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diff --git a/doc/format.adoc b/doc/format.adoc new file mode 100644 index 0000000..f81b6f8 --- /dev/null +++ b/doc/format.adoc @@ -0,0 +1,1067 @@ += Squashfs Binary Format +:toc: left +:toclevels: 4 +:sectnums: + +== About + +SquashFS is a compressed, read-only filesystem for Linux that can also be used +as a flexible, general purpose, compressed archive format, optimized for fast +random access with support for Unix permissions, sparse files and extended +attributes. + +SquashFS supports data and metadata compression through zlib, lz4, lzo, lzma, +xz or zstd. + +For fast random access, compressed files are split up in fixed size blocks +that are compressed separately. +The block size can be set between 4k and 1M (default for squashfs-tools and +squashfs-tools-ng is 128K). + +This document attempts to specify the on-disk format in detail. +It is based on a previous on-line version that was originally written by +Zachary Dremann and subsequently expanded by David Oberhollenzer during +reverse engineering attempts and available here: https://dr-emann.github.io/squashfs/. + +== Overview + +SquashFS always stores integers in little endian format. +The data blocks that make up the SquashFS archive are byte aligned, +i.e. they typically do not care for alignment. +The implementation in the Linux kernel requires the archive itself to +be a multiple of either 1k or 4k in size (called the device block size) +and user space tools typically use 4k to be compatible with both. + +A SquashFS archive consists of a maximum of nine parts: + +[%nowrap] +---- + _______________ +| | Important information about the archive, including +| Superblock | locations of other sections. +|_______________| +| | If non-default compression options have been used, +| Compression | they can optionally be stored here, to facilitate +| options | later, offline editing of the archive. +|_______________| +| | +| Data blocks | The contents of the files in the archive, +| & fragments | split into separately compressed blocks. +|_______________| +| | Metadata (ownership, permissions, etc) for +| Inode table | items in the archive. +|_______________| +| | +| Directory | Directory listings, including file names, and +| table | references to inodes. +|_______________| +| | +| Fragment | Description of fragment locations within the +| table | Datablocks & Fragments section. +|_______________| +| | A mapping from inode numbers to disk locations, +| Export table | required for NFS export. +|_______________| +| | +| UID/GID | A list of unique UID/GIDs. Inodes use an index into +| lookup table | this table to save memory. +|_______________| +| | +| Xattr | Extended attributes for items in the archive. +| table | +|_______________| +---- + +Although the super block details the exact positions of each section, most +implementations, including the one in the Linux kernel, insist on this exact +order. + +=== Packing File Data + +The file data is packed into the archive after the super block (and optional +compressor options). + +Files are divided into fixed size blocks that are separately compressed and +stored in order. SquashFS supports optional tail-end-packing of files that +are not an exact multiple of the block size. The remaining ends can either +be treated as a short block, or can be packed together with the tail ends of +other files in a single "fragment block". Files that are less than block size +are treated the same way. + +If the size of a data or fragment block would exceed the input size after +compression, the original, uncompressed data is stored, so that the size of a +block after compression never exceeds the input block size. + +=== Packing Metadata + +Metadata (e.g. inodes, directory listings, etc...) is treated as a continuous +stream of records that is chopped up into 8KiB blocks that are separately +compressed into special metadata blocks. + +The input size of 8KiB is fixed and independent of the data block size. +Similar to data blocks, if the compressed size would exceed 8KiB, the +uncompressed block is stored instead, so the on-disk size of a metadata +block never exceeds 8KiB. + +Individual entries are allowed to cross the block boundary, so e.g. an inode +may be located at the end of a metadata block with some part of it located at +the start of the next block. Both have to be read and decompressed when +reading this inode. If an entry is written across block boundaries, there +*MUST NOT* be any gap between the compressed metadata blocks on-disk. + + +In contrast to data blocks, every metadata block is preceded by a single, +16 bit unsigned integer. This integer holds the on-disk size of the block +that follows. The MSB is set if the block is stored uncompressed. Whenever +a metadata block is referenced, the position of this integer is given. + +To read a metadata block, seek to the indicated position and read the 16 bit +header. Sanity check that the lower 15 bit are less than 8KiB and proceed +to read that many bytes. If the highest bit of the header is cleared, +uncompress the data into an 8KiB buffer that *MUST NOT* overflow. + + +In the SquashFS archive format, metadata entries (e.g. inodes) are often +referenced using a 64 bit integer. The lower 16 bit hold an offset into the +uncompressed block and the upper 48 bit point to the on-disk location of the +block. + +The on-disk location is relative to the type of metadata entry, e.g. for +inodes it is relative to the start of the inode table given by the +super block. + +=== Storing Lookup Tables + +Lookup tables are arrays (i.e. sequences of identical sized records) that are +addressed by an index. + +Such tables are stored in the SquashFS format as metadata blocks, i.e. by +dividing the table data into 8KiB chunks that are separately compressed and +stored in sequence. + +To allow constant time lookup, a list of 64 bit unsigned integers is stored, +holding the on-disk locations of each metadata block. + +This list itself is stored uncompressed and not preceded by a header. + +When referring to a lookup table, the superblock gives the number of table +entries and points to this location list. + +Since the table entry size is a known, fixed value, the required number of +metadata blocks can be computed: + + block_count = ceil(table_count * entry_size / 8192) + +Which is also the number of 64 bit integers in the location list. + +When resolving a lookup table index, first work out the index of the +metadata block: + + meta_index = floor(index * entry_size / 8192) + +Using this index on the location list yields the on-disk location of +the metadata block containing the entry. + +After reading this metadata block, the byte offset into the block can +be computed to get the entry: + + offset = index * entry_size % 8192 + +The location list can be cached in memory. Resolving an index requires at +worst a single metadata block read (at most 8194 bytes fetched from an +unaligned on-disk location). + +=== Supported Compressors + +The SquashFS format supports the following compressors: + +* zlib deflate (referred to as "gzip" but only uses raw zlib streams) +* lzo +* lzma 1 (considered deprecated) +* lzma 2 (referred to as "xz") +* lz4 +* zstd + +The archive can only specify one compressor in the super block and has to use +it for both file data and metadata compression. Using one compressor for data +and switching to a different compressor for e.g. inodes is not supported. + +While it is technically not possible to pick a "null" compressor in the super +block, an implementation can still deliberately write only uncompressed blocks +to a SquashFS archive, or choose to store certain metadata blocks without +compression. + +The lzma 2 aka xz compressor *MUST* use `CRC32` checksums only. Using `SHA-256` is +not supported. + +== The Superblock + +The superblock is the first section of a SquashFS archive. It is always +96 bytes in size and contains important information about the archive, +including the locations of other sections. + +[cols="1,3,13a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | magic | Must be set to `0x73717368` ("hsqs" on disk). +| u32 | inode count | The number of inodes stored in the archive. +| u32 | mod time | Last modification time of the archive. Count seconds + since 00:00, Jan 1st 1970 UTC (not counting leap + seconds). This is unsigned, so it expires in the + year 2106 (as opposed to 2038). +| u32 | block size | The size of a data block in bytes. Must be a power + of two between 4096 (4k) and 1048576 (1 MiB). +| u32 | frag count | The number of entries in the fragment table. +| u16 | compressor | An ID designating the compressor used for both data + and meta data blocks. + +[cols=">1,2,8",frame="none",grid="none",options="header"] +!=== +! Value ! Name ! Comment +! 1 ! GZIP ! just zlib streams (no gzip headers\!) +! 2 ! LZO ! +! 3 ! LZMA ! LZMA version 1 +! 4 ! XZ ! LZMA version 2 as used by xz-utils +! 5 ! LZ4 ! +! 6 ! ZSTD ! +!=== + +| u16 | block log | The log~2~ of the block size. If the two fields do not + agree, the archive is considered corrupted. +| u16 | flags | Bit wise *OR* of the flag bits below. + + +[cols=">1m,10",frame="none",grid="none",options="header"] +!=== +! Value ! Meaning +! 0x0001 ! Inodes are stored uncompressed. +! 0x0002 ! Data blocks are stored uncompressed. +! 0x0004 ! Unused, should always be unset. +! 0x0008 ! Fragments are stored uncompressed. +! 0x0010 ! Fragments are not used. +! 0x0020 ! Fragments are always generated. +! 0x0040 ! Data has been deduplicated. +! 0x0080 ! NFS export table exists. +! 0x0100 ! Xattrs are stored uncompressed. +! 0x0200 ! There are no Xattrs in the archive. +! 0x0400 ! Compressor options are present. +! 0x0800 ! The ID table is uncompressed. +!=== + +| u16 | id count | The number of entries in the ID lookup table. +| u16 | version major | Major version of the format. Must be set to 4. +| u16 | version minor | Minor version of the format. Must be set to 0. +| u64 | root inode | A reference to the inode of the root directory. +| u64 | bytes used | The number of bytes used by the archive. Because + SquashFS archives must be padded to a multiple of the underlying + device block size, this can be less than the actual file size. +| u64 | ID table | The byte offset at which the id table starts. +| u64 | Xattr table | The byte offset at which the xattr id table starts. +| u64 | Inode table | The byte offset at which the inode table starts. +| u64 | Dir. table | The byte offset at which the directory table starts. +| u64 | Frag table | The byte offset at which the fragment table starts. +| u64 | Export table | The byte offset at which the export table starts. +|=== + + +The Xattr table, fragment table and export table are optional. If they are +omitted from the archive, the respective fields indicating their position +must be set to `0xFFFFFFFFFFFFFFFF` (i.e. all bits set). + +Most of the flags only serve an informational purpose and are only useful +when editing the archive to convey the original packer settings. + +The only flag that actually carries information is the "Compressor options are +present" flag. In fact, this is the only flag that the Linux kernel +implementation actually tests for. + +The compressor options, however, are also only there for informal purpose, as +most compression libraries understand their own stream format irregardless of +the options used to compress and in fact don't provide any options for the +decompressor. In the Linux kernel, the XZ decompressor is currently the only +one that processes those options to pre-allocate the LZMA dictionary if a +non-default size was used. + +=== Compression Options + +If the compressor options flag is set in the superblock, the superblock is +immediately followed by a single metadata block, which is always uncompressed. + +The data stored in this block is compressor dependent. + +There are two special cases: + +* For LZ4, the compressor options always have to be present. +* The LZMA compressor does not support compressor options, so this section + must never be present. + +For the compressors currently implemented, a 4 to 8 byte payload follows. + +The following sub sections outline the contents for each compressor that +supports options. The default values if the options are missing are outlined +as well. + +==== GZIP + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | compression level | In the range 1 to 9 (inclusive). Defaults to 9. +| u16 | window size | In the rage 8 to 15 (inclusive). Defaults to 15. +| u16 | strategies | A bit field describing the enabled strategies. + If no flags are set, the default strategy is + implicitly used. Please consult the ZLIB manual + for details on specific strategies. + +[cols=">1m,10",frame="none",grid="none",options="header"] +!=== +! Value ! Comment +! 0x0001 ! Default strategy. +! 0x0002 ! Filtered. +! 0x0004 ! Huffman Only. +! 0x0008 ! Run Length Encoded. +! 0x0010 ! Fixed. +!=== +|=== + +NOTE: The SquashFS writer typically tries all selected strategies (including +not setting any and letting zlib work with defaults) and stores the result +with the smallest size. + +==== XZ + + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | dictionary size | *SHOULD* be >= 8KiB, and must be either a power of + 2, or the sum of two consecutive powers of 2. +| u32 | Filters | A bit field describing the additional enabled + filters attempted to better compress executable + code. + +[cols=">1m,10",frame="none",grid="none",options="header"] +!=== +! Value ! Comment +! 0x0001 ! x86 +! 0x0002 ! PowerPC +! 0x0004 ! IA64 +! 0x0008 ! ARM +! 0x0010 ! ARM thumb +! 0x0020 ! SPARC +!=== +|=== + +NOTE: A SquashFS writer typically tries all selected VLI filters (including +not setting any and letting libxz work with defaults) and stores the resulting +block that has the smallest size. + +Also note that further options, such as XZ presets, are not included. The +compressor typically uses the libxz defaults, i.e. level 6 and not using the +extreme flag. Likewise for `lc`, `lp` and `pb` (defaults are 3, 0 and 2 +respectively). + +If the encoder chooses to change those values, the decoder will still be +able to read the data, but there is currently no way to convey that those +values were changed. + +This is specifically problematic for the compression level, since increasing +the level can result in drastically increasing the decoders memory consumption. + +==== LZ4 + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | Version | *MUST* be set to 1. +| u32 | Flags | A bit field describing the enabled LZ4 flags. + There is currently only one possible flag: + + +[cols=">1m,10",frame="none",grid="none",options="header"] +!=== +! Value ! Comment +! 0x0001 ! Use LZ4 High Compression(HC) mode. +!=== +|=== + +==== ZSTD + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | compression level | Should be in range 1 to 22 (inclusive). The real + maximum is the zstd defined ZSTD_maxCLevel(). + + + + + The default value is 15. +|=== + +==== LZO + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | algorithm | Which variant of LZO to use. + +[cols=">1m,10",frame="none",grid="none",options="header"] +!=== +! Value ! Comment +! 0 ! lzo1x_1 +! 1 ! lzo1x_1_11 +! 2 ! lzo1x_1_12 +! 3 ! lzo1x_1_15 +! 4 ! lzo1x_999 (default) +!=== + +| u32 | compression level | For lzo1x_999, this can be a value between 0 + and 9 inclusive (defaults to 8). *MUST* be 0 + for all other algorithms. +|=== + +== Data and Fragment Blocks + +As outlined in 2.1, file data is packed by dividing the input files into fixed +size chunks (the block size from the super block) that are stored in sequence. + +The picture below tries to illustrate this concept: + +.Packing of File Data +[%nowrap] +.... + _____ _____ _____ _ _____ _____ _ _ +File A: |__A__|__A__|__A__|A| File B: |__B__|__B__|B| File C: |C| + | | | | | | | | + | +---+ | | | | | | + | | +------+ | | | | | + | | | | | | | | + | | | +------|---------------+ | | | + | | | | +--|---------------------+ | | + | | | | | | | | + | | | | | +-----------------------+ | +------------+ + | | | | | | | | + V V V V V V V V + __ _ ___ ___ ___ __ Fragment block: |A|B|C| + Output: |_A|A|_A_|_B_|_B_|_F| | + __V__ + A |__F__| + | | + +------------------------+ +.... + +In the above diagram, file A consists of 3 blocks and a single tail end, file B has +2 blocks and one tail end while file C is smaller than block size. + + +For each file, the blocks are individually compressed and stored on disk +in order. + +The tail ends of A and B, together with the entire contents of C are packed +together into a fragment block F, that is compressed and stored on disk once +it is full. + +This tail-end-packing is completely optional. The tail ends (or in case of C +the entire file) can also be treated as truncated blocks that expand to less +than block size when uncompressed. + + +There are no headers in front of data or fragment blocks and there *MUST NOT* be +any gaps between data blocks from a single file, but a SquashFS packer is free +to leave gaps between two different files or fragment blocks. The packer is +also free to decide how to arrange fragments within a fragment block and what +fragments to pack together. + +To locate file data, the file inodes store the following information: + +* The uncompressed size of the file. From this, the number of blocks can + be computed: + + block_count = floor(file_size / block_size) # if tail end packing is used + block_count = ceil(file_size / block_size) # otherwise + +* The exact location of the first block, if one exists. +* For each consecutive block, the on-disk size. ++ +A 32 bit integer is used with bit 24 (i.e. `1 << 24`) set if the block +is stored uncompressed. + +* If tail-end-packing was done, the location of the fragment block and a + byte offset into the uncompressed fragment block. The size of the tail + end can be computed easily: + + tail_end_size = file_size % block_size + +Since a fragment block will likely be referred to by multiple files, inodes +don't store its on-disk location and size directly, but instead use a 32 bit +index into a fragment block lookup table (see the <<Fragment Table>>). + +If a data block other than the last one unpacks to less than block size, the +rest of the buffer is filled with 0 bytes. This way, sparse files are +implemented. Specifically if a block has an on-disk size of 0 this translates +to an entire block filled with 0 bytes without having to retrieve any data +from disk. + +The on-disk locations of file blocks *MAY* overlap and different file inodes are +free to refer to the same fragment. Typical SquashFS packers would explicitly +use this to for files that are duplicates of others. Doing so is NOT counted +as a hard link. + +If an inode references on-disk locations outside the data area, the result is +undefined. + +== Inode Table + +Inodes are packed into metadata blocks and are not aligned, i.e. they can span +the boundary between metadata blocks. To save space, there are different +inodes for each type (regular file, directory, device, etc.) of varying +contents and size. + +To further save more space, inodes come in two flavors: simple inode types +optimized for a simple, standard use case, and extended inode types where +extra information has to be stored. + +SquashFS more or less supports 32 bit UIDs and GIDs. As an optimization, those +IDs are stored in a lookup table (see <<ID Table>>) and the inodes themselves +hold a 16 bit index into this table. This allows to 32 bit UIDs/GIDs, but only +among 2^16^ unique values. + +The location of the first metadata block is indicated by the inode table start +in the superblock. The inode table ends at the start of the directory table. + +=== Common Inode Header + +All Inodes share a common header, which contains some common information, +as well as describing the type of Inode which follows. This header has the +following structure: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u16 | type | The type of item described by the inode which follows this header + +[cols=">1,10",frame="none",grid="none",options="header"] +!=== +! Value ! Comment +! 1 ! Basic Directory +! 2 ! Basic File +! 3 ! Basic Symlink +! 4 ! Basic Block Device +! 5 ! Basic Character Device +! 6 ! Basic Named Pipe (FIFO) +! 7 ! Basic Socked +! 8 ! Extended Directory +! 9 ! Extended File +! 10 ! Extended Symlink +! 11 ! Extended Block Device +! 12 ! Extended Character Device +! 13 ! Extended Named Pipe (FIFO) +! 14 ! Extended Socked +!=== + +| u16 | permissions | A bit mask representing Unix file system permissions + for the inode. This only stores permissions, not the + type. The type is reconstructed from the field above. +| u16 | uid | An index into the <<ID Table>>, giving the user ID of the owner. +| u16 | gid | An index into the <<ID Table>>, giving the group ID of the owner. +| u32 | mtime | The unsigned number of seconds (not counting leap + seconds) since 00:00, Jan 1st, 1970 UTC when the item + described by the inode was last modified. +| u32 | inode number | Unique node number. Must be at least 1 and at most + the inode count from the super block. +|=== + +=== Directory Inodes + +Directory inodes mainly contain a reference into the directory table where +the listing of entries is stored. + +A basic directory has an entry listing of at most 64k (uncompressed) and +no extended attributes. The layout of the inode data is as follows: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | block index | The location of the metadata block in the directory + table where the entry information starts. This is + relative to the directory table location. +| u32 | link count | The number of hard links to this directory. +| u16 | file size | Total (uncompressed) size in bytes of the entry + listing in the directory table, including headers. + + + + + This value is 3 bytes larger than the real listing. + The Linux kernel creates "." and ".." entries for + offsets 0 and 1, and only after 3 looks into the + listing, subtracting 3 from the size. +| u16 | block offset | The (uncompressed) offset within the metadata block + in the directory table where the directory listing + starts. +| u32 | parent inode | The inode number of the parent of this directory. If + this is the root directory, this *SHOULD* be 0. +|=== + +NOTE: For historical reasons, the hard link count of a directory includes +the number of entries in the directory and is initialized to 2 for an empty +directory. I.e. a directory with N entries has at least N + 2 link count. + +If the "file size" is set to a value < 4, the directory is empty and there is +no corresponding listing in the directory table. + +An extended directory can have a listing that is at most 4GiB in size, may +have extended attributes and can have an optional index for faster lookup: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | link count | Same as above. +| u32 | file size | Same as above. +| u32 | block index | Same as above. +| u32 | parent inode | Same as above. +| u16 | index count | The number of directory index entries following the + inode structure. +| u16 | block offset | Same as above. +| u32 | xattr index | An index into the <<Xattr Table>> or `0xFFFFFFFF` + if the inode has no extended attributes. +|=== + + +The index follows directly after the inode. See <<Directory Index>> for details on +how the directory index is structured. + +=== File Inodes + +Basic files can be at most 4 GiB in size (uncompressed), must be located +within the first 4 GiB of the SquashFS image, cannot have any extended +attributes and don't support hard-link or sparse file accounting: + + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | blocks start | The offset from the start of the archive to the first + data block. +| u32 | frag index | An index into the <<Fragment Table>> which describes the fragment + block that the tail end of this file is stored in. If not used, + this is set to `0xFFFFFFFF`. +| u32 | block offset | The (uncompressed) offset within the fragment block + where the tail end of this file is. See <<Data and Fragment Blocks>> + for details. +| u32 | file size | The (uncompressed) size of this file. +| u32[] | block sizes | An array of consecutive block sizes. See <<Data and Fragment Blocks>> for details. +|=== + +If 'frag index' is set to `0xFFFFFFFF`, the number of blocks is computed as + + ceil(file_size / block_size) + +otherwise, if 'frag index' is a valid fragment index, the block count is +computed as + + floor(file_size / block_size) + +and the size of the tail end is + + file_size % block_size + + +To access a data block, first compute the block index as + + index = floor(offset / block_size) + +then compute the on-disk location of the block by summing up the sizes of the +blocks that come before it: + + location = block_start + + for i = 0; i < index; i++ + location += block_sizes[i] & 0x00FFFFFF + + +The tail end, if present, is accessed by resolving the fragment index through +the fragment lookup table (see the <<Fragment Table>>), loading the fragment block and +using the given 'block offset' into the fragment block. + +Extended files have a 64 bit location and size, have additional counters for +sparse file accounting and hard links, and can have extended attributes: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u64 | blocks start | Same as above (but larger). +| u64 | file size | Same as above (but larger). +| u64 | sparse | The number of bytes saved by omitting zero bytes. + Used in the kernel for sparse file accounting. +| u32 | link count | The number of hard links to this node. +| u32 | frag index | Same as above. +| u32 | block offset | Same as above. +| u32 | xattr index | An index into the <<Xattr Table>> or `0xFFFFFFFF` + if the inode has no extended attributes. +| u32[] | block sizes | Same as above. +|=== + +=== Symbolic Links + +Symbolic links mainly have a target path stored directly after the inode +header, as well as a hard-link counter (yes, you can have hard links to +symlinks): + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | link count | The number of hard links to this symlink. +| u32 | target size | The size in bytes of the target path this symlink + points to. +| u8[] | target path | An array of bytes holding the target path this + symlink points to. The path is 'target size' bytes + long and NOT null-terminated. +|=== + +The extended symlink type adds an additional extended attribute index: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | link count | Same as above. +| u32 | target size | Same as above. +| u8[] | target path | Same as above. +| u32 | xattr index | An index into the <<Xattr Table>> +|=== + +=== Device Special Files + +Basic device special files only store a hard-link counter and a device number. +The layout is identical for both character and block devices: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | link count | The number of hard links to this entry. +| u32 | device number | The system specific device number. + + + + + On Linux, this consists of major and minor device + numbers that can be extracted as follows: + + major = (dev & 0xFFF00) >> 8. + minor = (dev & 0x000FF) | ((dev >> 12) & 0xFFF00) +|=== + +The extended device file inode adds an additional extended attribute index: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | link count | Same as above. +| u32 | device number | Same as above. +| u32 | xattr index | An index into the <<Xattr Table>> +|=== + +=== IPC Inodes (FIFO or Socket) + +Named pipe (FIFO) and socket special files only add a hard-link counter +after the inode header: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | link count | The number of hard links to this entry. +|=== + +The extended versions add an additional extended attribute index: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | link count | Same as above. +| u32 | xattr index | An index into the <<Xattr Table>> +|=== + +== Directory Table + +For each directory inode, the directory table stores a linear list of all +entries, with references back to the inodes that describe those entries. + +The entry list itself is sorted ASCIIbetically by entry name and split into +multiple runs, each preceded by a short header. + +The directory inodes store the total, uncompressed size of the entire listing, +including headers. Using this size, a SquashFS reader can determine if another +header with further entries should be following once it reaches the end of a +run. + +To save space, the header indicates a metadata block and a reference inode +number. The entries that follow simply store a difference to that inode number +and an offset into the specified metadata block. + +Every time, the inode block changes or the difference of the inode number +to the reference in the header cannot be encoded in 16 bits anymore, a new +header is emitted. + +A header must be followed by *AT MOST* 256 entries. If there are more entries, +a new header *MUST* be emitted. + +Typically, inode allocation strategies would sort the children of a directory +and then allocate inode numbers incrementally, to optimize directory entry +listings. + +Since hard links might be further further away than ±32k of the reference +number, they might require a new header to be emitted. Inode number allocation +and picking of the reference could of course be optimized to prevent this. + +The directory header has the following structure: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | count | Number of entries following the header. +| u32 | start | The location of the metadata block in the inode table + where the inodes are stored. This is relative to the + inode table start from the super block. +| 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: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u16 | offset | An offset into the uncompressed inode metadata block. +| s16 | inode offset | The difference of this inode's number to the reference + stored in the header. +| u16 | type | The inode type. For extended inodes, the basic type is stored + here instead. +| u16 | name size | One less than the size of the entry name. +| u8[] | name | The file name of the entry without a trailing null byte. Has + `name size` + 1 bytes. +|=== + +In the entry structure itself, the file names are stored without trailing null +bytes. Since a zero length name makes no sense, the name length is stored +off-by-one, i.e. the value 0 cannot be encoded. + +The inode type is stored in the entry, but always as the corresponding +basic type. + +While the field is technically 16 bits, the kernel implementation currently +imposes an arbitrary limit of 255 on the name size field. Since the field is +off-by-one, this means that a file name in SquashFS can be at most 256 +characters long. + +=== Directory Index + +To speed up lookups on directories with lots of entries, the extended +directory inode can store an index, holding the locations of all directory +headers and the name of the first entry after the header. + +When searching for an entry, the reader can then iterate over the index to +find a range of metadata blocks that should contain a given entry and then +only scan over the given range. + +To allow for even faster lookups, a new header should be emitted every time +the entry list crosses a metadata block boundary. This narrows the boundary +down to a single metadata block lookup in most cases. + +The index entries have the following structure: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | index | This stores a byte offset from the first directory + header to the current header, as if the uncompressed + directory metadata blocks were laid out in memory + consecutively. +| u32 | start | Start offset of a directory table metadata block, + relative to the directory table start. +| u32 | name size | One less than the size of the entry name. +| u8[] | name | The name of the first entry following the header + without a trailing null byte. +|=== + +== Fragment Table + +Tail-ends and smaller than block size files can be combined into fragment +blocks that are at most 'block size' bytes long. + +The fragment table describes the location and size of the fragment blocks +(not the tail-ends within them). + +This is a lookup table which stores entries of the following shape: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u64 | start | The offset within the archive where the fragment block starts +| u32 | size | The on-disk size of the fragment block. If the block is + uncompressed, bit 24 (i.e. `1 << 24`) is set. +| u32 | unused | *SHOULD* be set to 0. +|=== + +The table is stored on-disk as described in <<Storing Lookup Tables>>. + +The fragment table location in the superblock points to an array of 64 bit +integers that store the on-disk locations of the metadata blocks containing +the lookup table. + +Each metadata block can store up to 512 entries (`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. + +== Export Table + +To support NFS exports, SquashFS needs a fast way to resolve an inode number +to an inode structure. + +For this purpose, a SquashFS archive can optionally contain an export table, +which is basically a flat array of 64 bit inode references, with the inode +number being used as an index into the array. + +Because the inode number 0 is not used (reserved as a sentinel value), the +array actually starts at inode number 1 and the index is thus +inode_number - 1. + +The array itself is stored in a series of metadata blocks, as outlined in +<<Storing Lookup Tables>>. + +Since each block can store 1024 references (`8192 / 8`), there will be +`ceil(inode_count / 1024)` metadata blocks for the entire array. + +== ID Table + +As outlined in <<Common Inode Header>>, SquashFS supports 32 bit user and group IDs. To +compact the inode table, the unique UID/GID values are collected in a lookup +table and a 16 bit table index is stored in the inode instead. + +This lookup table is stored as outlined in <<Storing Lookup Tables>>. + +Each metadata block can store up to 2048 IDs (`8192 / 4`). + +[[_xattr_table,Xattr Table]] +== Extended Attribute Table + +Extended attributes are arbitrary key value pairs attached to inodes. The key +names use dots as separators to create a hierarchy of name spaces. + +The key value pairs of all inodes are stored consecutively in a series of +metadata blocks. + +The values can either be stored inline, i.e. a key entry is directly followed +by a value, or out-of-line to deduplicate identical values and use a reference +instead. Typically, the first occurrence of a value is stored in line and +every consecutive use of the same value uses an out-of-line reference back to +the first one. + +The keys are stored using the following data structure: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u16 | type | A prefix ID for the key name. If the value that follows + is stored out-of-line, the flag `0x0100` is **OR**ed to the + type ID. + +[cols=">1,10",frame="none",grid="none",options="header"] +!=== +! Value ! Comment +! 0 ! Prefix the name with `"user."` +! 1 ! Prefix the name with `"trusted."` +! 2 ! Prefix the name with `"security."` +!=== + +| u16 | name size | The number of key bytes the follows. +| u8[] | name | The remainder of the key without the prefix and without a + trailing null byte. +|=== + +Following the key, this structure is used to store the value: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u32 | value size | The size of the value string. If the value is stored + out of line, this is always 8, i.e. the size of an + unsigned 64 bit integer. +| u8[] | value | This is 'value size' bytes of arbitrary binary data. + If the value is stored out-of-line, this is a 64 bit + reference, i.e. a location of a metadata block, + shifted left by 16 and **OR**ed with an offset into the + uncompressed block, giving the location of another + value structure. +|=== + +The metadata block location given by an out-of-line reference is relative to +the location of the first block. + +To actually address a block of key value pairs associated with an inode, a +lookup table is used that specifies the start and size of a sequence of key +value pairs. + +All an inode needs to store is a 32 bit index into this table. If two inodes +have an identical attribute sets, the key/value sequence is only written once, +there is only one lookup table entry and both inodes have the same index. + +Each lookup table entry has the following structure: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u64 | xattr ref | A reference to the start of the key value block, i.e. + the metadata block location shifted left by 16, **OR**ed + with an offset into the uncompressed block. +| u32 | count | The number of key value pairs. +| u32 | size | The exact, uncompressed size in bytes of the entire + block of key value pairs, counting what has been + written to disk and including the key/value entry + structures. +|=== + +This lookup table is stored as outlined in <<Storing Lookup Tables>> + +Each metadata block can hold 512 (`8192 / 16`) entries. + +However, in contrast to <<Storing Lookup Tables>>, additional data is given before +the list of metadata block locations, to locate the key-value pairs, as well as the +actual number of lookup table entries that are not specified in the super +block. + +The 'Xattr table' entry in the superblock gives the absolute location of the +following data structure which is stored on-disk as is, uncompressed: + +[cols="1,4,20a",frame="none",grid="none",options="header"] +|=== +| Type | Name | Description +| u64 | kv start | The absolute position of the first metadata block holding the + key/value pairs. +| u32 | count | The number of entries in the lookup table. +| u32 | unused | *SHOULD* be set to 0, however Linux currently ignores + this field completely and squashfs-tools used to leak + stack data here, making it impossible for Linux to + ever re-purpose this field. +| u64[] | locations | An array holding the absolute on-disk location of each + metadata block of the lookup table. +|=== + +If an inode has a a valid xattr index (i.e. not `0xFFFFFFFF`), the metadata +block index is computed as + + block_idx = floor(index / 512) + +which is then used to retrieve the metadata block index from the locations +array. + +Once the block has been read from disk and uncompressed, the byte offset into +the metadata block can be computed as + + offset = (index * 16) % 8192 + +From this position, the structure can be read that holds a reference to the +metadata block that contains the key/value pairs (and byte offset into the +uncompressed block where the pairs start), as well as the number of key/value +pairs and their total, uncompressed size. |