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1===========================================================2LZO stream format as understood by Linux's LZO decompressor3===========================================================4 5Introduction6============7 8 This is not a specification. No specification seems to be publicly available9 for the LZO stream format. This document describes what input format the LZO10 decompressor as implemented in the Linux kernel understands. The file subject11 of this analysis is lib/lzo/lzo1x_decompress_safe.c. No analysis was made on12 the compressor nor on any other implementations though it seems likely that13 the format matches the standard one. The purpose of this document is to14 better understand what the code does in order to propose more efficient fixes15 for future bug reports.16 17Description18===========19 20 The stream is composed of a series of instructions, operands, and data. The21 instructions consist in a few bits representing an opcode, and bits forming22 the operands for the instruction, whose size and position depend on the23 opcode and on the number of literals copied by previous instruction. The24 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. These32 extra data can be a complement for the operand (eg: a length or a distance33 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, it36 seems to be optimized for literal use only, since there is no dictionary yet37 prior to that byte.38 39 Lengths are always encoded on a variable size starting with a small number40 of bits in the operand. If the number of bits isn't enough to represent the41 length, up to 255 may be added in increments by consuming more bytes with a42 rate of at most 255 per extra byte (thus the compression ratio cannot exceed43 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-byte50 }51 length += constant (generally 2 or 3)52 53 For references to the dictionary, distances are relative to the output54 pointer. Distances are encoded using very few bits belonging to certain55 ranges, resulting in multiple copy instructions using different encodings.56 Certain encodings involve one extra byte, others involve two extra bytes57 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 literals60 are copied before starting the next instruction. The number of literals that61 were copied may change the meaning and behaviour of the next instruction. In62 practice, only one instruction needs to know whether 0, less than 4, or more63 literals were copied. This is the information stored in the <state> variable64 in this implementation. This number of immediate literals to be copied is65 generally encoded in the last two bits of the instruction but may also be66 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 one69 instruction may encode this distance (0001HLLL), it takes one LE16 operand70 for the distance, thus requiring 3 bytes.71 72 .. important::73 74 In the code some length checks are missing because certain instructions75 are called under the assumption that a certain number of bytes follow76 because it has already been guaranteed before parsing the instructions.77 They just have to "refill" this credit if they consume extra bytes. This78 is an implementation design choice independent on the algorithm or79 encoding.80 81Versions82 830: Original version841: LZO-RLE85 86Version 1 of LZO implements an extension to encode runs of zeros using run87length encoding. This improves speed for data with many zeros, which is a88common case for zram. This modifies the bitstream in a backwards compatible way89(v1 can correctly decompress v0 compressed data, but v0 cannot read v1 data).90 91For maximum compatibility, both versions are available under different names92(lzo and lzo-rle). Differences in the encoding are noted in this document with93e.g.: version 1 only.94 95Byte sequences96==============97 98 First byte encoding::99 100 0..16 : follow regular instruction encoding, see below. It is worth101 noting that code 16 will represent a block copy from the102 dictionary which is empty, and that it will always be103 invalid at this place.104 105 17 : bitstream version. If the first byte is 17, and compressed106 stream length is at least 5 bytes (length of shortest possible107 versioned bitstream), the next byte gives the bitstream version108 (version 1 only).109 Otherwise, the bitstream version is 0.110 111 18..21 : copy 0..3 literals112 state = (byte - 17) = 0..3 [ copy <state> literals ]113 skip byte114 115 22..255 : copy literal string116 length = (byte - 17) = 4..238117 state = 4 [ don't copy extra literals ]118 skip byte119 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), this125 encoding will be a copy of 4 or more literal, and must be interpreted126 like this :127 128 0 0 0 0 L L L L (0..15) : copy long literal string129 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 in133 the instruction's opcode or distance), the instruction is a copy of a134 2-byte block from the dictionary within a 1kB distance. It is worth135 noting that this instruction provides little savings since it uses 2136 bytes to encode a copy of 2 other bytes but it encodes the number of137 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 distance140 length = 2141 state = S (copy S literals after this block)142 Always followed by exactly one byte : H H H H H H H H143 distance = (H << 2) + D + 1144 145 If last instruction used to copy 4 or more literals (as detected by146 state == 4), the instruction becomes a copy of a 3-byte block from the147 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 distance150 length = 3151 state = S (copy S literals after this block)152 Always followed by exactly one byte : H H H H H H H H153 distance = (H << 2) + D + 2049154 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 S159 distance = 16384 + (H << 14) + D160 state = S (copy S literals after this block)161 End of stream is reached if distance == 16384162 In version 1 only, to prevent ambiguity with the RLE case when163 ((distance & 0x803f) == 0x803f) && (261 <= length <= 264), the164 compressor must not emit block copies where distance and length165 meet these conditions.166 167 In version 1 only, this instruction is also used to encode a run of168 zeros if distance = 0xbfff, i.e. H = 1 and the D bits are all 1.169 In this case, it is followed by a fourth byte, X.170 run length = ((X << 3) | (0 0 0 0 0 L L L)) + 4171 172 0 0 1 L L L L L (32..63)173 Copy of small block within 16kB distance (preferably less than 34B)174 length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte)175 Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S176 distance = D + 1177 state = S (copy S literals after this block)178 179 0 1 L D D D S S (64..127)180 Copy 3-4 bytes from block within 2kB distance181 state = S (copy S literals after this block)182 length = 3 + L183 Always followed by exactly one byte : H H H H H H H H184 distance = (H << 3) + D + 1185 186 1 L L D D D S S (128..255)187 Copy 5-8 bytes from block within 2kB distance188 state = S (copy S literals after this block)189 length = 5 + L190 Always followed by exactly one byte : H H H H H H H H191 distance = (H << 3) + D + 1192 193Authors194=======195 196 This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an197 analysis of the decompression code available in Linux 3.16-rc5, and updated198 by Dave Rodgman <dave.rodgman@arm.com> on 2018/10/30 to introduce run-length199 encoding. The code is tricky, it is possible that this document contains200 mistakes or that a few corner cases were overlooked. In any case, please201 report any doubt, fix, or proposed updates to the author(s) so that the202 document can be updated.203