Checksums¶
Starting in early 2012, metadata checksums were added to all major ext4 and jbd2 data structures. The associated feature flag is metadata_csum. The desired checksum algorithm is indicated in the superblock, though as of October 2012 the only supported algorithm is crc32c. Some data structures did not have space to fit a full 32-bit checksum, so only the lower 16 bits are stored. Enabling the 64bit feature increases the data structure size so that full 32-bit checksums can be stored for many data structures. However, existing 32-bit filesystems cannot be extended to enable 64bit mode, at least not without the experimental resize2fs patches to do so.
Existing filesystems can have checksumming added by running
tune2fs -O metadata_csum against the underlying device. If tune2fs
encounters directory blocks that lack sufficient empty space to add a
checksum, it will request that you run e2fsck -D to have the
directories rebuilt with checksums. This has the added benefit of
removing slack space from the directory files and rebalancing the htree
indexes. If you _ignore_ this step, your directories will not be
protected by a checksum!
The following table describes the data elements that go into each type of checksum. The checksum function is whatever the superblock describes (crc32c as of October 2013) unless noted otherwise.
| Metadata | Length | Ingredients | 
|---|---|---|
| Superblock | __le32 | The entire superblock up to the checksum field. The UUID lives inside the superblock. | 
| MMP | __le32 | UUID + the entire MMP block up to the checksum field. | 
| Extended Attributes | __le32 | UUID + the entire extended attribute block. The checksum field is set to zero. | 
| Directory Entries | __le32 | UUID + inode number + inode generation + the directory block up to the fake entry enclosing the checksum field. | 
| HTREE Nodes | __le32 | UUID + inode number + inode generation + all valid extents + HTREE tail. The checksum field is set to zero. | 
| Extents | __le32 | UUID + inode number + inode generation + the entire extent block up to the checksum field. | 
| Bitmaps | __le32 or __le16 | UUID + the entire bitmap. Checksums are stored in the group descriptor, and truncated if the group descriptor size is 32 bytes (i.e. ^64bit) | 
| Inodes | __le32 | UUID + inode number + inode generation + the entire inode. The checksum field is set to zero. Each inode has its own checksum. | 
| Group Descriptors | __le16 | If metadata_csum, then UUID + group number + the entire descriptor; else if gdt_csum, then crc16(UUID + group number + the entire descriptor). In all cases, only the lower 16 bits are stored. |