1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 Checksums 3 Checksums
4 --------- 4 ---------
5 5
6 Starting in early 2012, metadata checksums wer 6 Starting in early 2012, metadata checksums were added to all major ext4
7 and jbd2 data structures. The associated featu 7 and jbd2 data structures. The associated feature flag is metadata_csum.
8 The desired checksum algorithm is indicated in 8 The desired checksum algorithm is indicated in the superblock, though as
9 of October 2012 the only supported algorithm i 9 of October 2012 the only supported algorithm is crc32c. Some data
10 structures did not have space to fit a full 32 10 structures did not have space to fit a full 32-bit checksum, so only the
11 lower 16 bits are stored. Enabling the 64bit f 11 lower 16 bits are stored. Enabling the 64bit feature increases the data
12 structure size so that full 32-bit checksums c 12 structure size so that full 32-bit checksums can be stored for many data
13 structures. However, existing 32-bit filesyste 13 structures. However, existing 32-bit filesystems cannot be extended to
14 enable 64bit mode, at least not without the ex 14 enable 64bit mode, at least not without the experimental resize2fs
15 patches to do so. 15 patches to do so.
16 16
17 Existing filesystems can have checksumming add 17 Existing filesystems can have checksumming added by running
18 ``tune2fs -O metadata_csum`` against the under 18 ``tune2fs -O metadata_csum`` against the underlying device. If tune2fs
19 encounters directory blocks that lack sufficie 19 encounters directory blocks that lack sufficient empty space to add a
20 checksum, it will request that you run ``e2fsc 20 checksum, it will request that you run ``e2fsck -D`` to have the
21 directories rebuilt with checksums. This has t 21 directories rebuilt with checksums. This has the added benefit of
22 removing slack space from the directory files 22 removing slack space from the directory files and rebalancing the htree
23 indexes. If you _ignore_ this step, your direc 23 indexes. If you _ignore_ this step, your directories will not be
24 protected by a checksum! 24 protected by a checksum!
25 25
26 The following table describes the data element 26 The following table describes the data elements that go into each type
27 of checksum. The checksum function is whatever 27 of checksum. The checksum function is whatever the superblock describes
28 (crc32c as of October 2013) unless noted other 28 (crc32c as of October 2013) unless noted otherwise.
29 29
30 .. list-table:: 30 .. list-table::
31 :widths: 20 8 50 31 :widths: 20 8 50
32 :header-rows: 1 32 :header-rows: 1
33 33
34 * - Metadata 34 * - Metadata
35 - Length 35 - Length
36 - Ingredients 36 - Ingredients
37 * - Superblock 37 * - Superblock
38 - __le32 38 - __le32
39 - The entire superblock up to the checksu 39 - The entire superblock up to the checksum field. The UUID lives inside
40 the superblock. 40 the superblock.
41 * - MMP 41 * - MMP
42 - __le32 42 - __le32
43 - UUID + the entire MMP block up to the c 43 - UUID + the entire MMP block up to the checksum field.
44 * - Extended Attributes 44 * - Extended Attributes
45 - __le32 45 - __le32
46 - UUID + the entire extended attribute bl 46 - UUID + the entire extended attribute block. The checksum field is set to
47 zero. 47 zero.
48 * - Directory Entries 48 * - Directory Entries
49 - __le32 49 - __le32
50 - UUID + inode number + inode generation 50 - UUID + inode number + inode generation + the directory block up to the
51 fake entry enclosing the checksum field 51 fake entry enclosing the checksum field.
52 * - HTREE Nodes 52 * - HTREE Nodes
53 - __le32 53 - __le32
54 - UUID + inode number + inode generation 54 - UUID + inode number + inode generation + all valid extents + HTREE tail.
55 The checksum field is set to zero. 55 The checksum field is set to zero.
56 * - Extents 56 * - Extents
57 - __le32 57 - __le32
58 - UUID + inode number + inode generation 58 - UUID + inode number + inode generation + the entire extent block up to
59 the checksum field. 59 the checksum field.
60 * - Bitmaps 60 * - Bitmaps
61 - __le32 or __le16 61 - __le32 or __le16
62 - UUID + the entire bitmap. Checksums are 62 - UUID + the entire bitmap. Checksums are stored in the group descriptor,
63 and truncated if the group descriptor s 63 and truncated if the group descriptor size is 32 bytes (i.e. ^64bit)
64 * - Inodes 64 * - Inodes
65 - __le32 65 - __le32
66 - UUID + inode number + inode generation 66 - UUID + inode number + inode generation + the entire inode. The checksum
67 field is set to zero. Each inode has it 67 field is set to zero. Each inode has its own checksum.
68 * - Group Descriptors 68 * - Group Descriptors
69 - __le16 69 - __le16
70 - If metadata_csum, then UUID + group num 70 - If metadata_csum, then UUID + group number + the entire descriptor;
71 else if gdt_csum, then crc16(UUID + gro 71 else if gdt_csum, then crc16(UUID + group number + the entire
72 descriptor). In all cases, only the low 72 descriptor). In all cases, only the lower 16 bits are stored.
73 73
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