Btrfs: prevent loops in the directory tree when creating snapshots

For a directory tree:

/mnt/subvolA/subvolB

btrfsctl -s /mnt/subvolA/subvolB /mnt

Will create a directory loop with subvolA under subvolB.  This
commit uses the forward refs for each subvol and snapshot to error out
before creating the loop.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.h

@@ -1801,6 +1801,9 @@ int btrfs_drop_subtree(struct btrfs_trans_handle *trans,
 			struct extent_buffer *node,
 			struct extent_buffer *parent);
 /* root-item.c */
+int btrfs_find_root_ref(struct btrfs_root *tree_root,
+		   struct btrfs_path *path,
+		   u64 root_id, u64 ref_id);
 int btrfs_add_root_ref(struct btrfs_trans_handle *trans,
 		       struct btrfs_root *tree_root,
 		       u64 root_id, u8 type, u64 ref_id,

fs/btrfs/disk-io.c

@@ -1129,7 +1129,7 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
 		kfree(root);
 		return ERR_PTR(ret);
 	}
-
+#if 0
 	ret = btrfs_sysfs_add_root(root);
 	if (ret) {
 		free_extent_buffer(root->node);
@@ -1137,6 +1137,7 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
 		kfree(root);
 		return ERR_PTR(ret);
 	}
+#endif
 	root->in_sysfs = 1;
 	return root;
 }
@@ -1963,8 +1964,10 @@ int btrfs_free_fs_root(struct btrfs_fs_info *fs_info, struct btrfs_root *root)
 		down_write(&root->anon_super.s_umount);
 		kill_anon_super(&root->anon_super);
 	}
+#if 0
 	if (root->in_sysfs)
 		btrfs_sysfs_del_root(root);
+#endif
 	if (root->node)
 		free_extent_buffer(root->node);
 	if (root->commit_root)

fs/btrfs/ioctl.c

@@ -284,6 +284,56 @@ static noinline int btrfs_mksubvol(struct path *parent, char *name,
 	 * subvolume with specific mode bits.
 	 */
 	if (snap_src) {
+		struct dentry *dir = dentry->d_parent;
+		struct dentry *test = dir->d_parent;
+		struct btrfs_path *path = btrfs_alloc_path();
+		int ret;
+		u64 test_oid;
+		u64 parent_oid = BTRFS_I(dir->d_inode)->root->root_key.objectid;
+
+		test_oid = snap_src->root_key.objectid;
+
+		ret = btrfs_find_root_ref(snap_src->fs_info->tree_root,
+					  path, parent_oid, test_oid);
+		if (ret == 0)
+			goto create;
+		btrfs_release_path(snap_src->fs_info->tree_root, path);
+
+		/* we need to make sure we aren't creating a directory loop
+		 * by taking a snapshot of something that has our current
+		 * subvol in its directory tree.  So, this loops through
+		 * the dentries and checks the forward refs for each subvolume
+		 * to see if is references the subvolume where we are
+		 * placing this new snapshot.
+		 */
+		while(1) {
+			if (!test ||
+			    dir == snap_src->fs_info->sb->s_root ||
+			    test == snap_src->fs_info->sb->s_root ||
+			    test->d_inode->i_sb != snap_src->fs_info->sb) {
+				break;
+			}
+			if (S_ISLNK(test->d_inode->i_mode)) {
+				printk("Symlink in snapshot path, failed\n");
+				error = -EMLINK;
+				btrfs_free_path(path);
+				goto out_drop_write;
+			}
+			test_oid =
+				BTRFS_I(test->d_inode)->root->root_key.objectid;
+			ret = btrfs_find_root_ref(snap_src->fs_info->tree_root,
+				  path, test_oid, parent_oid);
+			if (ret == 0) {
+				printk("Snapshot creation failed, looping\n");
+				error = -EMLINK;
+				btrfs_free_path(path);
+				goto out_drop_write;
+			}
+			btrfs_release_path(snap_src->fs_info->tree_root, path);
+			test = test->d_parent;
+		}
+create:
+		btrfs_free_path(path);
 		error = create_snapshot(snap_src, dentry, name, namelen);
 	} else {
 		error = create_subvol(BTRFS_I(parent->dentry->d_inode)->root,

fs/btrfs/root-tree.c

@@ -300,6 +300,22 @@ int btrfs_del_root_ref(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
+int btrfs_find_root_ref(struct btrfs_root *tree_root,
+		   struct btrfs_path *path,
+		   u64 root_id, u64 ref_id)
+{
+	struct btrfs_key key;
+	int ret;
+
+	key.objectid = root_id;
+	key.type = BTRFS_ROOT_REF_KEY;
+	key.offset = ref_id;
+
+	ret = btrfs_search_slot(NULL, tree_root, &key, path, 0, 0);
+	return ret;
+}
+
+
 /*
  * add a btrfs_root_ref item.  type is either BTRFS_ROOT_REF_KEY
  * or BTRFS_ROOT_BACKREF_KEY.