace.
>>        See the EXAMPLE section for an example of the use of these  opera‐
>>        tions.

So, after playing with this a bit, I have a question. 

I gather that in order to, for example, elaborate the tree of user
namespaces on the system, one would use NS_GET_PARENT on each of
the /proc/*/ns/user files and match up the results. Right?
What happens if one of the parent user namespaces contains no
processes? That is, the parent namespace exists by virtue of being
pinned because a proc/PID/ns/user file is open or bind mounted.
(Chrome seems to do this sort of dance with user namespaces, for
example.) How do we find the ancestor of *that* user namespace?



Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:

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