Documenting the ioctl interfaces to discover relationships between namespaces

Michael Kerrisk (man-pages) mtk.manpages at
Mon Dec 12 06:13:18 UTC 2016

[Fixing Serge's address in my original CC]

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.

Ah yes. Thanks for catching that. I've adjusted the text,
and the example program.



>>        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.
>>    [...]
> Eric

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

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