linux-imx

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LZO stream format as understood by Linux's LZO decompressor

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Introduction

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  This is not a specification. No specification seems to be publicly available

  for the LZO stream format. This document describes what input format the LZO

  decompressor as implemented in the Linux kernel understands. The file subject

  of this analysis is lib/lzo/lzo1x_decompress_safe.c. No analysis was made on

  the compressor nor on any other implementations though it seems likely that

  the format matches the standard one. The purpose of this document is to

  better understand what the code does in order to propose more efficient fixes

  for future bug reports.

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Description

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  The stream is composed of a series of instructions, operands, and data. The

  instructions consist in a few bits representing an opcode, and bits forming

  the operands for the instruction, whose size and position depend on the

  opcode and on the number of literals copied by previous instruction. The

  operands are used to indicate:

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    - a distance when copying data from the dictionary (past output buffer)

    - a length (number of bytes to copy from dictionary)

    - the number of literals to copy, which is retained in variable "state"

      as a piece of information for next instructions.

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  Optionally depending on the opcode and operands, extra data may follow. These

  extra data can be a complement for the operand (eg: a length or a distance

  encoded on larger values), or a literal to be copied to the output buffer.

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  The first byte of the block follows a different encoding from other bytes, it

  seems to be optimized for literal use only, since there is no dictionary yet

  prior to that byte.

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  Lengths are always encoded on a variable size starting with a small number

  of bits in the operand. If the number of bits isn't enough to represent the

  length, up to 255 may be added in increments by consuming more bytes with a

  rate of at most 255 per extra byte (thus the compression ratio cannot exceed

  around 255:1). The variable length encoding using #bits is always the same::

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       length = byte & ((1 << #bits) - 1)

       if (!length) {

               length = ((1 << #bits) - 1)

               length += 255*(number of zero bytes)

               length += first-non-zero-byte

       }

       length += constant (generally 2 or 3)

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  For references to the dictionary, distances are relative to the output

  pointer. Distances are encoded using very few bits belonging to certain

  ranges, resulting in multiple copy instructions using different encodings.

  Certain encodings involve one extra byte, others involve two extra bytes

  forming a little-endian 16-bit quantity (marked LE16 below).

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  After any instruction except the large literal copy, 0, 1, 2 or 3 literals

  are copied before starting the next instruction. The number of literals that

  were copied may change the meaning and behaviour of the next instruction. In

  practice, only one instruction needs to know whether 0, less than 4, or more

  literals were copied. This is the information stored in the <state> variable