summaryrefslogtreecommitdiffstats
path: root/Documentation/filesystems/ext4/checksums.rst
diff options
context:
space:
mode:
authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-27 10:05:51 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-27 10:05:51 +0000
commit5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 (patch)
treea94efe259b9009378be6d90eb30d2b019d95c194 /Documentation/filesystems/ext4/checksums.rst
parentInitial commit. (diff)
downloadlinux-upstream/5.10.209.tar.xz
linux-upstream/5.10.209.zip
Adding upstream version 5.10.209.upstream/5.10.209upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'Documentation/filesystems/ext4/checksums.rst')
-rw-r--r--Documentation/filesystems/ext4/checksums.rst73
1 files changed, 73 insertions, 0 deletions
diff --git a/Documentation/filesystems/ext4/checksums.rst b/Documentation/filesystems/ext4/checksums.rst
new file mode 100644
index 000000000..5519e2538
--- /dev/null
+++ b/Documentation/filesystems/ext4/checksums.rst
@@ -0,0 +1,73 @@
+.. 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.
+