checksums.rst (2934B)
1.. SPDX-License-Identifier: GPL-2.0 2 3Checksums 4--------- 5 6Starting in early 2012, metadata checksums were added to all major ext4 7and jbd2 data structures. The associated feature flag is metadata_csum. 8The desired checksum algorithm is indicated in the superblock, though as 9of October 2012 the only supported algorithm is crc32c. Some data 10structures did not have space to fit a full 32-bit checksum, so only the 11lower 16 bits are stored. Enabling the 64bit feature increases the data 12structure size so that full 32-bit checksums can be stored for many data 13structures. However, existing 32-bit filesystems cannot be extended to 14enable 64bit mode, at least not without the experimental resize2fs 15patches to do so. 16 17Existing filesystems can have checksumming added by running 18``tune2fs -O metadata_csum`` against the underlying device. If tune2fs 19encounters directory blocks that lack sufficient empty space to add a 20checksum, it will request that you run ``e2fsck -D`` to have the 21directories rebuilt with checksums. This has the added benefit of 22removing slack space from the directory files and rebalancing the htree 23indexes. If you _ignore_ this step, your directories will not be 24protected by a checksum! 25 26The following table describes the data elements that go into each type 27of checksum. The checksum function is whatever the superblock describes 28(crc32c as of October 2013) unless noted otherwise. 29 30.. list-table:: 31 :widths: 20 8 50 32 :header-rows: 1 33 34 * - Metadata 35 - Length 36 - Ingredients 37 * - Superblock 38 - __le32 39 - The entire superblock up to the checksum field. The UUID lives inside 40 the superblock. 41 * - MMP 42 - __le32 43 - UUID + the entire MMP block up to the checksum field. 44 * - Extended Attributes 45 - __le32 46 - UUID + the entire extended attribute block. The checksum field is set to 47 zero. 48 * - Directory Entries 49 - __le32 50 - UUID + inode number + inode generation + the directory block up to the 51 fake entry enclosing the checksum field. 52 * - HTREE Nodes 53 - __le32 54 - UUID + inode number + inode generation + all valid extents + HTREE tail. 55 The checksum field is set to zero. 56 * - Extents 57 - __le32 58 - UUID + inode number + inode generation + the entire extent block up to 59 the checksum field. 60 * - Bitmaps 61 - __le32 or __le16 62 - UUID + the entire bitmap. Checksums are stored in the group descriptor, 63 and truncated if the group descriptor size is 32 bytes (i.e. ^64bit) 64 * - Inodes 65 - __le32 66 - UUID + inode number + inode generation + the entire inode. The checksum 67 field is set to zero. Each inode has its own checksum. 68 * - Group Descriptors 69 - __le16 70 - If metadata_csum, then UUID + group number + the entire descriptor; 71 else if gdt_csum, then crc16(UUID + group number + the entire 72 descriptor). In all cases, only the lower 16 bits are stored. 73