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 81Byte sequences 82============== 83 84 First byte encoding:: 85 86 0..17 : follow regular instruction encoding, see below. It is worth 87 noting that codes 16 and 17 will represent a block copy from 88 the dictionary which is empty, and that they will always be 89 invalid at this place. 90 91 18..21 : copy 0..3 literals 92 state = (byte - 17) = 0..3 [ copy <state> literals ] 93 skip byte 94 95 22..255 : copy literal string 96 length = (byte - 17) = 4..238 97 state = 4 [ don't copy extra literals ] 98 skip byte 99 100 Instruction encoding:: 101 102 0 0 0 0 X X X X (0..15) 103 Depends on the number of literals copied by the last instruction. 104 If last instruction did not copy any literal (state == 0), this 105 encoding will be a copy of 4 or more literal, and must be interpreted 106 like this : 107 108 0 0 0 0 L L L L (0..15) : copy long literal string 109 length = 3 + (L ?: 15 + (zero_bytes * 255) + non_zero_byte) 110 state = 4 (no extra literals are copied) 111 112 If last instruction used to copy between 1 to 3 literals (encoded in 113 the instruction's opcode or distance), the instruction is a copy of a 114 2-byte block from the dictionary within a 1kB distance. It is worth 115 noting that this instruction provides little savings since it uses 2 116 bytes to encode a copy of 2 other bytes but it encodes the number of 117 following literals for free. It must be interpreted like this : 118 119 0 0 0 0 D D S S (0..15) : copy 2 bytes from <= 1kB distance 120 length = 2 121 state = S (copy S literals after this block) 122 Always followed by exactly one byte : H H H H H H H H 123 distance = (H << 2) + D + 1 124 125 If last instruction used to copy 4 or more literals (as detected by 126 state == 4), the instruction becomes a copy of a 3-byte block from the 127 dictionary from a 2..3kB distance, and must be interpreted like this : 128 129 0 0 0 0 D D S S (0..15) : copy 3 bytes from 2..3 kB distance 130 length = 3 131 state = S (copy S literals after this block) 132 Always followed by exactly one byte : H H H H H H H H 133 distance = (H << 2) + D + 2049 134 135 0 0 0 1 H L L L (16..31) 136 Copy of a block within 16..48kB distance (preferably less than 10B) 137 length = 2 + (L ?: 7 + (zero_bytes * 255) + non_zero_byte) 138 Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S 139 distance = 16384 + (H << 14) + D 140 state = S (copy S literals after this block) 141 End of stream is reached if distance == 16384 142 143 0 0 1 L L L L L (32..63) 144 Copy of small block within 16kB distance (preferably less than 34B) 145 length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte) 146 Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S 147 distance = D + 1 148 state = S (copy S literals after this block) 149 150 0 1 L D D D S S (64..127) 151 Copy 3-4 bytes from block within 2kB distance 152 state = S (copy S literals after this block) 153 length = 3 + L 154 Always followed by exactly one byte : H H H H H H H H 155 distance = (H << 3) + D + 1 156 157 1 L L D D D S S (128..255) 158 Copy 5-8 bytes from block within 2kB distance 159 state = S (copy S literals after this block) 160 length = 5 + L 161 Always followed by exactly one byte : H H H H H H H H 162 distance = (H << 3) + D + 1 163 164Authors 165======= 166 167 This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an 168 analysis of the decompression code available in Linux 3.16-rc5. The code is 169 tricky, it is possible that this document contains mistakes or that a few 170 corner cases were overlooked. In any case, please report any doubt, fix, or 171 proposed updates to the author(s) so that the document can be updated. 172
