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Linux/Documentation/lzo.txt

  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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