nsfs: Add an ioctl() to return the namespace type

Linux 4.9 added two ioctl() operations that can be used to discover: * the parental relationships for hierarchical namespaces (user and PID) [NS_GET_PARENT] * the user namespaces that owns a specified non-user-namespace [NS_GET_USERNS] For no good reason that I can glean, NS_GET_USERNS was made synonymous with NS_GET_PARENT for user namespaces. It might have been better if NS_GET_USERNS had returned an error if the supplied file descriptor referred to a user namespace, since it suggests that the caller may be confused. More particularly, if it had generated an error, then I wouldn't need the new ioctl() operation proposed here. (On the other hand, what I propose here may be more generally useful.) I would like to write code that discovers namespace relationships for the purpose of understanding the namespace setup on a running system. In particular, given a file descriptor (or pathname) for a namespace, N, I'd like to obtain the corresponding user namespace. Namespace N might be a user namespace (in which case my code would just use N) or a non-user namespace (in which case my code will use NS_GET_USERNS to get the user namespace associated with N). The problem is that there is no way to tell the difference by looking at the file descriptor (and if I try to use NS_GET_USERNS on an N that is a user namespace, I get the parent user namespace of N, which is not what I want). This patch therefore adds a new ioctl(), NS_GET_NSTYPE, which, given a file descriptor that refers to a user namespace, returns the namespace type (one of the CLONE_NEW* constants).

MKMichael Kerrisk (man-pages) committed 9 years ago