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