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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

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	Byte 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	
164	Authors
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.
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