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Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -03001===========================================================
Willy Tarreaud98a0522014-09-27 12:31:35 +02002LZO stream format as understood by Linux's LZO decompressor
3===========================================================
4
5Introduction
Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -03006============
Willy Tarreaud98a0522014-09-27 12:31:35 +02007
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
Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -030018===========
Willy Tarreaud98a0522014-09-27 12:31:35 +020019
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
Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -030024 operands are used to indicate:
Willy Tarreaud98a0522014-09-27 12:31:35 +020025
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
Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -030043 around 255:1). The variable length encoding using #bits is always the same::
Willy Tarreaud98a0522014-09-27 12:31:35 +020044
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
Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -030072 .. 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.
Willy Tarreaud98a0522014-09-27 12:31:35 +020080
81Byte sequences
Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -030082==============
Willy Tarreaud98a0522014-09-27 12:31:35 +020083
Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -030084 First byte encoding::
Willy Tarreaud98a0522014-09-27 12:31:35 +020085
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
Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -0300100 Instruction encoding::
Willy Tarreaud98a0522014-09-27 12:31:35 +0200101
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
Mauro Carvalho Chehab7b001bf2017-05-14 21:02:59 -0300165=======
Willy Tarreaud98a0522014-09-27 12:31:35 +0200166
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.