path: root/doc/format.txt

                            Squashfs Binary Format
                            **********************

 0) Index
 ********

   0............Index
   1............About
   2............Overview
     2.1........Packing File Data
     2.2........Packing Metadata
     2.3........Storing Lookup Tables
   3............The Superblock
     3.1........Compression Options
       3.1.1....GZIP
       3.1.2....XZ
       3.1.3....LZ4
       3.1.4....ZSTD
       3.1.5....LZO
   4............Data and Fragment Blocks
   5............Inode Table
     5.1........Common Inode Header
     5.2........Directory inodes
     5.3........File Inodes
     5.4........Symbolic Links
     5.5........Device Special Files
     5.6........IPC inodes (FIFO or Socket)
   6............Directory Table
     6.1........Directory Index
   7............Fragment Table
   8............Export Table
   9............ID Table
   10...........Extended Attribute Table

 1) 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 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/


 2) Overview
 ***********

 SquashFS always stores integers in little endian format.

 A SquashFS archive consists of a maximum of nine parts:

         _______________
        |               |  Important information about the archive, including
        |  Superblock 	|  locations of other sections.
        |_______________|
        |               |
        |  Compression  |  If non-default compression options have been used,
        |    options    |  then these are stored here.
        |_______________|
        |               |
        |  Data blocks  |  The contents of the files in the archive,
        |  & fragments  |  split into 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.

 The archive is usually padded with null bytes to make the size a multiple of
 1024 or 4096 bytes, called the "device block size". Some implementations
 insists on the size to be a multiple of the device block size (particularly
 the one in the Linux kernel where the device block size is a configure option).

 The individual parts don't have to be aligned and it is perfectly fine to
 cram them together at single byte alignment.


 2.1) 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.


 2.2) Packing Metadata

 Metadata (e.g. inodes, directory listings, etc...) is stored in special
 metadata blocks.

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

 In contrast to data blocks, metadata blocks are prefixed 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.


 To read a metadata block, seek to the indicated position and read the 16 bit
 header. Then read the size indicated by the lower 15 bit. If the highest bit
 of the header is set, uncompress the data you just read.


 In the SquashFS archive format, metadata is often referenced using a 64 bit
 integer, consisting of the on-disk location of the metadata block, shifted
 left by 16, and OR-ed with a byte offset into the uncompressed block.

 The location is relative to the type of metadata, i.e. for inodes it is an
 offset relative to the start of the inode table.

 The location of a metadata block always points to the 16 bit header.


 2.3) Storing Lookup Tables

 Lookup tables are arrays (i.e. sequences of identical sized data) that are
 addressed by an index in constant time.

 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 is itself uncompressed and not preceded by a header. It is just a
 block of raw values.

 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 / 8129)

 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).


 2.4) Supported Compressors

 The SquashFS format supports the following compressors:

  - zlib deflate (referred to as "gzip" but only uses raw deflate streams)
  - lzo
  - lzma 1
  - 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.

 A data or metadata block is only stored compressed, if compressing actually
 shrinks the input data. If not, the original uncompressed block is stored.
 So 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 file.

 If compatibility with the Linux implementation is desired, the lzma 2 aka xz
 compressor should only use CRC32 checksums. The decompressor in the kernel
 cannot process the data if checksummed with SHA-256.


 3) 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.

 +======+===============+=====================================================+
 | 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.                               |
 |      |               |                                                     |
 |      |               +-------+------+--------------------------------------+
 |      |               | Value | Name | Comment                              |
 |      |               +-------+------+--------------------------------------+
 |      |               | 1     | GZIP | just zlib deflate (no gzip headers!) |
 |      |               | 2     | LZO  |                                      |
 |      |               | 3     | LZMA | LZMA version 1                       |
 |      |               | 4     | XZ   | LZMA version 2 (no XZ headers!)      |
 |      |               | 5     | LZ4  |                                      |
 |      |               | 6     | ZSTD |                                      |
 +------+---------------+-------+------+--------------------------------------+
 | u16  | block log     | The log2 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.                 |
 |      |               |                                                     |
 |      |               +--------+--------------------------------------------+
 |      |               | Value  | Meaing                                     |
 |      |               +--------+--------------------------------------------+
 |      |               | 0x0001 | Inodes are stored uncompressed.            |
 |      |               | 0x0002 | Data blocks are stored uncompressed.       |
 |      |               | 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 (1k or 4k, default |
 |      |               | is 4k), this can be less than the 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).

 Please note that most of the flags are either redundant, or entirely useless
 and only serve an informational purpose.

 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.



 3.1) 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.


 3.1.1) GZIP

 +======+===================+=================================================+
 | Type | Name              | Description                                     |
 +======+===================+=================================================+
 | u32  | compression level | In the range 1 to 9 (inclusive). Defaults to 9. |
 +------+-------------------+-------------------------------------------------+
 | u16  | window size       | In the range 8 to 15 (inclusive) Defaults to 15.|
 +------+-------------------+-------------------------------------------------+
 | u16  | strategies        | A bitfield 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.             |
 |      |                   |                                                 |
 |      |                   +--------+----------------------------------------+
 |      |                   | Value  | Comment                                |
 |      |                   +--------+----------------------------------------+
 |      |                   | 0x0001 | Default strategy.                      |
 |      |                   | 0x0002 | Filtered.                              |
 |      |                   | 0x0004 | Huffman Only.                          |
 |      |                   | 0x0008 | Run Length Encoded.                    |
 |      |                   | 0x0010 | Fixed.                                 |
 +------+-------------------+--------+----------------------------------------+

 Note: If multiple strategies are selected, the SquashFS writer tries all of
 them (including not setting any and letting zlib work with defaults) and
 selects the resulting block that has the smallest size.


 3.1.2) XZ

 +======+===================+=================================================+
 | Type | Name              | Description                                     |
 +======+===================+=================================================+
 | u32  | dictionary size   | Should be > 8KiB, and must be either a power of |
 |      |                   | two, or the sum of two sequential powers of two.|
 +------+-------------------+-------------------------------------------------+
 | u32  | Filters           | A bitfield describing the additional enabled    |
 |      |                   | filters attempted to better compress executable |
 |      |                   | code.                                           |
 |      |                   |                                                 |
 |      |                   +--------+----------------------------------------+
 |      |                   | Value  | Comment                                |
 |      |                   +--------+----------------------------------------+
 |      |                   | 0x0001 | x86                                    |
 |      |                   | 0x0002 | PowerPC                                |
 |      |                   | 0x0004 | IA64                                   |
 |      |                   | 0x0008 | ARM                                    |
 |      |                   | 0x0010 | ARM thumb                              |
 |      |                   | 0x0020 | SPARC                                  |
 +------+-------------------+--------+----------------------------------------+

 Note: If multiple filters are selected, the SquashFS writer tries all of
 them (including not setting any and letting libxz work with defaults) and
 selects the resulting block that has the smallest size.


 3.1.3) LZ4

 +======+===================+=================================================+
 | Type | Name              | Description                                     |
 +======+===================+=================================================+
 | u32  | Version           | Must be set to 1.                               |
 +------+-------------------+-------------------------------------------------+
 | u32  | Flags             | A bitfield describing the enabled LZ4 flags.    |
 |      |                   | There is currently only one possible flag:      |
 |      |                   |                                                 |
 |      |                   +--------+----------------------------------------+
 |      |                   | Value  | Comment                                |
 |      |                   +--------+----------------------------------------+
 |      |                   | 0x0001 | Use LZ4 High Compression(HC) mode.     |
 +------+-------------------+--------+----------------------------------------+

 3.1.4) ZSTD

 +======+===================+=================================================+
 | Type | Name              | Description                                     |
 +======+===================+=================================================+
 | u32  | compression level | Should be in range 1 to 22 (inclusive). The real|
 |      |                   | maximum is the zstd defined ZSTD_maxCLevel().   |
 +------+-------------------+-------------------------------------------------+

 3.1.5) LZO

 +======+===================+=================================================+
 | Type | Name              | Description                                     |
 +======+===================+=================================================+
 | u32  | algorithm         | Which variant of LZO to use.                    |
 |      |                   |                                                 |
 |      |                   +--------+----------------------------------------+
 |      |                   | 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). Has to be 0    |
 |      |                   | for all other algorithms.                       |
 +------+-------------------+-------------------------------------------------+



 4) 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:


           _____ _____ _____ _             _____ _____ _              _
  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__|
			      |                        |
			      +------------------------+

 Figure 1: Packing of File Data.


 In Figure 1, 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 compressed in sequence and stored on disk.

 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. Additional meta
 information, stored in the inodes, is required to locate the blocks of a
 file.


 To locate file data, the 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)    if NOT

  - 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 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


 To access fragment blocks, a lookup table is built (see section 7) that stores
 the on-disk location and size of all fragment blocks. Inodes use an index into
 this table together with a byte offset into the uncompressed fragment block to
 access tail ends.


 To support sparse files, an on-disk block size of 0 is used to indicate that
 no data was written to disk and the SquashFS reader should supply a single
 block filled with 0 bytes instead.


 Typical SquashFS packers would also perform file deduplication. If the
 compressed blocks of a file are identical to blocks already written to disk,
 the already written blocks are reused. Similarly, the tail end can be
 deduplicated independently, by pointing to an existing tail end.


 5) 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 frequently occurring items, 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 section 9) 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.


 5.1) 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:

 +======+==============+======================================================+
 | Type | Name         | Description                                          |
 +======+==============+======================================================+
 | u16  | type         | The type of item described by the inode which follows|
 |      |              | this 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 Socket                                 |
 |      |              | 8     | Extended Directory                           |
 |      |              | 9     | Extended File                                |
 |      |              | 10    | Extended Symlink                             |
 |      |              | 11    | Extended Block Device                        |
 |      |              | 12    | Extended Character Device                    |
 |      |              | 13    | Extended Named Pipe (FIFO)                   |
 |      |              | 14    | Extended Socket                              |
 +------+--------------+-------+----------------------------------------------+
 | u16  | permissions  | A bitmask 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.                |
 +------+--------------+------------------------------------------------------+

 Depending on the type, additional data follows, outlined in sections 4.2
 to 4.6.



 5.2) 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:

 +======+==============+======================================================+
 | 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.   |
 +------+--------------+------------------------------------------------------+
 | 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 will be 0.          |
 +------+--------------+------------------------------------------------------+


 Note that 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. an empty directory has a link count of 2, a directory with one
 entry has 3, a directory with two entries has 4 and so on.


 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:

 +======+==============+======================================================+
 | 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  | One less than the number of directory index entries  |
 |      |              | following the inode structure.                       |
 +------+--------------+------------------------------------------------------+
 | u16  | block offset | Same as above.                                       |
 +------+--------------+------------------------------------------------------+
 | u32  | xattr index  | An index into the xattr lookup table or 0xFFFFFFFF   |
 |      |              | if the inode has no extended attributes.             |
 +------+--------------+------------------------------------------------------+

 The index follows directly after the inode. See section 6.1 for details on
 how the directory index is structured.


 5.3) 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:

 +======+==============+======================================================+
 | 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 section 3    |
 |      |              | for details.                                         |
 +------+--------------+------------------------------------------------------+
 | u32  | file size    | The (uncompressed) size of this file.                |
 +------+--------------+------------------------------------------------------+
 | u32[]| block sizes  | An array of consecutive block sizes. See section 3   |
 |      |              | 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


 There is one corner case: if the on-disk block size is 0, the block does not
 exist on-disk. The reader has to substitute a single block full of 0 bytes
 instead.


 The tail end, if present, is accessed by resolving the fragment index through
 the fragment lookup table (see section 7), 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:


 +======+==============+======================================================+
 | 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 blocks of 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 lookup table or 0xFFFFFFFF   |
 |      |              | if the inode has no extended attributes.             |
 +------+--------------+------------------------------------------------------+
 | u32[]| block sizes  | Same as above.                                       |
 +------+--------------+------------------------------------------------------+


 5.4) 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):

 +======+==============+======================================================+
 | 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:

 +======+==============+=======================================+
 | 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 lookup table. |
 +------+--------------+---------------------------------------+


 5.5) 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:

 +======+===============+=====================================================+
 | 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:

 +======+===============+=========================================+
 | Type | Name          | Description                             |
 +======+===============+=========================================+
 | u32  | link count    | The number of hard links to this entry. |
 +------+---------------+-----------------------------------------+
 | u32  | device number | Same as above.                          |
 +------+---------------+-----------------------------------------+
 | u32  | xattr index   | An index into the xattr lookup table.   |
 +------+---------------+-----------------------------------------+


 5.6) IPC inodes (FIFO or Socket)

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

 +======+=============+=========================================+
 | Type | Name        | Description                             |
 +======+=============+=========================================+
 | u32  | link count  | The number of hard links to this entry. |
 +------+-------------+-----------------------------------------+

 The extended versions add an additional extended attribute index:

 +======+=============+=========================================+
 | Type | Name        | Description                             |
 +======+=============+=========================================+
 | u32  | link count  | Same as above.                          |
 +------+-------------+-----------------------------------------+
 | u32  | xattr index | An index into the xattr lookup table.   |
 +------+-------------+-----------------------------------------+



 6) 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 that are 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. All 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 not be followed by more than 256 entries. If there are more
 entries, a new header is emitted.

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

 Hard links of course break the sequence and require a new header if they are
 further away than +/- 32k of the reference number in the header. Inode number
 allocation and picking of the reference could of course be optimized to
 prevent this.

 The directory header has the following structure:

 +======+==============+======================================================+
 | 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.              |
 +------+--------------+------------------------------------------------------+
 | s32  | inode number | An arbitrary inode number. The entries that follow   |
 |      |              | store their inode number as a difference to this.    |
 +======+==============+======================================================+

 And is followed by entries that each have this structure:

 +======+==============+======================================================+
 | Type | Name         | Description                                          |
 +======+==============+======================================================+
 | u16  | offset       | An offset into the uncompressed inode metadata block.|
 +------+--------------+------------------------------------------------------+
 | s16  | inode offset | The difference of this inodes 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.

 Also note, that the inode type is stored in the entry, but always as a basic
 type!


 6.1) 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:

 +======+==============+======================================================+
 | 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.                        |
 +------+--------------+------------------------------------------------------+


 7) 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:

 +======+==============+======================================================+
 | 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       | Must be set to 0.                                    |
 +------+--------------+------------------------------------------------------+


 The table is stored on-disk, as described in section 2.3.

 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 (= 8129 / 16).


 8) 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
 section 2.3.

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


 9) ID Table
 ***********

 As outlined in section 5.1, 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 section 2.3.

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


 10) 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 be 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:

 +======+===========+=========================================================+
 | 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 ORed to the   |
 |      |           | type ID.                                                |
 |      |           |                                                         |
 |      |           +-------+-------------------------------------------------+
 |      |           | 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 that follows.                   |
 +------+-----------+---------------------------------------------------------+
 | u8[] | name      | The remainder of the key without the prefix and without |
 |      |           | trailing null byte.                                     |
 +------+-----------+---------------------------------------------------------+


 After a key, the following structure follows to store the value:

 +======+============+========================================================+
 | 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 block of key
 value pairs.

 All an inode needs to store is a 32 bit index into this table. If two inodes
 have the identical attribute sets, the key/value block 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:

 +======+============+========================================================+
 | 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 am 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 section 2.3.

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

 However, in contrast to section 2.3, additional data is given before the list
 of metdata 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:

 +=======+===========+========================================================+
 | 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    | Always set this to 0.                                  |
 +-------+-----------+--------------------------------------------------------+
 | 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.