vfs: more mnt_parent cleanups

a) mount --move is checking that ->mnt_parent is non-NULL before
looking if that parent happens to be shared; ->mnt_parent is never
NULL and it's not even an misspelled !mnt_has_parent()

b) pivot_root open-codes is_path_reachable(), poorly.

c) so does path_is_under(), while we are at it.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -2853,31 +2853,6 @@ int is_subdir(struct dentry *new_dentry, struct dentry *old_dentry)
 	return result;
 }
 
-int path_is_under(struct path *path1, struct path *path2)
-{
-	struct vfsmount *mnt = path1->mnt;
-	struct dentry *dentry = path1->dentry;
-	int res;
-
-	br_read_lock(vfsmount_lock);
-	if (mnt != path2->mnt) {
-		for (;;) {
-			if (!mnt_has_parent(mnt)) {
-				br_read_unlock(vfsmount_lock);
-				return 0;
-			}
-			if (mnt->mnt_parent == path2->mnt)
-				break;
-			mnt = mnt->mnt_parent;
-		}
-		dentry = mnt->mnt_mountpoint;
-	}
-	res = is_subdir(dentry, path2->dentry);
-	br_read_unlock(vfsmount_lock);
-	return res;
-}
-EXPORT_SYMBOL(path_is_under);
-
 void d_genocide(struct dentry *root)
 {
 	struct dentry *this_parent;

fs/namespace.c

@@ -1876,8 +1876,7 @@ static int do_move_mount(struct path *path, char *old_name)
 	/*
 	 * Don't move a mount residing in a shared parent.
 	 */
-	if (old_path.mnt->mnt_parent &&
-	    IS_MNT_SHARED(old_path.mnt->mnt_parent))
+	if (IS_MNT_SHARED(old_path.mnt->mnt_parent))
 		goto out1;
 	/*
 	 * Don't move a mount tree containing unbindable mounts to a destination
@@ -2533,6 +2532,31 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
 	return ret;
 }
 
+/*
+ * Return true if path is reachable from root
+ *
+ * namespace_sem or vfsmount_lock is held
+ */
+bool is_path_reachable(struct vfsmount *mnt, struct dentry *dentry,
+			 const struct path *root)
+{
+	while (mnt != root->mnt && mnt_has_parent(mnt)) {
+		dentry = mnt->mnt_mountpoint;
+		mnt = mnt->mnt_parent;
+	}
+	return mnt == root->mnt && is_subdir(dentry, root->dentry);
+}
+
+int path_is_under(struct path *path1, struct path *path2)
+{
+	int res;
+	br_read_lock(vfsmount_lock);
+	res = is_path_reachable(path1->mnt, path1->dentry, path2);
+	br_read_unlock(vfsmount_lock);
+	return res;
+}
+EXPORT_SYMBOL(path_is_under);
+
 /*
  * pivot_root Semantics:
  * Moves the root file system of the current process to the directory put_old,
@@ -2561,7 +2585,6 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
 SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
 		const char __user *, put_old)
 {
-	struct vfsmount *tmp;
 	struct path new, old, parent_path, root_parent, root;
 	int error;
 
@@ -2611,18 +2634,7 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
 	if (!mnt_has_parent(new.mnt))
 		goto out4; /* not attached */
 	/* make sure we can reach put_old from new_root */
-	tmp = old.mnt;
-	if (tmp != new.mnt) {
-		for (;;) {
-			if (!mnt_has_parent(tmp))
-				goto out4; /* already mounted on put_old */
-			if (tmp->mnt_parent == new.mnt)
-				break;
-			tmp = tmp->mnt_parent;
-		}
-		if (!is_subdir(tmp->mnt_mountpoint, new.dentry))
-			goto out4;
-	} else if (!is_subdir(old.dentry, new.dentry))
+	if (!is_path_reachable(old.mnt, old.dentry, &new))
 		goto out4;
 	br_write_lock(vfsmount_lock);
 	detach_mnt(new.mnt, &parent_path);

fs/pnode.c

@@ -28,21 +28,6 @@ static inline struct vfsmount *next_slave(struct vfsmount *p)
 	return list_entry(p->mnt_slave.next, struct vfsmount, mnt_slave);
 }
 
-/*
- * Return true if path is reachable from root
- *
- * namespace_sem is held, and mnt is attached
- */
-static bool is_path_reachable(struct vfsmount *mnt, struct dentry *dentry,
-			 const struct path *root)
-{
-	while (mnt != root->mnt && mnt_has_parent(mnt)) {
-		dentry = mnt->mnt_mountpoint;
-		mnt = mnt->mnt_parent;
-	}
-	return mnt == root->mnt && is_subdir(dentry, root->dentry);
-}
-
 static struct vfsmount *get_peer_under_root(struct vfsmount *mnt,
 					    struct mnt_namespace *ns,
 					    const struct path *root)

fs/pnode.h

@@ -42,4 +42,6 @@ void mnt_set_mountpoint(struct vfsmount *, struct dentry *,
 void release_mounts(struct list_head *);
 void umount_tree(struct vfsmount *, int, struct list_head *);
 struct vfsmount *copy_tree(struct vfsmount *, struct dentry *, int);
+bool is_path_reachable(struct vfsmount *, struct dentry *,
+			 const struct path *root);
 #endif /* _LINUX_PNODE_H */