nsfs: add missing id retrieval support

The mount namespace has supported id retrieval for a while already.
Add support for the other types as well.

Signed-off-by: Christian Brauner <[e-mail]>

fs/nsfs.c

···
 	case NS_GET_TGID_FROM_PIDNS:
 	case NS_GET_PID_IN_PIDNS:
 	case NS_GET_TGID_IN_PIDNS:
+	case NS_GET_ID:
 		return true;
 	}
 
···
 		argp = (uid_t __user *) arg;
 		uid = from_kuid_munged(current_user_ns(), user_ns->owner);
 		return put_user(uid, argp);
-	case NS_GET_MNTNS_ID: {
-		__u64 __user *idp;
-		__u64 id;
-
-		if (ns->ops->type != CLONE_NEWNS)
-			return -EINVAL;
-
-		mnt_ns = container_of(ns, struct mnt_namespace, ns);
-		idp = (__u64 __user *)arg;
-		id = mnt_ns->ns.ns_id;
-		return put_user(id, idp);
-	}
 	case NS_GET_PID_FROM_PIDNS:
 		fallthrough;
 	case NS_GET_TGID_FROM_PIDNS:
···
 		if (!ret)
 			ret = -ESRCH;
 		return ret;
+	}
+	case NS_GET_MNTNS_ID:
+		if (ns->ops->type != CLONE_NEWNS)
+			return -EINVAL;
+		fallthrough;
+	case NS_GET_ID: {
+		__u64 __user *idp;
+		__u64 id;
+
+		idp = (__u64 __user *)arg;
+		id = ns->ns_id;
+		return put_user(id, idp);
 	}
 	}
 

include/uapi/linux/nsfs.h

···
 #define NS_GET_NSTYPE		_IO(NSIO, 0x3)
 /* Get owner UID (in the caller's user namespace) for a user namespace */
 #define NS_GET_OWNER_UID	_IO(NSIO, 0x4)
-/* Get the id for a mount namespace */
-#define NS_GET_MNTNS_ID		_IOR(NSIO, 0x5, __u64)
 /* Translate pid from target pid namespace into the caller's pid namespace. */
 #define NS_GET_PID_FROM_PIDNS	_IOR(NSIO, 0x6, int)
 /* Return thread-group leader id of pid in the callers pid namespace. */
···
 #define NS_MNT_GET_NEXT		_IOR(NSIO, 11, struct mnt_ns_info)
 /* Get previous namespace. */
 #define NS_MNT_GET_PREV		_IOR(NSIO, 12, struct mnt_ns_info)
+
+/* Retrieve namespace identifiers. */
+#define NS_GET_MNTNS_ID		_IOR(NSIO, 5,  __u64)
+#define NS_GET_ID		_IOR(NSIO, 13, __u64)
 
 enum init_ns_ino {
 	IPC_NS_INIT_INO		= 0xEFFFFFFFU,