hfs: fix oops on mount with corrupted btree extent records

A particular fsfuzzer run caused an hfs file system to crash on mount.
This is due to a corrupted MDB extent record causing a miscalculation of
HFS_I(inode)->first_blocks for the extent tree.  If the extent records are
zereod out, it won't trigger the first_blocks special case.  Instead it
falls through to the extent code which we're still in the middle of
initializing.

This patch catches the 0 size extent records, reports the corruption, and
fails the mount.
Reported-by: NRamon de Carvalho Valle <rcvalle@linux.vnet.ibm.com>
Signed-off-by: NJeff Mahoney <jeffm@suse.com>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/hfs/btree.c

@@ -58,6 +58,11 @@ struct hfs_btree *hfs_btree_open(struct super_block *sb, u32 id, btree_keycmp ke
 	}
 	unlock_new_inode(tree->inode);
 
+	if (!HFS_I(tree->inode)->first_blocks) {
+		printk(KERN_ERR "hfs: invalid btree extent records (0 size).\n");
+		goto free_inode;
+	}
+
 	mapping = tree->inode->i_mapping;
 	page = read_mapping_page(mapping, 0, NULL);
 	if (IS_ERR(page))