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>

1 files changed, 16 insertions, 0 deletions

diff --git a/fs/btrfs/root-tree.c b/fs/btrfs/root-tree.c
index e9be3abcb368..dbe20d4c6ea4 100644
--- a/fs/btrfs/root-tree.c
+++ b/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.