path: root/doc/format.adoc

= Squashfs Binary Format
:toc: left
:toclevels: 4
:sectnums:

== About

SquashFS is a compressed, read-only filesystem for Linux that can also be used
as a flexible, general purpose, compressed archive format, optimized for fast
random access with support for Unix permissions, sparse files and extended
attributes.

SquashFS supports data and metadata compression through zlib, lz4, lzo, lzma,
xz or zstd.

For fast random access, compressed files are split up in fixed size blocks
that are compressed separately.
The block size can be set between 4k and 1M (default for squashfs-tools and
squashfs-tools-ng is 128K).

This document attempts to specify the on-disk format in detail.
It is based on a previous on-line version that was originally written by
Zachary Dremann and subsequently expanded by David Oberhollenzer during
reverse engineering attempts and available here: https://dr-emann.github.io/squashfs/.

== Overview

SquashFS always stores integers in little endian format.
The data blocks that make up the SquashFS archive are byte aligned,
i.e.  they typically do not care for alignment.
The implementation in the Linux kernel requires the archive itself to
be a multiple of either 1k or 4k in size (called the device block size)
and user space tools typically use 4k to be compatible with both.

A SquashFS archive consists of a maximum of nine parts:

[%nowrap]
----
 _______________
|               |  Important information about the archive, including
|  Superblock   |  locations of other sections.
|_______________|
|               |  If non-default compression options have been used,
|  Compression  |  they can optionally be stored here, to facilitate
|    options    |  later, offline editing of the archive.
|_______________|
|               |
|  Data blocks  |  The contents of the files in the archive,
|  & fragments  |  split into separately compressed blocks.
|_______________|
|               |  Metadata (ownership, permissions, etc) for
|  Inode table  |  items in the archive.
|_______________|
|               |
|   Directory   |  Directory listings, including file names, and
|     table     |  references to inodes.
|_______________|
|               |
|   Fragment    |  Description of fragment locations within the
|    table      |  Datablocks & Fragments section.
|_______________|
|               |  A mapping from inode numbers to disk locations,
| Export table  |  required for NFS export.
|_______________|
|               |
|    UID/GID    |  A list of unique UID/GIDs. Inodes use an index into
|  lookup table |  this table to save memory.
|_______________|
|               |
|     Xattr     |  Extended attributes for items in the archive.
|     table     |
|_______________|
----

Although the super block details the exact positions of each section, most
implementations, including the one in the Linux kernel, insist on this exact
order.

=== Packing File Data

The file data is packed into the archive after the super block (and optional
compressor options).

Files are divided into fixed size blocks that are separately compressed and
stored in order. SquashFS supports optional tail-end-packing of files that
are not an exact multiple of the block size. The remaining ends can either
be treated as a short block, or can be packed together with the tail ends of
other files in a single "fragment block". Files that are less than block size
are treated the same way.

If the size of a data or fragment block would exceed the input size after
compression, the original, uncompressed data is stored, so that the size of a
block after compression never exceeds the input block size.

=== Packing Metadata

Metadata (e.g. inodes, directory listings, etc...) is treated as a continuous
stream of records that is chopped up into 8KiB blocks that are separately
compressed into special metadata blocks.

The input size of 8KiB is fixed and independent of the data block size.
Similar to data blocks, if the compressed size would exceed 8KiB, the
uncompressed block is stored instead, so the on-disk size of a metadata
block never exceeds 8KiB.

Individual entries are allowed to cross the block boundary, so e.g. an inode
may be located at the end of a metadata block with some part of it located at
the start of the next block. Both have to be read and decompressed when
reading this inode. If an entry is written across block boundaries, there
*MUST NOT* be any gap between the compressed metadata blocks on-disk.


In contrast to data blocks, every metadata block is preceded by a single,
16 bit unsigned integer. This integer holds the on-disk size of the block
that follows. The MSB is set if the block is stored uncompressed. Whenever
a metadata block is referenced, the position of this integer is given.

To read a metadata block, seek to the indicated position and read the 16 bit
header. Sanity check that the lower 15 bit are less than 8KiB and proceed
to read that many bytes. If the highest bit of the header is cleared,
uncompress the data into an 8KiB buffer that *MUST NOT* overflow.


In the SquashFS archive format, metadata entries (e.g. inodes) are often
referenced using a 64 bit integer. The lower 16 bit hold an offset into the
uncompressed block and the upper 48 bit point to the on-disk location of the
block.

The on-disk location is relative to the type of metadata entry, e.g. for
inodes it is relative to the start of the inode table given by the
super block.

=== Storing Lookup Tables

Lookup tables are arrays (i.e. sequences of identical sized records) that are
addressed by an index.

Such tables are stored in the SquashFS format as metadata blocks, i.e. by
dividing the table data into 8KiB chunks that are separately compressed and
stored in sequence.

To allow constant time lookup, a list of 64 bit unsigned integers is stored,
holding the on-disk locations of each metadata block.

This list itself is stored uncompressed and not preceded by a header.

When referring to a lookup table, the superblock gives the number of table
entries and points to this location list.

Since the table entry size is a known, fixed value, the required number of
metadata blocks can be computed:

  block_count = ceil(table_count * entry_size / 8192)

Which is also the number of 64 bit integers in the location list.

When resolving a lookup table index, first work out the index of the
metadata block:

  meta_index = floor(index * entry_size / 8192)

Using this index on the location list yields the on-disk location of
the metadata block containing the entry.

After reading this metadata block, the byte offset into the block can
be computed to get the entry:

  offset = index * entry_size % 8192

The location list can be cached in memory. Resolving an index requires at
worst a single metadata block read (at most 8194 bytes fetched from an
unaligned on-disk location).

=== Supported Compressors

The SquashFS format supports the following compressors:

* zlib deflate (referred to as "gzip" but only uses raw zlib streams)
* lzo
* lzma 1 (considered deprecated)
* lzma 2 (referred to as "xz")
* lz4
* zstd

The archive can only specify one compressor in the super block and has to use
it for both file data and metadata compression. Using one compressor for data
and switching to a different compressor for e.g. inodes is not supported.

While it is technically not possible to pick a "null" compressor in the super
block, an implementation can still deliberately write only uncompressed blocks
to a SquashFS archive, or choose to store certain metadata blocks without
compression.

The lzma 2 aka xz compressor *MUST* use `CRC32` checksums only. Using `SHA-256` is
not supported.

== The Superblock

The superblock is the first section of a SquashFS archive. It is always
96 bytes in size and contains important information about the archive,
including the locations of other sections.

[cols="1,3,13a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | magic | Must be set to `0x73717368` ("hsqs" on disk).
| u32 | inode count | The number of inodes stored in the archive.
| u32 | mod time | Last modification time of the archive. Count seconds
                   since 00:00, Jan 1st 1970 UTC (not counting leap
                   seconds). This is unsigned, so it expires in the
                   year 2106 (as opposed to 2038).
| u32 | block size | The size of a data block in bytes. Must be a power
                     of two between 4096 (4k) and 1048576 (1 MiB).
| u32 | frag count | The number of entries in the fragment table.
| u16 | compressor | An ID designating the compressor used for both data
                     and meta data blocks.

[cols=">1,2,8",frame="none",grid="none",options="header"]
!===
! Value ! Name ! Comment
! 1 ! GZIP ! just zlib streams (no gzip headers\!)
! 2 ! LZMA ! LZMA version 1
! 3 ! LZO  !
! 4 ! XZ   ! LZMA version 2 as used by xz-utils
! 5 ! LZ4  !
! 6 ! ZSTD !
!===

| u16 | block log | The log~2~ of the block size. If the two fields do not
                    agree, the archive is considered corrupted.
| u16 | flags | Bit wise *OR* of the flag bits below.


[cols=">1m,10",frame="none",grid="none",options="header"]
!===
! Value ! Meaning
! 0x0001 ! Inodes are stored uncompressed.
! 0x0002 ! Data blocks are stored uncompressed.
! 0x0004 ! Unused, should always be unset.
! 0x0008 ! Fragments are stored uncompressed.
! 0x0010 ! Fragments are not used.
! 0x0020 ! Fragments are always generated.
! 0x0040 ! Data has been deduplicated.
! 0x0080 ! NFS export table exists.
! 0x0100 ! Xattrs are stored uncompressed.
! 0x0200 ! There are no Xattrs in the archive.
! 0x0400 ! Compressor options are present.
! 0x0800 ! The ID table is uncompressed.
!===

| u16 | id count | The number of entries in the ID lookup table.
| u16 | version major | Major version of the format. Must be set to 4.
| u16 | version minor | Minor version of the format. Must be set to 0.
| u64 | root inode | A reference to the inode of the root directory.
| u64 | bytes used | The number of bytes used by the archive. Because
                     SquashFS archives must be padded to a multiple of the underlying
                     device block size, this can be less than the actual file size.
| u64 | ID table | The byte offset at which the id table starts.
| u64 | Xattr table | The byte offset at which the xattr id table starts.
| u64 | Inode table | The byte offset at which the inode table starts.
| u64 | Dir. table | The byte offset at which the directory table starts.
| u64 | Frag table | The byte offset at which the fragment table starts.
| u64 | Export table | The byte offset at which the export table starts.
|===


The Xattr table, fragment table and export table are optional. If they are
omitted from the archive, the respective fields indicating their position
must be set to `0xFFFFFFFFFFFFFFFF` (i.e. all bits set).

Most of the flags only serve an informational purpose and are only useful
when editing the archive to convey the original packer settings.

The only flag that actually carries information is the "Compressor options are
present" flag. In fact, this is the only flag that the Linux kernel
implementation actually tests for.

The compressor options, however, are also only there for informal purpose, as
most compression libraries understand their own stream format irregardless of
the options used to compress and in fact don't provide any options for the
decompressor. In the Linux kernel, the XZ decompressor is currently the only
one that processes those options to pre-allocate the LZMA dictionary if a
non-default size was used.

=== Compression Options

If the compressor options flag is set in the superblock, the superblock is
immediately followed by a single metadata block, which is always uncompressed.

The data stored in this block is compressor dependent.

There are two special cases:

* For LZ4, the compressor options always have to be present.
* The LZMA compressor does not support compressor options, so this section
  must never be present.

For the compressors currently implemented, a 4 to 8 byte payload follows.

The following sub sections outline the contents for each compressor that
supports options. The default values if the options are missing are outlined
as well.

==== GZIP

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | compression level | In the range 1 to 9 (inclusive). Defaults to 9.
| u16 | window size | In the rage 8 to 15 (inclusive). Defaults to 15.
| u16 | strategies | A bit field describing the enabled strategies.
                     If no flags are set, the default strategy is
                     implicitly used. Please consult the ZLIB manual
                     for details on specific strategies.

[cols=">1m,10",frame="none",grid="none",options="header"]
!===
! Value ! Comment
! 0x0001 ! Default strategy.
! 0x0002 ! Filtered.
! 0x0004 ! Huffman Only.
! 0x0008 ! Run Length Encoded.
! 0x0010 ! Fixed.
!===
|===

NOTE: The SquashFS writer typically tries all selected strategies (including
not setting any and letting zlib work with defaults) and stores the result
with the smallest size.

==== XZ


[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | dictionary size | *SHOULD* be >= 8KiB, and must be either a power of
                          2, or the sum of two consecutive powers of 2.
| u32 | Filters | A bit field describing the additional enabled
                  filters attempted to better compress executable
                  code.

[cols=">1m,10",frame="none",grid="none",options="header"]
!===
! Value ! Comment
! 0x0001 ! x86
! 0x0002 ! PowerPC
! 0x0004 ! IA64
! 0x0008 ! ARM
! 0x0010 ! ARM thumb
! 0x0020 ! SPARC
!===
|===

NOTE: A SquashFS writer typically tries all selected VLI filters (including
not setting any and letting libxz work with defaults) and stores the resulting
block that has the smallest size.

Also note that further options, such as XZ presets, are not included. The
compressor typically uses the libxz defaults, i.e. level 6 and not using the
extreme flag. Likewise for `lc`, `lp` and `pb` (defaults are 3, 0 and 2
respectively).

If the encoder chooses to change those values, the decoder will still be
able to read the data, but there is currently no way to convey that those
values were changed.

This is specifically problematic for the compression level, since increasing
the level can result in drastically increasing the decoders memory consumption.

==== LZ4

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | Version | *MUST* be set to 1.
| u32 | Flags |  A bit field describing the enabled LZ4 flags.
                 There is currently only one possible flag:


[cols=">1m,10",frame="none",grid="none",options="header"]
!===
! Value ! Comment
! 0x0001 ! Use LZ4 High Compression(HC) mode.
!===
|===

==== ZSTD

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | compression level | Should be in range 1 to 22 (inclusive). The real
                            maximum is the zstd defined ZSTD_maxCLevel().
                            +
                            +
                            The default value is 15.
|===

==== LZO

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | algorithm | Which variant of LZO to use.

[cols=">1m,10",frame="none",grid="none",options="header"]
!===
! Value ! Comment
! 0 ! lzo1x_1
! 1 ! lzo1x_1_11
! 2 ! lzo1x_1_12
! 3 ! lzo1x_1_15
! 4 ! lzo1x_999 (default)
!===

| u32 | compression level | For lzo1x_999, this can be a value between 0
                            and 9 inclusive (defaults to 8). *MUST* be 0
                            for all other algorithms.
|===

== Data and Fragment Blocks

As outlined in 2.1, file data is packed by dividing the input files into fixed
size chunks (the block size from the super block) that are stored in sequence.

The picture below tries to illustrate this concept:

.Packing of File Data
[%nowrap]
....
         _____ _____ _____ _             _____ _____ _              _
File A: |__A__|__A__|__A__|A|   File B: |__B__|__B__|B|    File C: |C|
           |     |     |   |               |     |   |              |
           | +---+     |   |               |     |   |              |
           | |  +------+   |               |     |   |              |
           | |  |          |               |     |   |              |
           | |  |   +------|---------------+     |   |              |
           | |  |   |   +--|---------------------+   |              |
           | |  |   |   |  |                         |              |
           | |  |   |   |  +-----------------------+ | +------------+
           | |  |   |   |                          | | |
           V V  V   V   V                          V V V
          __ _ ___ ___ ___ __     Fragment block: |A|B|C|
 Output: |_A|A|_A_|_B_|_B_|_F|                       |
                                                   __V__
                            A                     |__F__|
                            |                        |
                            +------------------------+
....

In the above diagram, file A consists of 3 blocks and a single tail end, file B has
2 blocks and one tail end while file C is smaller than block size.


For each file, the blocks are individually compressed and stored on disk
in order.

The tail ends of A and B, together with the entire contents of C are packed
together into a fragment block F, that is compressed and stored on disk once
it is full.

This tail-end-packing is completely optional. The tail ends (or in case of C
the entire file) can also be treated as truncated blocks that expand to less
than block size when uncompressed.


There are no headers in front of data or fragment blocks and there *MUST NOT* be
any gaps between data blocks from a single file, but a SquashFS packer is free
to leave gaps between two different files or fragment blocks. The packer is
also free to decide how to arrange fragments within a fragment block and what
fragments to pack together.

To locate file data, the file inodes store the following information:

* The uncompressed size of the file. From this, the number of blocks can
  be computed:

    block_count = floor(file_size / block_size)   # if tail end packing is used
    block_count = ceil(file_size / block_size)    # otherwise

* The exact location of the first block, if one exists.
* For each consecutive block, the on-disk size.
+
A 32 bit integer is used with bit 24 (i.e. `1 << 24`) set if the block
is stored uncompressed.

* If tail-end-packing was done, the location of the fragment block and a
  byte offset into the uncompressed fragment block. The size of the tail
  end can be computed easily:

    tail_end_size = file_size % block_size

Since a fragment block will likely be referred to by multiple files, inodes
don't store its on-disk location and size directly, but instead use a 32 bit
index into a fragment block lookup table (see the <<Fragment Table>>).

If a data block other than the last one unpacks to less than block size, the
rest of the buffer is filled with 0 bytes. This way, sparse files are
implemented. Specifically if a block has an on-disk size of 0 this translates
to an entire block filled with 0 bytes without having to retrieve any data
from disk.

The on-disk locations of file blocks *MAY* overlap and different file inodes are
free to refer to the same fragment. Typical SquashFS packers would explicitly
use this to for files that are duplicates of others. Doing so is NOT counted
as a hard link.

If an inode references on-disk locations outside the data area, the result is
undefined.

== Inode Table

Inodes are packed into metadata blocks and are not aligned, i.e. they can span
the boundary between metadata blocks. To save space, there are different
inodes for each type (regular file, directory, device, etc.) of varying
contents and size.

To further save more space, inodes come in two flavors: simple inode types
optimized for a simple, standard use case, and extended inode types where
extra information has to be stored.

SquashFS more or less supports 32 bit UIDs and GIDs. As an optimization, those
IDs are stored in a lookup table (see <<ID Table>>) and the inodes themselves
hold a 16 bit index into this table. This allows to 32 bit UIDs/GIDs, but only
among 2^16^ unique values.

The location of the first metadata block is indicated by the inode table start
in the superblock. The inode table ends at the start of the directory table.

=== Common Inode Header

All Inodes share a common header, which contains some common information,
as well as describing the type of Inode which follows. This header has the
following structure:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u16 | type | The type of item described by the inode which follows this header

[cols=">1,10",frame="none",grid="none",options="header"]
!===
! Value ! Comment
!  1 ! Basic Directory
!  2 ! Basic File
!  3 ! Basic Symlink
!  4 ! Basic Block Device
!  5 ! Basic Character Device
!  6 ! Basic Named Pipe (FIFO)
!  7 ! Basic Socked
!  8 ! Extended Directory
!  9 ! Extended File
! 10 ! Extended Symlink
! 11 ! Extended Block Device
! 12 ! Extended Character Device
! 13 ! Extended Named Pipe (FIFO)
! 14 ! Extended Socked
!===

| u16 | permissions | A bit mask representing Unix file system permissions
                      for the inode. This only stores permissions, not the
                      type. The type is reconstructed from the field above.
| u16 | uid | An index into the <<ID Table>>, giving the user ID of the owner.
| u16 | gid | An index into the <<ID Table>>, giving the group ID of the owner.
| u32 | mtime | The unsigned number of seconds (not counting leap
                seconds) since 00:00, Jan 1st, 1970 UTC when the item
                described by the inode was last modified.
| u32 | inode number | Unique node number. Must be at least 1 and at most
                       the inode count from the super block.
|===

=== Directory Inodes

Directory inodes mainly contain a reference into the directory table where
the listing of entries is stored.

A basic directory has an entry listing of at most 64k (uncompressed) and
no extended attributes. The layout of the inode data is as follows:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | block index | The location of the metadata block in the directory
                      table where the entry information starts. This is
                      relative to the directory table location.
| u32 | link count | The number of hard links to this directory.
| u16 | file size | Total (uncompressed) size in bytes of the entry
                    listing in the directory table, including headers.
                    +
                    +
                    This value is 3 bytes larger than the real listing.
                    The Linux kernel creates "." and ".." entries for
                    offsets 0 and 1, and only after 3 looks into the
                    listing, subtracting 3 from the size.
| u16 | block offset | The (uncompressed) offset within the metadata block
                       in the directory table where the directory listing
                       starts.
| u32 | parent inode | The inode number of the parent of this directory. If
                       this is the root directory, this *SHOULD* be 0.
|===

NOTE: For historical reasons, the hard link count of a directory includes
the number of entries in the directory and is initialized to 2 for an empty
directory. I.e. a directory with N entries has at least N + 2 link count.

If the "file size" is set to a value < 4, the directory is empty and there is
no corresponding listing in the directory table.

An extended directory can have a listing that is at most 4GiB in size, may
have extended attributes and can have an optional index for faster lookup:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | link count | Same as above.
| u32 | file size | Same as above.
| u32 | block index | Same as above.
| u32 | parent inode | Same as above.
| u16 | index count | The number of directory index entries following the
                      inode structure.
| u16 | block offset | Same as above.
| u32 | xattr index | An index into the <<Xattr Table>> or `0xFFFFFFFF`
                      if the inode has no extended attributes.
|===


The index follows directly after the inode. See <<Directory Index>> for details on
how the directory index is structured.

=== File Inodes

Basic files can be at most 4 GiB in size (uncompressed), must be located
within the first 4 GiB of the SquashFS image, cannot have any extended
attributes and don't support hard-link or sparse file accounting:


[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | blocks start | The offset from the start of the archive to the first
                       data block.
| u32 | frag index | An index into the <<Fragment Table>> which describes the fragment
                     block that the tail end of this file is stored in. If not used,
                     this is set to `0xFFFFFFFF`.
| u32 | block offset | The (uncompressed) offset within the fragment block
                       where the tail end of this file is. See <<Data and Fragment Blocks>>
                       for details.
| u32 | file size | The (uncompressed) size of this file.
| u32[] | block sizes | An array of consecutive block sizes. See <<Data and Fragment Blocks>> for details.
|===

If 'frag index' is set to `0xFFFFFFFF`, the number of blocks is computed as

  ceil(file_size / block_size)

otherwise, if 'frag index' is a valid fragment index, the block count is
computed as

  floor(file_size / block_size)

and the size of the tail end is

  file_size % block_size


To access a data block, first compute the block index as

  index = floor(offset / block_size)

then compute the on-disk location of the block by summing up the sizes of the
blocks that come before it:

  location = block_start

  for i = 0; i < index; i++
      location += block_sizes[i] & 0x00FFFFFF


The tail end, if present, is accessed by resolving the fragment index through
the fragment lookup table (see the <<Fragment Table>>), loading the fragment block and
using the given 'block offset' into the fragment block.

Extended files have a 64 bit location and size, have additional counters for
sparse file accounting and hard links, and can have extended attributes:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u64 | blocks start | Same as above (but larger).
| u64 | file size | Same as above (but larger).
| u64 | sparse | The number of bytes saved by omitting zero bytes.
                 Used in the kernel for sparse file accounting.
| u32 | link count | The number of hard links to this node.
| u32 | frag index | Same as above.
| u32 | block offset | Same as above.
| u32 | xattr index | An index into the <<Xattr Table>> or `0xFFFFFFFF`
                      if the inode has no extended attributes.
| u32[] | block sizes | Same as above.
|===

=== Symbolic Links

Symbolic links mainly have a target path stored directly after the inode
header, as well as a hard-link counter (yes, you can have hard links to
symlinks):

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | link count | The number of hard links to this symlink.
| u32 | target size | The size in bytes of the target path this symlink
                      points to.
| u8[] | target path | An array of bytes holding the target path this
                       symlink points to. The path is 'target size' bytes
                       long and NOT null-terminated.
|===

The extended symlink type adds an additional extended attribute index:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | link count | Same as above.
| u32 | target size | Same as above.
| u8[] | target path | Same as above.
| u32 | xattr index | An index into the <<Xattr Table>>
|===

=== Device Special Files

Basic device special files only store a hard-link counter and a device number.
The layout is identical for both character and block devices:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | link count | The number of hard links to this entry.
| u32 | device number | The system specific device number.
                        +
                        +
                        On Linux, this consists of major and minor device
                        numbers that can be extracted as follows:

                          major = (dev & 0xFFF00) >> 8.
                          minor = (dev & 0x000FF) | ((dev >> 12) & 0xFFF00)
|===

The extended device file inode adds an additional extended attribute index:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | link count | Same as above.
| u32 | device number | Same as above.
| u32 | xattr index | An index into the <<Xattr Table>>
|===

=== IPC Inodes (FIFO or Socket)

Named pipe (FIFO) and socket special files only add a hard-link counter
after the inode header:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | link count | The number of hard links to this entry.
|===

The extended versions add an additional extended attribute index:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | link count | Same as above.
| u32 | xattr index | An index into the <<Xattr Table>>
|===

== Directory Table

For each directory inode, the directory table stores a linear list of all
entries, with references back to the inodes that describe those entries.

The entry list itself is sorted ASCIIbetically by entry name and split into
multiple runs, each preceded by a short header.

The directory inodes store the total, uncompressed size of the entire listing,
including headers. Using this size, a SquashFS reader can determine if another
header with further entries should be following once it reaches the end of a
run.

To save space, the header indicates a metadata block and a reference inode
number. The entries that follow simply store a difference to that inode number
and an offset into the specified metadata block.

Every time, the inode block changes or the difference of the inode number
to the reference in the header cannot be encoded in 16 bits anymore, a new
header is emitted.

A header must be followed by *AT MOST* 256 entries. If there are more entries,
a new header *MUST* be emitted.

Typically, inode allocation strategies would sort the children of a directory
and then allocate inode numbers incrementally, to optimize directory entry
listings.

Since hard links might be further further away than ±32k of the reference
number, they might require a new header to be emitted. Inode number allocation
and picking of the reference could of course be optimized to prevent this.

The directory header has the following structure:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | count | Number of entries following the header.
| u32 | start | The location of the metadata block in the inode table
                where the inodes are stored. This is relative to the
                inode table start from the super block.
| u32 | inode number | An arbitrary inode number. The entries that follow
                       store their inode number as a difference to this.
|===

The counter is stored off-by-one, i.e. a value of 0 indicates 1 entry follows.
This also makes it impossible to encode a size of 0, which wouldn't make any
sense. Empty directories simply have their size set to 0 in the inode instead,
so no extra dummy header has to be stored or looked up.

The header is followed by multiple entries that each have this structure:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u16 | offset | An offset into the uncompressed inode metadata block.
| s16 | inode offset | The difference of this inode's number to the reference
                       stored in the header.
| u16 | type | The inode type. For extended inodes, the basic type is stored
               here instead.
| u16 | name size | One less than the size of the entry name.
| u8[] | name | The file name of the entry without a trailing null byte. Has
                `name size` + 1 bytes.
|===

In the entry structure itself, the file names are stored without trailing null
bytes. Since a zero length name makes no sense, the name length is stored
off-by-one, i.e. the value 0 cannot be encoded.

The inode type is stored in the entry, but always as the corresponding
basic type.

While the field is technically 16 bits, the kernel implementation currently
imposes an arbitrary limit of 255 on the name size field. Since the field is
off-by-one, this means that a file name in SquashFS can be at most 256
characters long.

=== Directory Index

To speed up lookups on directories with lots of entries, the extended
directory inode can store an index, holding the locations of all directory
headers and the name of the first entry after the header.

When searching for an entry, the reader can then iterate over the index to
find a range of metadata blocks that should contain a given entry and then
only scan over the given range.

To allow for even faster lookups, a new header should be emitted every time
the entry list crosses a metadata block boundary. This narrows the boundary
down to a single metadata block lookup in most cases.

The index entries have the following structure:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | index | This stores a byte offset from the first directory
                header to the current header, as if the uncompressed
                directory metadata blocks were laid out in memory
                consecutively.
| u32 | start | Start offset of a directory table metadata block,
                relative to the directory table start.
| u32 | name size | One less than the size of the entry name.
| u8[] | name | The name of the first entry following the header
                without a trailing null byte.
|===

== Fragment Table

Tail-ends and smaller than block size files can be combined into fragment
blocks that are at most 'block size' bytes long.

The fragment table describes the location and size of the fragment blocks
(not the tail-ends within them).

This is a lookup table which stores entries of the following shape:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u64 | start | The offset within the archive where the fragment block starts
| u32 | size | The on-disk size of the fragment block. If the block is
               uncompressed, bit 24 (i.e. `1 << 24`) is set.
| u32 | unused | *SHOULD* be set to 0.
|===

The table is stored on-disk as described in <<Storing Lookup Tables>>.

The fragment table location in the superblock points to an array of 64 bit
integers that store the on-disk locations of the metadata blocks containing
the lookup table.

Each metadata block can store up to 512 entries (`8192 / 16`).

The "unused" field is there for alignment and *SHOULD* be set to 0, however the
Linux kernel currently ignores this field completely, making it impossible for
Linux to ever re-purpose this field.

== Export Table

To support NFS exports, SquashFS needs a fast way to resolve an inode number
to an inode structure.

For this purpose, a SquashFS archive can optionally contain an export table,
which is basically a flat array of 64 bit inode references, with the inode
number being used as an index into the array.

Because the inode number 0 is not used (reserved as a sentinel value), the
array actually starts at inode number 1 and the index is thus
inode_number - 1.

The array itself is stored in a series of metadata blocks, as outlined in
<<Storing Lookup Tables>>.

Since each block can store 1024 references (`8192 / 8`), there will be
`ceil(inode_count / 1024)` metadata blocks for the entire array.

== ID Table

As outlined in <<Common Inode Header>>, SquashFS supports 32 bit user and group IDs. To
compact the inode table, the unique UID/GID values are collected in a lookup
table and a 16 bit table index is stored in the inode instead.

This lookup table is stored as outlined in <<Storing Lookup Tables>>.

Each metadata block can store up to 2048 IDs (`8192 / 4`).

[[_xattr_table,Xattr Table]]
== Extended Attribute Table

Extended attributes are arbitrary key value pairs attached to inodes. The key
names use dots as separators to create a hierarchy of name spaces.

The key value pairs of all inodes are stored consecutively in a series of
metadata blocks.

The values can either be stored inline, i.e. a key entry is directly followed
by a value, or out-of-line to deduplicate identical values and use a reference
instead. Typically, the first occurrence of a value is stored in line and
every consecutive use of the same value uses an out-of-line reference back to
the first one.

The keys are stored using the following data structure:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u16 | type | A prefix ID for the key name. If the value that follows
               is stored out-of-line, the flag `0x0100` is **OR**ed to the
               type ID.

[cols=">1,10",frame="none",grid="none",options="header"]
!===
! Value ! Comment
! 0 ! Prefix the name with `"user."`
! 1 ! Prefix the name with `"trusted."`
! 2 ! Prefix the name with `"security."`
!===

| u16 | name size | The number of key bytes the follows.
| u8[] | name | The remainder of the key without the prefix and without a
                trailing null byte.
|===

Following the key, this structure is used to store the value:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u32 | value size | The size of the value string. If the value is stored
                     out of line, this is always 8, i.e. the size of an
                     unsigned 64 bit integer.
| u8[] | value | This is 'value size' bytes of arbitrary binary data.
                 If the value is stored out-of-line, this is a 64 bit
                 reference, i.e. a location of a metadata block,
                 shifted left by 16 and **OR**ed with an offset into the
                 uncompressed block, giving the location of another
                 value structure.
|===

The metadata block location given by an out-of-line reference is relative to
the location of the first block.

To actually address a block of key value pairs associated with an inode, a
lookup table is used that specifies the start and size of a sequence of key
value pairs.

All an inode needs to store is a 32 bit index into this table. If two inodes
have an identical attribute sets, the key/value sequence is only written once,
there is only one lookup table entry and both inodes have the same index.

Each lookup table entry has the following structure:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u64 | xattr ref | A reference to the start of the key value block, i.e.
                    the metadata block location shifted left by 16, **OR**ed
                    with an offset into the uncompressed block.
| u32 | count | The number of key value pairs.
| u32 | size | The exact, uncompressed size in bytes of the entire
               block of key value pairs, counting what has been
               written to disk and including the key/value entry
               structures.
|===

This lookup table is stored as outlined in <<Storing Lookup Tables>>

Each metadata block can hold 512 (`8192 / 16`) entries.

However, in contrast to <<Storing Lookup Tables>>, additional data is given before
the list of metadata block locations, to locate the key-value pairs, as well as the
actual number of lookup table entries that are not specified in the super
block.

The 'Xattr table' entry in the superblock gives the absolute location of the
following data structure which is stored on-disk as is, uncompressed:

[cols="1,4,20a",frame="none",grid="none",options="header"]
|===
| Type | Name | Description
| u64 | kv start | The absolute position of the first metadata block holding the
                   key/value pairs.
| u32 | count | The number of entries in the lookup table.
| u32 | unused | *SHOULD* be set to 0, however Linux currently ignores
                 this field completely and squashfs-tools used to leak
                 stack data here, making it impossible for Linux to
                 ever re-purpose this field.
| u64[] | locations | An array holding the absolute on-disk location of each
                      metadata block of the lookup table.
|===

If an inode has a a valid xattr index (i.e. not `0xFFFFFFFF`), the metadata
block index is computed as

  block_idx = floor(index / 512)

which is then used to retrieve the metadata block index from the locations
array.

Once the block has been read from disk and uncompressed, the byte offset into
the metadata block can be computed as

  offset = (index * 16) % 8192

From this position, the structure can be read that holds a reference to the
metadata block that contains the key/value pairs (and byte offset into the
uncompressed block where the pairs start), as well as the number of key/value
pairs and their total, uncompressed size.