mount: add helper mntns_get_root_by_mnt_id

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files-reg.c

@@ -676,7 +676,6 @@ int open_path(struct file_desc *d,
 		int(*open_cb)(int mntns_root, struct reg_file_info *, void *), void *arg)
 {
 	struct reg_file_info *rfi;
-	struct ns_id *ns;
 	int tmp, mntns_root;
 	char *orig_path = NULL;
 
@@ -722,12 +721,7 @@ int open_path(struct file_desc *d,
 		}
 	}
 
-	ns = lookup_nsid_by_mnt_id(rfi->rfe->mnt_id);
-	if (ns == NULL)
-		return -1;
-
-	mntns_root = mntns_get_root_fd(ns);
-
+	mntns_root = mntns_get_root_by_mnt_id(rfi->rfe->mnt_id);
 	tmp = open_cb(mntns_root, rfi, arg);
 	if (tmp < 0) {
 		pr_perror("Can't open file %s", rfi->path);

include/mount.h

@@ -6,6 +6,7 @@ extern struct mount_info *mntinfo;
 struct ns_id;
 extern int __mntns_get_root_fd(pid_t pid);
 extern int mntns_get_root_fd(struct ns_id *ns);
+extern int mntns_get_root_by_mnt_id(int mnt_id);
 extern struct ns_id *lookup_nsid_by_mnt_id(int mnt_id);
 
 struct proc_mountinfo;

mount.c

@@ -2062,6 +2062,16 @@ struct ns_id *lookup_nsid_by_mnt_id(int mnt_id)
 	return mi->nsid;
 }
 
+int mntns_get_root_by_mnt_id(int mnt_id)
+{
+	struct ns_id *mntns;
+
+	mntns = lookup_nsid_by_mnt_id(mnt_id);
+	BUG_ON(mntns == NULL);
+
+	return mntns_get_root_fd(mntns);
+}
+
 static int walk_mnt_ns(int (*cb)(struct ns_id *, struct mount_info *, void *), void *arg)
 {
 	struct mount_info *pms;