Checksums

.. SPDX-License-Identifier: GPL-2.0

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.

.. list-table:: :widths: 20 8 50 :header-rows: 1

• • 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.