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 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/.

2. 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:

 _______________
|               |  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.

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 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.

2.3. 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).

2.4. 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.

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 streams (no gzip headers!)

2

LZO

3

LZMA

LZMA version 1

4

XZ

LZMA version 2 as used by xz-utils

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 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.

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 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.

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.

3.1.2. XZ

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.

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.

3.1.3. LZ4

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:

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().

The default value is 15.

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). MUST 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:

Packing of File Data
         _____ _____ _____ _             _____ _____ _              _
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.

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 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 216 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 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.

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.

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:

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.

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 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:

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 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 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)

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

Type Name Description

u32

link count

Same as above.

u32

device number

Same as above.

u32

xattr index

An index into the Xattr 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 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, 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:

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.

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:

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.

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

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 (8129 / 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.

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 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.

9. 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).

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 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 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:

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 ORed 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:

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, ORed 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:

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.