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SubjectRe: Documenting the ioctl interfaces to discover relationships between namespaces
"Michael Kerrisk (man-pages)" <> 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.
> Returns a file descriptor that refers to the owning user
> namespace for the namespace referred to by fd.
> 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.
> [...]


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