1 ===========================================================
2 LZO stream format as understood by Linux's LZO decompressor
3 ===========================================================
4
5 Introduction
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
17 Description
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
81 Versions
82
83 0: Original version
84 1: LZO-RLE
85
86 Version 1 of LZO implements an extension to encode runs of zeros using run
87 length encoding. This improves speed for data with many zeros, which is a
88 common 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
91 For maximum compatibility, both versions are available under different names
92 (lzo and lzo-rle). Differences in the encoding are noted in this document with
93 e.g.: version 1 only.
94
95 Byte 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 In version 1 only, to prevent ambiguity with the RLE case when
163 ((distance & 0x803f) == 0x803f) && (261 <= length <= 264), the
164 compressor must not emit block copies where distance and length
165 meet these conditions.
166
167 In version 1 only, this instruction is also used to encode a run of
168 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)) + 4
171
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 S
176 distance = D + 1
177 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 distance
181 state = S (copy S literals after this block)
182 length = 3 + L
183 Always followed by exactly one byte : H H H H H H H H
184 distance = (H << 3) + D + 1
185
186 1 L L D D D S S (128..255)
187 Copy 5-8 bytes from block within 2kB distance
188 state = S (copy S literals after this block)
189 length = 5 + L
190 Always followed by exactly one byte : H H H H H H H H
191 distance = (H << 3) + D + 1
192
193 Authors
194 =======
195
196 This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an
197 analysis of the decompression code available in Linux 3.16-rc5, and updated
198 by Dave Rodgman <dave.rodgman@arm.com> on 2018/10/30 to introduce run-length
199 encoding. The code is tricky, it is possible that this document contains
200 mistakes or that a few corner cases were overlooked. In any case, please
201 report any doubt, fix, or proposed updates to the author(s) so that the
202 document can be updated.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.