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Based on kernel version 4.9. Page generated on 2016-12-21 14:35 EST.

1	
2	LZO stream format as understood by Linux's LZO decompressor
3	===========================================================
4	
5	Introduction
6	
7	  This is not a specification. No specification seems to be publicly available
8	  for the LZO stream format. This document describes what input format the LZO
9	  decompressor as implemented in the Linux kernel understands. The file subject
10	  of this analysis is lib/lzo/lzo1x_decompress_safe.c. No analysis was made on
11	  the compressor nor on any other implementations though it seems likely that
12	  the format matches the standard one. The purpose of this document is to
13	  better understand what the code does in order to propose more efficient fixes
14	  for future bug reports.
15	
16	Description
17	
18	  The stream is composed of a series of instructions, operands, and data. The
19	  instructions consist in a few bits representing an opcode, and bits forming
20	  the operands for the instruction, whose size and position depend on the
21	  opcode and on the number of literals copied by previous instruction. The
22	  operands are used to indicate :
23	
24	    - a distance when copying data from the dictionary (past output buffer)
25	    - a length (number of bytes to copy from dictionary)
26	    - the number of literals to copy, which is retained in variable "state"
27	      as a piece of information for next instructions.
28	
29	  Optionally depending on the opcode and operands, extra data may follow. These
30	  extra data can be a complement for the operand (eg: a length or a distance
31	  encoded on larger values), or a literal to be copied to the output buffer.
32	
33	  The first byte of the block follows a different encoding from other bytes, it
34	  seems to be optimized for literal use only, since there is no dictionary yet
35	  prior to that byte.
36	
37	  Lengths are always encoded on a variable size starting with a small number
38	  of bits in the operand. If the number of bits isn't enough to represent the
39	  length, up to 255 may be added in increments by consuming more bytes with a
40	  rate of at most 255 per extra byte (thus the compression ratio cannot exceed
41	  around 255:1). The variable length encoding using #bits is always the same :
42	
43	       length = byte & ((1 << #bits) - 1)
44	       if (!length) {
45	               length = ((1 << #bits) - 1)
46	               length += 255*(number of zero bytes)
47	               length += first-non-zero-byte
48	       }
49	       length += constant (generally 2 or 3)
50	
51	  For references to the dictionary, distances are relative to the output
52	  pointer. Distances are encoded using very few bits belonging to certain
53	  ranges, resulting in multiple copy instructions using different encodings.
54	  Certain encodings involve one extra byte, others involve two extra bytes
55	  forming a little-endian 16-bit quantity (marked LE16 below).
56	
57	  After any instruction except the large literal copy, 0, 1, 2 or 3 literals
58	  are copied before starting the next instruction. The number of literals that
59	  were copied may change the meaning and behaviour of the next instruction. In
60	  practice, only one instruction needs to know whether 0, less than 4, or more
61	  literals were copied. This is the information stored in the <state> variable
62	  in this implementation. This number of immediate literals to be copied is
63	  generally encoded in the last two bits of the instruction but may also be
64	  taken from the last two bits of an extra operand (eg: distance).
65	
66	  End of stream is declared when a block copy of distance 0 is seen. Only one
67	  instruction may encode this distance (0001HLLL), it takes one LE16 operand
68	  for the distance, thus requiring 3 bytes.
69	
70	  IMPORTANT NOTE : in the code some length checks are missing because certain
71	  instructions are called under the assumption that a certain number of bytes
72	  follow because it has already been guaranteed before parsing the instructions.
73	  They just have to "refill" this credit if they consume extra bytes. This is
74	  an implementation design choice independent on the algorithm or encoding.
75	
76	Byte sequences
77	
78	  First byte encoding :
79	
80	      0..17   : follow regular instruction encoding, see below. It is worth
81	                noting that codes 16 and 17 will represent a block copy from
82	                the dictionary which is empty, and that they will always be
83	                invalid at this place.
84	
85	      18..21  : copy 0..3 literals
86	                state = (byte - 17) = 0..3  [ copy <state> literals ]
87	                skip byte
88	
89	      22..255 : copy literal string
90	                length = (byte - 17) = 4..238
91	                state = 4 [ don't copy extra literals ]
92	                skip byte
93	
94	  Instruction encoding :
95	
96	      0 0 0 0 X X X X  (0..15)
97	        Depends on the number of literals copied by the last instruction.
98	        If last instruction did not copy any literal (state == 0), this
99	        encoding will be a copy of 4 or more literal, and must be interpreted
100	        like this :
101	
102	           0 0 0 0 L L L L  (0..15)  : copy long literal string
103	           length = 3 + (L ?: 15 + (zero_bytes * 255) + non_zero_byte)
104	           state = 4  (no extra literals are copied)
105	
106	        If last instruction used to copy between 1 to 3 literals (encoded in
107	        the instruction's opcode or distance), the instruction is a copy of a
108	        2-byte block from the dictionary within a 1kB distance. It is worth
109	        noting that this instruction provides little savings since it uses 2
110	        bytes to encode a copy of 2 other bytes but it encodes the number of
111	        following literals for free. It must be interpreted like this :
112	
113	           0 0 0 0 D D S S  (0..15)  : copy 2 bytes from <= 1kB distance
114	           length = 2
115	           state = S (copy S literals after this block)
116	         Always followed by exactly one byte : H H H H H H H H
117	           distance = (H << 2) + D + 1
118	
119	        If last instruction used to copy 4 or more literals (as detected by
120	        state == 4), the instruction becomes a copy of a 3-byte block from the
121	        dictionary from a 2..3kB distance, and must be interpreted like this :
122	
123	           0 0 0 0 D D S S  (0..15)  : copy 3 bytes from 2..3 kB distance
124	           length = 3
125	           state = S (copy S literals after this block)
126	         Always followed by exactly one byte : H H H H H H H H
127	           distance = (H << 2) + D + 2049
128	
129	      0 0 0 1 H L L L  (16..31)
130	           Copy of a block within 16..48kB distance (preferably less than 10B)
131	           length = 2 + (L ?: 7 + (zero_bytes * 255) + non_zero_byte)
132	        Always followed by exactly one LE16 :  D D D D D D D D : D D D D D D S S
133	           distance = 16384 + (H << 14) + D
134	           state = S (copy S literals after this block)
135	           End of stream is reached if distance == 16384
136	
137	      0 0 1 L L L L L  (32..63)
138	           Copy of small block within 16kB distance (preferably less than 34B)
139	           length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte)
140	        Always followed by exactly one LE16 :  D D D D D D D D : D D D D D D S S
141	           distance = D + 1
142	           state = S (copy S literals after this block)
143	
144	      0 1 L D D D S S  (64..127)
145	           Copy 3-4 bytes from block within 2kB distance
146	           state = S (copy S literals after this block)
147	           length = 3 + L
148	         Always followed by exactly one byte : H H H H H H H H
149	           distance = (H << 3) + D + 1
150	
151	      1 L L D D D S S  (128..255)
152	           Copy 5-8 bytes from block within 2kB distance
153	           state = S (copy S literals after this block)
154	           length = 5 + L
155	         Always followed by exactly one byte : H H H H H H H H
156	           distance = (H << 3) + D + 1
157	
158	Authors
159	
160	  This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an
161	  analysis of the decompression code available in Linux 3.16-rc5. The code is
162	  tricky, it is possible that this document contains mistakes or that a few
163	  corner cases were overlooked. In any case, please report any doubt, fix, or
164	  proposed updates to the author(s) so that the document can be updated.
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