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).
Signed-off-by: Michael Kerrisk <mtk-manpages@gmail.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

fs/nsfs.c

@@ -172,6 +172,8 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
 		if (!ns->ops->get_parent)
 			return -EINVAL;
 		return open_related_ns(ns, ns->ops->get_parent);
+	case NS_GET_NSTYPE:
+		return ns->ops->type;
 	default:
 		return -ENOTTY;
 	}

include/uapi/linux/nsfs.h

@@ -9,5 +9,8 @@
 #define NS_GET_USERNS	_IO(NSIO, 0x1)
 /* Returns a file descriptor that refers to a parent namespace */
 #define NS_GET_PARENT	_IO(NSIO, 0x2)
+/* Returns the type of namespace (CLONE_NEW* value) referred to by
+   file descriptor */
+#define NS_GET_NSTYPE	_IO(NSIO, 0x3)
 
 #endif /* __LINUX_NSFS_H */