203 lines
9.4 KiB
ReStructuredText
203 lines
9.4 KiB
ReStructuredText
===========================================================
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LZO stream format as understood by Linux's LZO decompressor
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===========================================================
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Introduction
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============
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This is not a specification. No specification seems to be publicly available
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for the LZO stream format. This document describes what input format the LZO
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decompressor as implemented in the Linux kernel understands. The file subject
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of this analysis is lib/lzo/lzo1x_decompress_safe.c. No analysis was made on
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the compressor nor on any other implementations though it seems likely that
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the format matches the standard one. The purpose of this document is to
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better understand what the code does in order to propose more efficient fixes
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for future bug reports.
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Description
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===========
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The stream is composed of a series of instructions, operands, and data. The
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instructions consist in a few bits representing an opcode, and bits forming
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the operands for the instruction, whose size and position depend on the
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opcode and on the number of literals copied by previous instruction. The
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operands are used to indicate:
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- a distance when copying data from the dictionary (past output buffer)
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- a length (number of bytes to copy from dictionary)
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- the number of literals to copy, which is retained in variable "state"
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as a piece of information for next instructions.
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Optionally depending on the opcode and operands, extra data may follow. These
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extra data can be a complement for the operand (eg: a length or a distance
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encoded on larger values), or a literal to be copied to the output buffer.
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The first byte of the block follows a different encoding from other bytes, it
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seems to be optimized for literal use only, since there is no dictionary yet
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prior to that byte.
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Lengths are always encoded on a variable size starting with a small number
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of bits in the operand. If the number of bits isn't enough to represent the
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length, up to 255 may be added in increments by consuming more bytes with a
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rate of at most 255 per extra byte (thus the compression ratio cannot exceed
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around 255:1). The variable length encoding using #bits is always the same::
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length = byte & ((1 << #bits) - 1)
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if (!length) {
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length = ((1 << #bits) - 1)
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length += 255*(number of zero bytes)
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length += first-non-zero-byte
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}
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length += constant (generally 2 or 3)
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For references to the dictionary, distances are relative to the output
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pointer. Distances are encoded using very few bits belonging to certain
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ranges, resulting in multiple copy instructions using different encodings.
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Certain encodings involve one extra byte, others involve two extra bytes
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forming a little-endian 16-bit quantity (marked LE16 below).
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After any instruction except the large literal copy, 0, 1, 2 or 3 literals
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are copied before starting the next instruction. The number of literals that
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were copied may change the meaning and behaviour of the next instruction. In
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practice, only one instruction needs to know whether 0, less than 4, or more
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literals were copied. This is the information stored in the <state> variable
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in this implementation. This number of immediate literals to be copied is
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generally encoded in the last two bits of the instruction but may also be
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taken from the last two bits of an extra operand (eg: distance).
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End of stream is declared when a block copy of distance 0 is seen. Only one
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instruction may encode this distance (0001HLLL), it takes one LE16 operand
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for the distance, thus requiring 3 bytes.
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.. important::
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In the code some length checks are missing because certain instructions
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are called under the assumption that a certain number of bytes follow
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because it has already been guaranteed before parsing the instructions.
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They just have to "refill" this credit if they consume extra bytes. This
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is an implementation design choice independent on the algorithm or
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encoding.
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Versions
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0: Original version
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1: LZO-RLE
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Version 1 of LZO implements an extension to encode runs of zeros using run
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length encoding. This improves speed for data with many zeros, which is a
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common case for zram. This modifies the bitstream in a backwards compatible way
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(v1 can correctly decompress v0 compressed data, but v0 cannot read v1 data).
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For maximum compatibility, both versions are available under different names
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(lzo and lzo-rle). Differences in the encoding are noted in this document with
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e.g.: version 1 only.
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Byte sequences
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==============
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First byte encoding::
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0..16 : follow regular instruction encoding, see below. It is worth
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noting that code 16 will represent a block copy from the
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dictionary which is empty, and that it will always be
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invalid at this place.
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17 : bitstream version. If the first byte is 17, and compressed
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stream length is at least 5 bytes (length of shortest possible
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versioned bitstream), the next byte gives the bitstream version
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(version 1 only).
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Otherwise, the bitstream version is 0.
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18..21 : copy 0..3 literals
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state = (byte - 17) = 0..3 [ copy <state> literals ]
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skip byte
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22..255 : copy literal string
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length = (byte - 17) = 4..238
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state = 4 [ don't copy extra literals ]
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skip byte
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Instruction encoding::
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0 0 0 0 X X X X (0..15)
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Depends on the number of literals copied by the last instruction.
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If last instruction did not copy any literal (state == 0), this
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encoding will be a copy of 4 or more literal, and must be interpreted
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like this :
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0 0 0 0 L L L L (0..15) : copy long literal string
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length = 3 + (L ?: 15 + (zero_bytes * 255) + non_zero_byte)
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state = 4 (no extra literals are copied)
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If last instruction used to copy between 1 to 3 literals (encoded in
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the instruction's opcode or distance), the instruction is a copy of a
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2-byte block from the dictionary within a 1kB distance. It is worth
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noting that this instruction provides little savings since it uses 2
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bytes to encode a copy of 2 other bytes but it encodes the number of
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following literals for free. It must be interpreted like this :
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0 0 0 0 D D S S (0..15) : copy 2 bytes from <= 1kB distance
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length = 2
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state = S (copy S literals after this block)
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Always followed by exactly one byte : H H H H H H H H
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distance = (H << 2) + D + 1
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If last instruction used to copy 4 or more literals (as detected by
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state == 4), the instruction becomes a copy of a 3-byte block from the
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dictionary from a 2..3kB distance, and must be interpreted like this :
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0 0 0 0 D D S S (0..15) : copy 3 bytes from 2..3 kB distance
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length = 3
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state = S (copy S literals after this block)
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Always followed by exactly one byte : H H H H H H H H
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distance = (H << 2) + D + 2049
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0 0 0 1 H L L L (16..31)
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Copy of a block within 16..48kB distance (preferably less than 10B)
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length = 2 + (L ?: 7 + (zero_bytes * 255) + non_zero_byte)
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Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S
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distance = 16384 + (H << 14) + D
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state = S (copy S literals after this block)
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End of stream is reached if distance == 16384
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In version 1 only, to prevent ambiguity with the RLE case when
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((distance & 0x803f) == 0x803f) && (261 <= length <= 264), the
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compressor must not emit block copies where distance and length
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meet these conditions.
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In version 1 only, this instruction is also used to encode a run of
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zeros if distance = 0xbfff, i.e. H = 1 and the D bits are all 1.
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In this case, it is followed by a fourth byte, X.
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run length = ((X << 3) | (0 0 0 0 0 L L L)) + 4
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0 0 1 L L L L L (32..63)
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Copy of small block within 16kB distance (preferably less than 34B)
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length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte)
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Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S
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distance = D + 1
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state = S (copy S literals after this block)
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0 1 L D D D S S (64..127)
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Copy 3-4 bytes from block within 2kB distance
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state = S (copy S literals after this block)
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length = 3 + L
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Always followed by exactly one byte : H H H H H H H H
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distance = (H << 3) + D + 1
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1 L L D D D S S (128..255)
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Copy 5-8 bytes from block within 2kB distance
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state = S (copy S literals after this block)
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length = 5 + L
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Always followed by exactly one byte : H H H H H H H H
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distance = (H << 3) + D + 1
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Authors
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=======
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This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an
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analysis of the decompression code available in Linux 3.16-rc5, and updated
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by Dave Rodgman <dave.rodgman@arm.com> on 2018/10/30 to introduce run-length
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encoding. The code is tricky, it is possible that this document contains
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mistakes or that a few corner cases were overlooked. In any case, please
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report any doubt, fix, or proposed updates to the author(s) so that the
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document can be updated.
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