Commit e5ff5ce6 authored Jan 25, 2017 by Michael Kerrisk (man-pages) Committed by Eric W. Biederman Jan 25, 2017

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>

parent 68eb94f1

fs/nsfs.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -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

+3 −0

Original line number	Diff line number	Diff line
		@@ -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 */