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 garanteed before parsing the instructions.
  73   They just have to "refill" this credit if they consume extra bytes. This is
  74   an implementation design choice independant 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.