Re: Documenting the ioctl interfaces to discover relationships between namespaces

"Michael Kerrisk (man-pages)" [off-list ref] 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.

   [...]

Eric
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