[PATCH] Btrfs: detect corruption when non-root leaf has zero item

[Liu Bo <bo.li.liu@oracle.com>]

Right now we treat leaf which has zero item as a valid one
because we could have an empty tree, that is, a root that is
also a leaf without any item, however, in the same case but
when the leaf is not a root, we can end up with hitting the
BUG_ON(1) in btrfs_extend_item() called by
setup_inline_extent_backref().

This makes us check the situation as a corruption if leaf is
not its own root.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
---
 fs/btrfs/disk-io.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index a5a22be..dfaeb96 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -559,8 +559,28 @@ static noinline int check_leaf(struct btrfs_root *root,
 	u32 nritems = btrfs_header_nritems(leaf);
 	int slot;
 
-	if (nritems == 0)
+	if (nritems == 0) {
+		struct btrfs_root *r;
+
+		key.objectid = btrfs_header_owner(leaf);
+		key.type = BTRFS_ROOT_ITEM_KEY;
+		key.offset = -1ULL;
+
+		r = btrfs_get_fs_root(root->fs_info, &key, false);
+		/*
+		 * The only reason we also check NULL here is that during
+		 * open_ctree() some roots has not yet been set up.
+		 */
+		if (!IS_ERR_OR_NULL(r)) {
+			/* if leaf is the root, then it's fine */
+			if (leaf->start != btrfs_root_bytenr(&r->root_item)) {
+				CORRUPT("non-root leaf's nritems is 0",
+					leaf, root, 0);
+				return -EIO;
+			}
+		}
 		return 0;
+	}
 
 	/* Check the 0 item */
 	if (btrfs_item_offset_nr(leaf, 0) + btrfs_item_size_nr(leaf, 0) !=
-- 
2.5.5