btrfs-progs: ctree: Add extra level check for read_node_slot()

Strangely, we have level check in btrfs_print_tree() while we don't have
the same check in read_node_slot().

That's to say, for the following corruption, btrfs_search_slot() or
btrfs_next_leaf() can return invalid leaf:

Parent eb:
  node XXXXXX level 1
              ^^^^^^^
              Child should be leaf (level 0)
  ...
  key (XXX XXX XXX) block YYYYYY

Child eb:
  leaf YYYYYY level 1
              ^^^^^^^
              Something went wrong now

And for the corrupted leaf returned, later caller can be screwed up
easily.

Reported-by: Ralph Gauges <ralphgauges@googlemail.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

ctree.c

@@ -22,6 +22,7 @@
 #include "repair.h"
 #include "internal.h"
 #include "sizes.h"
+#include "messages.h"
 
 static int split_node(struct btrfs_trans_handle *trans, struct btrfs_root
 		      *root, struct btrfs_path *path, int level);
@@ -642,7 +643,9 @@ static int bin_search(struct extent_buffer *eb, struct btrfs_key *key,
 struct extent_buffer *read_node_slot(struct btrfs_fs_info *fs_info,
 				   struct extent_buffer *parent, int slot)
 {
+	struct extent_buffer *ret;
 	int level = btrfs_header_level(parent);
+
 	if (slot < 0)
 		return NULL;
 	if (slot >= btrfs_header_nritems(parent))
@@ -651,8 +654,20 @@ struct extent_buffer *read_node_slot(struct btrfs_fs_info *fs_info,
 	if (level == 0)
 		return NULL;
 
-	return read_tree_block(fs_info, btrfs_node_blockptr(parent, slot),
+	ret = read_tree_block(fs_info, btrfs_node_blockptr(parent, slot),
 		       btrfs_node_ptr_generation(parent, slot));
+	if (!extent_buffer_uptodate(ret))
+		return ERR_PTR(-EIO);
+
+	if (btrfs_header_level(ret) != level - 1) {
+		error(
+"child eb corrupted: parent bytenr=%llu item=%d parent level=%d child level=%d",
+		      btrfs_header_bytenr(parent), slot,
+		      btrfs_header_level(parent), btrfs_header_level(ret));
+		free_extent_buffer(ret);
+		return ERR_PTR(-EIO);
+	}
+	return ret;
 }
 
 static int balance_level(struct btrfs_trans_handle *trans,