OpenCloudOS-Kernel/Documentation/filesystems/ext4/directory.rst

.. SPDX-License-Identifier: GPL-2.0

Directory Entries
-----------------

In an ext4 filesystem, a directory is more or less a flat file that maps
an arbitrary byte string (usually ASCII) to an inode number on the
filesystem. There can be many directory entries across the filesystem
that reference the same inode number--these are known as hard links, and
that is why hard links cannot reference files on other filesystems. As
such, directory entries are found by reading the data block(s)
associated with a directory file for the particular directory entry that
is desired.

Linear (Classic) Directories
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default, each directory lists its entries in an “almost-linear”
array. I write “almost” because it's not a linear array in the memory
sense because directory entries are not split across filesystem blocks.
Therefore, it is more accurate to say that a directory is a series of
data blocks and that each block contains a linear array of directory
entries. The end of each per-block array is signified by reaching the
end of the block; the last entry in the block has a record length that
takes it all the way to the end of the block. The end of the entire
directory is of course signified by reaching the end of the file. Unused
directory entries are signified by inode = 0. By default the filesystem
uses ``struct ext4_dir_entry_2`` for directory entries unless the
“filetype” feature flag is not set, in which case it uses
``struct ext4_dir_entry``.

The original directory entry format is ``struct ext4_dir_entry``, which
is at most 263 bytes long, though on disk you'll need to reference
``dirent.rec_len`` to know for sure.

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Size
     - Name
     - Description
   * - 0x0
     - \_\_le32
     - inode
     - Number of the inode that this directory entry points to.
   * - 0x4
     - \_\_le16
     - rec\_len
     - Length of this directory entry. Must be a multiple of 4.
   * - 0x6
     - \_\_le16
     - name\_len
     - Length of the file name.
   * - 0x8
     - char
     - name[EXT4\_NAME\_LEN]
     - File name.

Since file names cannot be longer than 255 bytes, the new directory
entry format shortens the name\_len field and uses the space for a file
type flag, probably to avoid having to load every inode during directory
tree traversal. This format is ``ext4_dir_entry_2``, which is at most
263 bytes long, though on disk you'll need to reference
``dirent.rec_len`` to know for sure.

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Size
     - Name
     - Description
   * - 0x0
     - \_\_le32
     - inode
     - Number of the inode that this directory entry points to.
   * - 0x4
     - \_\_le16
     - rec\_len
     - Length of this directory entry.
   * - 0x6
     - \_\_u8
     - name\_len
     - Length of the file name.
   * - 0x7
     - \_\_u8
     - file\_type
     - File type code, see ftype_ table below.
   * - 0x8
     - char
     - name[EXT4\_NAME\_LEN]
     - File name.

.. _ftype:

The directory file type is one of the following values:

.. list-table::
   :widths: 16 64
   :header-rows: 1

   * - Value
     - Description
   * - 0x0
     - Unknown.
   * - 0x1
     - Regular file.
   * - 0x2
     - Directory.
   * - 0x3
     - Character device file.
   * - 0x4
     - Block device file.
   * - 0x5
     - FIFO.
   * - 0x6
     - Socket.
   * - 0x7
     - Symbolic link.

In order to add checksums to these classic directory blocks, a phony
``struct ext4_dir_entry`` is placed at the end of each leaf block to
hold the checksum. The directory entry is 12 bytes long. The inode
number and name\_len fields are set to zero to fool old software into
ignoring an apparently empty directory entry, and the checksum is stored
in the place where the name normally goes. The structure is
``struct ext4_dir_entry_tail``:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Size
     - Name
     - Description
   * - 0x0
     - \_\_le32
     - det\_reserved\_zero1
     - Inode number, which must be zero.
   * - 0x4
     - \_\_le16
     - det\_rec\_len
     - Length of this directory entry, which must be 12.
   * - 0x6
     - \_\_u8
     - det\_reserved\_zero2
     - Length of the file name, which must be zero.
   * - 0x7
     - \_\_u8
     - det\_reserved\_ft
     - File type, which must be 0xDE.
   * - 0x8
     - \_\_le32
     - det\_checksum
     - Directory leaf block checksum.

The leaf directory block checksum is calculated against the FS UUID, the
directory's inode number, the directory's inode generation number, and
the entire directory entry block up to (but not including) the fake
directory entry.

Hash Tree Directories
~~~~~~~~~~~~~~~~~~~~~

A linear array of directory entries isn't great for performance, so a
new feature was added to ext3 to provide a faster (but peculiar)
balanced tree keyed off a hash of the directory entry name. If the
EXT4\_INDEX\_FL (0x1000) flag is set in the inode, this directory uses a
hashed btree (htree) to organize and find directory entries. For
backwards read-only compatibility with ext2, this tree is actually
hidden inside the directory file, masquerading as “empty” directory data
blocks! It was stated previously that the end of the linear directory
entry table was signified with an entry pointing to inode 0; this is
(ab)used to fool the old linear-scan algorithm into thinking that the
rest of the directory block is empty so that it moves on.

The root of the tree always lives in the first data block of the
directory. By ext2 custom, the '.' and '..' entries must appear at the
beginning of this first block, so they are put here as two
``struct ext4_dir_entry_2``\ s and not stored in the tree. The rest of
the root node contains metadata about the tree and finally a hash->block
map to find nodes that are lower in the htree. If
``dx_root.info.indirect_levels`` is non-zero then the htree has two
levels; the data block pointed to by the root node's map is an interior
node, which is indexed by a minor hash. Interior nodes in this tree
contains a zeroed out ``struct ext4_dir_entry_2`` followed by a
minor\_hash->block map to find leafe nodes. Leaf nodes contain a linear
array of all ``struct ext4_dir_entry_2``; all of these entries
(presumably) hash to the same value. If there is an overflow, the
entries simply overflow into the next leaf node, and the
least-significant bit of the hash (in the interior node map) that gets
us to this next leaf node is set.

To traverse the directory as a htree, the code calculates the hash of
the desired file name and uses it to find the corresponding block
number. If the tree is flat, the block is a linear array of directory
entries that can be searched; otherwise, the minor hash of the file name
is computed and used against this second block to find the corresponding
third block number. That third block number will be a linear array of
directory entries.

To traverse the directory as a linear array (such as the old code does),
the code simply reads every data block in the directory. The blocks used
for the htree will appear to have no entries (aside from '.' and '..')
and so only the leaf nodes will appear to have any interesting content.

The root of the htree is in ``struct dx_root``, which is the full length
of a data block:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - \_\_le32
     - dot.inode
     - inode number of this directory.
   * - 0x4
     - \_\_le16
     - dot.rec\_len
     - Length of this record, 12.
   * - 0x6
     - u8
     - dot.name\_len
     - Length of the name, 1.
   * - 0x7
     - u8
     - dot.file\_type
     - File type of this entry, 0x2 (directory) (if the feature flag is set).
   * - 0x8
     - char
     - dot.name[4]
     - “.\\0\\0\\0”
   * - 0xC
     - \_\_le32
     - dotdot.inode
     - inode number of parent directory.
   * - 0x10
     - \_\_le16
     - dotdot.rec\_len
     - block\_size - 12. The record length is long enough to cover all htree
       data.
   * - 0x12
     - u8
     - dotdot.name\_len
     - Length of the name, 2.
   * - 0x13
     - u8
     - dotdot.file\_type
     - File type of this entry, 0x2 (directory) (if the feature flag is set).
   * - 0x14
     - char
     - dotdot\_name[4]
     - “..\\0\\0”
   * - 0x18
     - \_\_le32
     - struct dx\_root\_info.reserved\_zero
     - Zero.
   * - 0x1C
     - u8
     - struct dx\_root\_info.hash\_version
     - Hash type, see dirhash_ table below.
   * - 0x1D
     - u8
     - struct dx\_root\_info.info\_length
     - Length of the tree information, 0x8.
   * - 0x1E
     - u8
     - struct dx\_root\_info.indirect\_levels
     - Depth of the htree. Cannot be larger than 3 if the INCOMPAT\_LARGEDIR
       feature is set; cannot be larger than 2 otherwise.
   * - 0x1F
     - u8
     - struct dx\_root\_info.unused\_flags
     -
   * - 0x20
     - \_\_le16
     - limit
     - Maximum number of dx\_entries that can follow this header, plus 1 for
       the header itself.
   * - 0x22
     - \_\_le16
     - count
     - Actual number of dx\_entries that follow this header, plus 1 for the
       header itself.
   * - 0x24
     - \_\_le32
     - block
     - The block number (within the directory file) that goes with hash=0.
   * - 0x28
     - struct dx\_entry
     - entries[0]
     - As many 8-byte ``struct dx_entry`` as fits in the rest of the data block.

.. _dirhash:

The directory hash is one of the following values:

.. list-table::
   :widths: 16 64
   :header-rows: 1

   * - Value
     - Description
   * - 0x0
     - Legacy.
   * - 0x1
     - Half MD4.
   * - 0x2
     - Tea.
   * - 0x3
     - Legacy, unsigned.
   * - 0x4
     - Half MD4, unsigned.
   * - 0x5
     - Tea, unsigned.

Interior nodes of an htree are recorded as ``struct dx_node``, which is
also the full length of a data block:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - \_\_le32
     - fake.inode
     - Zero, to make it look like this entry is not in use.
   * - 0x4
     - \_\_le16
     - fake.rec\_len
     - The size of the block, in order to hide all of the dx\_node data.
   * - 0x6
     - u8
     - name\_len
     - Zero. There is no name for this “unused” directory entry.
   * - 0x7
     - u8
     - file\_type
     - Zero. There is no file type for this “unused” directory entry.
   * - 0x8
     - \_\_le16
     - limit
     - Maximum number of dx\_entries that can follow this header, plus 1 for
       the header itself.
   * - 0xA
     - \_\_le16
     - count
     - Actual number of dx\_entries that follow this header, plus 1 for the
       header itself.
   * - 0xE
     - \_\_le32
     - block
     - The block number (within the directory file) that goes with the lowest
       hash value of this block. This value is stored in the parent block.
   * - 0x12
     - struct dx\_entry
     - entries[0]
     - As many 8-byte ``struct dx_entry`` as fits in the rest of the data block.

The hash maps that exist in both ``struct dx_root`` and
``struct dx_node`` are recorded as ``struct dx_entry``, which is 8 bytes
long:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - \_\_le32
     - hash
     - Hash code.
   * - 0x4
     - \_\_le32
     - block
     - Block number (within the directory file, not filesystem blocks) of the
       next node in the htree.

(If you think this is all quite clever and peculiar, so does the
author.)

If metadata checksums are enabled, the last 8 bytes of the directory
block (precisely the length of one dx\_entry) are used to store a
``struct dx_tail``, which contains the checksum. The ``limit`` and
``count`` entries in the dx\_root/dx\_node structures are adjusted as
necessary to fit the dx\_tail into the block. If there is no space for
the dx\_tail, the user is notified to run e2fsck -D to rebuild the
directory index (which will ensure that there's space for the checksum.
The dx\_tail structure is 8 bytes long and looks like this:

.. list-table::
   :widths: 8 8 24 40
   :header-rows: 1

   * - Offset
     - Type
     - Name
     - Description
   * - 0x0
     - u32
     - dt\_reserved
     - Zero.
   * - 0x4
     - \_\_le32
     - dt\_checksum
     - Checksum of the htree directory block.

The checksum is calculated against the FS UUID, the htree index header
(dx\_root or dx\_node), all of the htree indices (dx\_entry) that are in
use, and the tail block (dx\_tail).