1=========================================================== 2LZO stream format as understood by Linux's LZO decompressor 3=========================================================== 4 5Introduction 6============ 7 8 This is not a specification. No specification seems to be publicly available 9 for the LZO stream format. This document describes what input format the LZO 10 decompressor as implemented in the Linux kernel understands. The file subject 11 of this analysis is lib/lzo/lzo1x_decompress_safe.c. No analysis was made on 12 the compressor nor on any other implementations though it seems likely that 13 the format matches the standard one. The purpose of this document is to 14 better understand what the code does in order to propose more efficient fixes 15 for future bug reports. 16 17Description 18=========== 19 20 The stream is composed of a series of instructions, operands, and data. The 21 instructions consist in a few bits representing an opcode, and bits forming 22 the operands for the instruction, whose size and position depend on the 23 opcode and on the number of literals copied by previous instruction. The 24 operands are used to indicate: 25 26 - a distance when copying data from the dictionary (past output buffer) 27 - a length (number of bytes to copy from dictionary) 28 - the number of literals to copy, which is retained in variable "state" 29 as a piece of information for next instructions. 30 31 Optionally depending on the opcode and operands, extra data may follow. These 32 extra data can be a complement for the operand (eg: a length or a distance 33 encoded on larger values), or a literal to be copied to the output buffer. 34 35 The first byte of the block follows a different encoding from other bytes, it 36 seems to be optimized for literal use only, since there is no dictionary yet 37 prior to that byte. 38 39 Lengths are always encoded on a variable size starting with a small number 40 of bits in the operand. If the number of bits isn't enough to represent the 41 length, up to 255 may be added in increments by consuming more bytes with a 42 rate of at most 255 per extra byte (thus the compression ratio cannot exceed 43 around 255:1). The variable length encoding using #bits is always the same:: 44 45 length = byte & ((1 << #bits) - 1) 46 if (!length) { 47 length = ((1 << #bits) - 1) 48 length += 255*(number of zero bytes) 49 length += first-non-zero-byte 50 } 51 length += constant (generally 2 or 3) 52 53 For references to the dictionary, distances are relative to the output 54 pointer. Distances are encoded using very few bits belonging to certain 55 ranges, resulting in multiple copy instructions using different encodings. 56 Certain encodings involve one extra byte, others involve two extra bytes 57 forming a little-endian 16-bit quantity (marked LE16 below). 58 59 After any instruction except the large literal copy, 0, 1, 2 or 3 literals 60 are copied before starting the next instruction. The number of literals that 61 were copied may change the meaning and behaviour of the next instruction. In 62 practice, only one instruction needs to know whether 0, less than 4, or more 63 literals were copied. This is the information stored in the <state> variable 64 in this implementation. This number of immediate literals to be copied is 65 generally encoded in the last two bits of the instruction but may also be 66 taken from the last two bits of an extra operand (eg: distance). 67 68 End of stream is declared when a block copy of distance 0 is seen. Only one 69 instruction may encode this distance (0001HLLL), it takes one LE16 operand 70 for the distance, thus requiring 3 bytes. 71 72 .. important:: 73 74 In the code some length checks are missing because certain instructions 75 are called under the assumption that a certain number of bytes follow 76 because it has already been guaranteed before parsing the instructions. 77 They just have to "refill" this credit if they consume extra bytes. This 78 is an implementation design choice independent on the algorithm or 79 encoding. 80 81Versions 82 830: Original version 841: LZO-RLE 85 86Version 1 of LZO implements an extension to encode runs of zeros using run 87length encoding. This improves speed for data with many zeros, which is a 88common case for zram. This modifies the bitstream in a backwards compatible way 89(v1 can correctly decompress v0 compressed data, but v0 cannot read v1 data). 90 91For maximum compatibility, both versions are available under different names 92(lzo and lzo-rle). Differences in the encoding are noted in this document with 93e.g.: version 1 only. 94 95Byte sequences 96============== 97 98 First byte encoding:: 99 100 0..16 : follow regular instruction encoding, see below. It is worth 101 noting that code 16 will represent a block copy from the 102 dictionary which is empty, and that it will always be 103 invalid at this place. 104 105 17 : bitstream version. If the first byte is 17, and compressed 106 stream length is at least 5 bytes (length of shortest possible 107 versioned bitstream), the next byte gives the bitstream version 108 (version 1 only). 109 Otherwise, the bitstream version is 0. 110 111 18..21 : copy 0..3 literals 112 state = (byte - 17) = 0..3 [ copy <state> literals ] 113 skip byte 114 115 22..255 : copy literal string 116 length = (byte - 17) = 4..238 117 state = 4 [ don't copy extra literals ] 118 skip byte 119 120 Instruction encoding:: 121 122 0 0 0 0 X X X X (0..15) 123 Depends on the number of literals copied by the last instruction. 124 If last instruction did not copy any literal (state == 0), this 125 encoding will be a copy of 4 or more literal, and must be interpreted 126 like this : 127 128 0 0 0 0 L L L L (0..15) : copy long literal string 129 length = 3 + (L ?: 15 + (zero_bytes * 255) + non_zero_byte) 130 state = 4 (no extra literals are copied) 131 132 If last instruction used to copy between 1 to 3 literals (encoded in 133 the instruction's opcode or distance), the instruction is a copy of a 134 2-byte block from the dictionary within a 1kB distance. It is worth 135 noting that this instruction provides little savings since it uses 2 136 bytes to encode a copy of 2 other bytes but it encodes the number of 137 following literals for free. It must be interpreted like this : 138 139 0 0 0 0 D D S S (0..15) : copy 2 bytes from <= 1kB distance 140 length = 2 141 state = S (copy S literals after this block) 142 Always followed by exactly one byte : H H H H H H H H 143 distance = (H << 2) + D + 1 144 145 If last instruction used to copy 4 or more literals (as detected by 146 state == 4), the instruction becomes a copy of a 3-byte block from the 147 dictionary from a 2..3kB distance, and must be interpreted like this : 148 149 0 0 0 0 D D S S (0..15) : copy 3 bytes from 2..3 kB distance 150 length = 3 151 state = S (copy S literals after this block) 152 Always followed by exactly one byte : H H H H H H H H 153 distance = (H << 2) + D + 2049 154 155 0 0 0 1 H L L L (16..31) 156 Copy of a block within 16..48kB distance (preferably less than 10B) 157 length = 2 + (L ?: 7 + (zero_bytes * 255) + non_zero_byte) 158 Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S 159 distance = 16384 + (H << 14) + D 160 state = S (copy S literals after this block) 161 End of stream is reached if distance == 16384 162 163 In version 1 only, this instruction is also used to encode a run of 164 zeros if distance = 0xbfff, i.e. H = 1 and the D bits are all 1. 165 In this case, it is followed by a fourth byte, X. 166 run length = ((X << 3) | (0 0 0 0 0 L L L)) + 4. 167 168 0 0 1 L L L L L (32..63) 169 Copy of small block within 16kB distance (preferably less than 34B) 170 length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte) 171 Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S 172 distance = D + 1 173 state = S (copy S literals after this block) 174 175 0 1 L D D D S S (64..127) 176 Copy 3-4 bytes from block within 2kB distance 177 state = S (copy S literals after this block) 178 length = 3 + L 179 Always followed by exactly one byte : H H H H H H H H 180 distance = (H << 3) + D + 1 181 182 1 L L D D D S S (128..255) 183 Copy 5-8 bytes from block within 2kB distance 184 state = S (copy S literals after this block) 185 length = 5 + L 186 Always followed by exactly one byte : H H H H H H H H 187 distance = (H << 3) + D + 1 188 189Authors 190======= 191 192 This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an 193 analysis of the decompression code available in Linux 3.16-rc5, and updated 194 by Dave Rodgman <dave.rodgman@arm.com> on 2018/10/30 to introduce run-length 195 encoding. The code is tricky, it is possible that this document contains 196 mistakes or that a few corner cases were overlooked. In any case, please 197 report any doubt, fix, or proposed updates to the author(s) so that the 198 document can be updated. 199
