74 lines
2.9 KiB
ReStructuredText
74 lines
2.9 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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Checksums
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---------
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Starting in early 2012, metadata checksums were added to all major ext4
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and jbd2 data structures. The associated feature flag is metadata\_csum.
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The desired checksum algorithm is indicated in the superblock, though as
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of October 2012 the only supported algorithm is crc32c. Some data
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structures did not have space to fit a full 32-bit checksum, so only the
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lower 16 bits are stored. Enabling the 64bit feature increases the data
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structure size so that full 32-bit checksums can be stored for many data
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structures. However, existing 32-bit filesystems cannot be extended to
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enable 64bit mode, at least not without the experimental resize2fs
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patches to do so.
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Existing filesystems can have checksumming added by running
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``tune2fs -O metadata_csum`` against the underlying device. If tune2fs
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encounters directory blocks that lack sufficient empty space to add a
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checksum, it will request that you run ``e2fsck -D`` to have the
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directories rebuilt with checksums. This has the added benefit of
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removing slack space from the directory files and rebalancing the htree
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indexes. If you \_ignore\_ this step, your directories will not be
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protected by a checksum!
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The following table describes the data elements that go into each type
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of checksum. The checksum function is whatever the superblock describes
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(crc32c as of October 2013) unless noted otherwise.
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.. list-table::
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:widths: 20 8 50
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:header-rows: 1
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* - Metadata
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- Length
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- Ingredients
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* - Superblock
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- \_\_le32
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- The entire superblock up to the checksum field. The UUID lives inside
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the superblock.
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* - MMP
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- \_\_le32
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- UUID + the entire MMP block up to the checksum field.
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* - Extended Attributes
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- \_\_le32
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- UUID + the entire extended attribute block. The checksum field is set to
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zero.
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* - Directory Entries
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- \_\_le32
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- UUID + inode number + inode generation + the directory block up to the
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fake entry enclosing the checksum field.
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* - HTREE Nodes
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- \_\_le32
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- UUID + inode number + inode generation + all valid extents + HTREE tail.
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The checksum field is set to zero.
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* - Extents
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- \_\_le32
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- UUID + inode number + inode generation + the entire extent block up to
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the checksum field.
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* - Bitmaps
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- \_\_le32 or \_\_le16
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- UUID + the entire bitmap. Checksums are stored in the group descriptor,
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and truncated if the group descriptor size is 32 bytes (i.e. ^64bit)
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* - Inodes
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- \_\_le32
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- UUID + inode number + inode generation + the entire inode. The checksum
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field is set to zero. Each inode has its own checksum.
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* - Group Descriptors
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- \_\_le16
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- If metadata\_csum, then UUID + group number + the entire descriptor;
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else if gdt\_csum, then crc16(UUID + group number + the entire
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descriptor). In all cases, only the lower 16 bits are stored.
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