ext4: clean up GET_BLOCKS_PRE_IO error handling

If the call to ext4_split_convert_extents() fails in the
EXT4_GET_BLOCKS_PRE_IO case within ext4_ext_handle_unwritten_extents(),
error out through the exit point at function end rather than jumping
through an intermediate point.  Fix the error handling in the event
ext4_split_convert_extents() returns 0, which it shouldn't do when
splitting an existing extent.  The current code returns the passed in
value of allocated (which is likely non-zero) while failing to set
m_flags, m_pblk, and m_len.
Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Link: https://lore.kernel.org/r/20200430185320.23001-4-enwlinux@gmail.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/extents.c

@@ -3818,12 +3818,25 @@ ext4_ext_handle_unwritten_extents(handle_t *handle, struct inode *inode,
 	trace_ext4_ext_handle_unwritten_extents(inode, map, flags,
 						    allocated, newblock);
 
-	/* get_block() before submit the IO, split the extent */
+	/* get_block() before submitting IO, split the extent */
 	if (flags & EXT4_GET_BLOCKS_PRE_IO) {
 		ret = ext4_split_convert_extents(handle, inode, map, ppath,
 					 flags | EXT4_GET_BLOCKS_CONVERT);
-		if (ret <= 0)
-			goto out;
+		if (ret < 0) {
+			err = ret;
+			goto out2;
+		}
+		/*
+		 * shouldn't get a 0 return when splitting an extent unless
+		 * m_len is 0 (bug) or extent has been corrupted
+		 */
+		if (unlikely(ret == 0)) {
+			EXT4_ERROR_INODE(inode,
+					 "unexpected ret == 0, m_len = %u",
+					 map->m_len);
+			err = -EFSCORRUPTED;
+			goto out2;
+		}
 		map->m_flags |= EXT4_MAP_UNWRITTEN;
 		goto out;
 	}
@@ -3863,12 +3876,13 @@ ext4_ext_handle_unwritten_extents(handle_t *handle, struct inode *inode,
 	ret = ext4_ext_convert_to_initialized(handle, inode, map, ppath, flags);
 	if (ret >= 0)
 		ext4_update_inode_fsync_trans(handle, inode, 1);
-out:
+
 	if (ret <= 0) {
 		err = ret;
 		goto out2;
-	} else
-		allocated = ret;
+	}
+out:
+	allocated = ret;
 	map->m_flags |= EXT4_MAP_NEW;
 map_out:
 	map->m_flags |= EXT4_MAP_MAPPED;