1 .. SPDX-License-Identifier: GPL-2.0
6 In an ext4 filesystem, a directory is more or less a flat file that maps
7 an arbitrary byte string (usually ASCII) to an inode number on the
8 filesystem. There can be many directory entries across the filesystem
9 that reference the same inode number--these are known as hard links, and
10 that is why hard links cannot reference files on other filesystems. As
11 such, directory entries are found by reading the data block(s)
12 associated with a directory file for the particular directory entry that
15 Linear (Classic) Directories
16 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
18 By default, each directory lists its entries in an “almost-linear”
19 array. I write “almost” because it's not a linear array in the memory
20 sense because directory entries are not split across filesystem blocks.
21 Therefore, it is more accurate to say that a directory is a series of
22 data blocks and that each block contains a linear array of directory
23 entries. The end of each per-block array is signified by reaching the
24 end of the block; the last entry in the block has a record length that
25 takes it all the way to the end of the block. The end of the entire
26 directory is of course signified by reaching the end of the file. Unused
27 directory entries are signified by inode = 0. By default the filesystem
28 uses ``struct ext4_dir_entry_2`` for directory entries unless the
29 “filetype” feature flag is not set, in which case it uses
30 ``struct ext4_dir_entry``.
32 The original directory entry format is ``struct ext4_dir_entry``, which
33 is at most 263 bytes long, though on disk you'll need to reference
34 ``dirent.rec_len`` to know for sure.
47 - Number of the inode that this directory entry points to.
51 - Length of this directory entry. Must be a multiple of 4.
55 - Length of the file name.
58 - name[EXT4\_NAME\_LEN]
61 Since file names cannot be longer than 255 bytes, the new directory
62 entry format shortens the name\_len field and uses the space for a file
63 type flag, probably to avoid having to load every inode during directory
64 tree traversal. This format is ``ext4_dir_entry_2``, which is at most
65 263 bytes long, though on disk you'll need to reference
66 ``dirent.rec_len`` to know for sure.
79 - Number of the inode that this directory entry points to.
83 - Length of this directory entry.
87 - Length of the file name.
91 - File type code, see ftype_ table below.
94 - name[EXT4\_NAME\_LEN]
99 The directory file type is one of the following values:
114 - Character device file.
124 To support directories that are both encrypted and casefolded directories, we
125 must also include hash information in the directory entry. We append
126 ``ext4_extended_dir_entry_2`` to ``ext4_dir_entry_2`` except for the entries
127 for dot and dotdot, which are kept the same. The structure follows immediately
128 after ``name`` and is included in the size listed by ``rec_len`` If a directory
129 entry uses this extension, it may be up to 271 bytes.
142 - The hash of the directory name
146 - The minor hash of the directory name
149 In order to add checksums to these classic directory blocks, a phony
150 ``struct ext4_dir_entry`` is placed at the end of each leaf block to
151 hold the checksum. The directory entry is 12 bytes long. The inode
152 number and name\_len fields are set to zero to fool old software into
153 ignoring an apparently empty directory entry, and the checksum is stored
154 in the place where the name normally goes. The structure is
155 ``struct ext4_dir_entry_tail``:
167 - det\_reserved\_zero1
168 - Inode number, which must be zero.
172 - Length of this directory entry, which must be 12.
175 - det\_reserved\_zero2
176 - Length of the file name, which must be zero.
180 - File type, which must be 0xDE.
184 - Directory leaf block checksum.
186 The leaf directory block checksum is calculated against the FS UUID, the
187 directory's inode number, the directory's inode generation number, and
188 the entire directory entry block up to (but not including) the fake
191 Hash Tree Directories
192 ~~~~~~~~~~~~~~~~~~~~~
194 A linear array of directory entries isn't great for performance, so a
195 new feature was added to ext3 to provide a faster (but peculiar)
196 balanced tree keyed off a hash of the directory entry name. If the
197 EXT4\_INDEX\_FL (0x1000) flag is set in the inode, this directory uses a
198 hashed btree (htree) to organize and find directory entries. For
199 backwards read-only compatibility with ext2, this tree is actually
200 hidden inside the directory file, masquerading as “empty” directory data
201 blocks! It was stated previously that the end of the linear directory
202 entry table was signified with an entry pointing to inode 0; this is
203 (ab)used to fool the old linear-scan algorithm into thinking that the
204 rest of the directory block is empty so that it moves on.
206 The root of the tree always lives in the first data block of the
207 directory. By ext2 custom, the '.' and '..' entries must appear at the
208 beginning of this first block, so they are put here as two
209 ``struct ext4_dir_entry_2``\ s and not stored in the tree. The rest of
210 the root node contains metadata about the tree and finally a hash->block
211 map to find nodes that are lower in the htree. If
212 ``dx_root.info.indirect_levels`` is non-zero then the htree has two
213 levels; the data block pointed to by the root node's map is an interior
214 node, which is indexed by a minor hash. Interior nodes in this tree
215 contains a zeroed out ``struct ext4_dir_entry_2`` followed by a
216 minor\_hash->block map to find leafe nodes. Leaf nodes contain a linear
217 array of all ``struct ext4_dir_entry_2``; all of these entries
218 (presumably) hash to the same value. If there is an overflow, the
219 entries simply overflow into the next leaf node, and the
220 least-significant bit of the hash (in the interior node map) that gets
221 us to this next leaf node is set.
223 To traverse the directory as a htree, the code calculates the hash of
224 the desired file name and uses it to find the corresponding block
225 number. If the tree is flat, the block is a linear array of directory
226 entries that can be searched; otherwise, the minor hash of the file name
227 is computed and used against this second block to find the corresponding
228 third block number. That third block number will be a linear array of
231 To traverse the directory as a linear array (such as the old code does),
232 the code simply reads every data block in the directory. The blocks used
233 for the htree will appear to have no entries (aside from '.' and '..')
234 and so only the leaf nodes will appear to have any interesting content.
236 The root of the htree is in ``struct dx_root``, which is the full length
250 - inode number of this directory.
254 - Length of this record, 12.
258 - Length of the name, 1.
262 - File type of this entry, 0x2 (directory) (if the feature flag is set).
270 - inode number of parent directory.
274 - block\_size - 12. The record length is long enough to cover all htree
279 - Length of the name, 2.
283 - File type of this entry, 0x2 (directory) (if the feature flag is set).
290 - struct dx\_root\_info.reserved\_zero
294 - struct dx\_root\_info.hash\_version
295 - Hash type, see dirhash_ table below.
298 - struct dx\_root\_info.info\_length
299 - Length of the tree information, 0x8.
302 - struct dx\_root\_info.indirect\_levels
303 - Depth of the htree. Cannot be larger than 3 if the INCOMPAT\_LARGEDIR
304 feature is set; cannot be larger than 2 otherwise.
307 - struct dx\_root\_info.unused\_flags
312 - Maximum number of dx\_entries that can follow this header, plus 1 for
317 - Actual number of dx\_entries that follow this header, plus 1 for the
322 - The block number (within the directory file) that goes with hash=0.
326 - As many 8-byte ``struct dx_entry`` as fits in the rest of the data block.
330 The directory hash is one of the following values:
347 - Half MD4, unsigned.
353 Interior nodes of an htree are recorded as ``struct dx_node``, which is
354 also the full length of a data block:
367 - Zero, to make it look like this entry is not in use.
371 - The size of the block, in order to hide all of the dx\_node data.
375 - Zero. There is no name for this “unused” directory entry.
379 - Zero. There is no file type for this “unused” directory entry.
383 - Maximum number of dx\_entries that can follow this header, plus 1 for
388 - Actual number of dx\_entries that follow this header, plus 1 for the
393 - The block number (within the directory file) that goes with the lowest
394 hash value of this block. This value is stored in the parent block.
398 - As many 8-byte ``struct dx_entry`` as fits in the rest of the data block.
400 The hash maps that exist in both ``struct dx_root`` and
401 ``struct dx_node`` are recorded as ``struct dx_entry``, which is 8 bytes
419 - Block number (within the directory file, not filesystem blocks) of the
420 next node in the htree.
422 (If you think this is all quite clever and peculiar, so does the
425 If metadata checksums are enabled, the last 8 bytes of the directory
426 block (precisely the length of one dx\_entry) are used to store a
427 ``struct dx_tail``, which contains the checksum. The ``limit`` and
428 ``count`` entries in the dx\_root/dx\_node structures are adjusted as
429 necessary to fit the dx\_tail into the block. If there is no space for
430 the dx\_tail, the user is notified to run e2fsck -D to rebuild the
431 directory index (which will ensure that there's space for the checksum.
432 The dx\_tail structure is 8 bytes long and looks like this:
449 - Checksum of the htree directory block.
451 The checksum is calculated against the FS UUID, the htree index header
452 (dx\_root or dx\_node), all of the htree indices (dx\_entry) that are in
453 use, and the tail block (dx\_tail).
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