From e5ff5ce6e20ee22511398bb31fb912466cf82a36 Mon Sep 17 00:00:00 2001 From: "Michael Kerrisk (man-pages)" Date: Wed, 25 Jan 2017 14:03:36 +1300 Subject: [PATCH] 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 Signed-off-by: Eric W. Biederman --- fs/nsfs.c | 2 ++ include/uapi/linux/nsfs.h | 3 +++ 2 files changed, 5 insertions(+) diff --git a/fs/nsfs.c b/fs/nsfs.c index 8c9fb29c6673..5d534763c662 100644 --- a/fs/nsfs.c +++ b/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; } diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h index 3af617230d1b..2b48df11056a 100644 --- a/include/uapi/linux/nsfs.h +++ b/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 */ -- 2.20.1