74 lines
2.9 KiB
ReStructuredText
74 lines
2.9 KiB
ReStructuredText
|
.. 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.
|
||
|
|
|