fs: ext4: Problem with ext4load and sparse files

Hi,

when I try to load a sparse file via ext4load, I am getting the error message
'invalid extent'

After a deeper look in the code, it seems to be an issue in the function ext4fs_get_extent_block in fs/ext4/ext4_common.c:

The file starts with 1k of zeros. The blocksize is 1024. So the first extend block contains the following information:

eh_entries: 1
eh_depth: 1
ei_block 1

When the upper layer (ext4fs_read_file) asks for fileblock 0, we are running in the 'invalid extent' error message.
For me it seems, that the code is not prepared for handling a sparse block at the beginning of the file. The following change, solved my problem:

I am really not an expert in ext4 filesystems. Can somebody please have a look at this issue and give me a feedback, if I am totally wrong or not?

fs/ext4/ext4_common.c

@@ -1547,8 +1547,12 @@ static struct ext4_extent_header *ext4fs_get_extent_block
 				break;
 		} while (fileblock >= le32_to_cpu(index[i].ei_block));
 
-		if (--i < 0)
-			return NULL;
+		/*
+		 * If first logical block number is higher than requested fileblock,
+		 * it is a sparse file. This is handled on upper layer.
+		 */
+		if (i > 0)
+			i--;
 
 		block = le16_to_cpu(index[i].ei_leaf_hi);
 		block = (block << 32) + le32_to_cpu(index[i].ei_leaf_lo);