| |
| LZO stream format as understood by Linux's LZO decompressor |
| =========================================================== |
| |
| Introduction |
| |
| 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. |
| |
| Description |
| |
| 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 : |
| |
| - 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. |
| |
| 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. |
| |
| 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. |
| |
| 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 : |
| |
| 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) |
| |
| 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). |
| |
| 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 |
| in this implementation. This number of immediate literals to be copied is |
| generally encoded in the last two bits of the instruction but may also be |
| taken from the last two bits of an extra operand (eg: distance). |
| |
| End of stream is declared when a block copy of distance 0 is seen. Only one |
| instruction may encode this distance (0001HLLL), it takes one LE16 operand |
| for the distance, thus requiring 3 bytes. |
| |
| IMPORTANT NOTE : in the code some length checks are missing because certain |
| instructions are called under the assumption that a certain number of bytes |
| follow because it has already been guaranteed before parsing the instructions. |
| They just have to "refill" this credit if they consume extra bytes. This is |
| an implementation design choice independent on the algorithm or encoding. |
| |
| Byte sequences |
| |
| First byte encoding : |
| |
| 0..17 : follow regular instruction encoding, see below. It is worth |
| noting that codes 16 and 17 will represent a block copy from |
| the dictionary which is empty, and that they will always be |
| invalid at this place. |
| |
| 18..21 : copy 0..3 literals |
| state = (byte - 17) = 0..3 [ copy <state> literals ] |
| skip byte |
| |
| 22..255 : copy literal string |
| length = (byte - 17) = 4..238 |
| state = 4 [ don't copy extra literals ] |
| skip byte |
| |
| Instruction encoding : |
| |
| 0 0 0 0 X X X X (0..15) |
| Depends on the number of literals copied by the last instruction. |
| If last instruction did not copy any literal (state == 0), this |
| encoding will be a copy of 4 or more literal, and must be interpreted |
| like this : |
| |
| 0 0 0 0 L L L L (0..15) : copy long literal string |
| length = 3 + (L ?: 15 + (zero_bytes * 255) + non_zero_byte) |
| state = 4 (no extra literals are copied) |
| |
| If last instruction used to copy between 1 to 3 literals (encoded in |
| the instruction's opcode or distance), the instruction is a copy of a |
| 2-byte block from the dictionary within a 1kB distance. It is worth |
| noting that this instruction provides little savings since it uses 2 |
| bytes to encode a copy of 2 other bytes but it encodes the number of |
| following literals for free. It must be interpreted like this : |
| |
| 0 0 0 0 D D S S (0..15) : copy 2 bytes from <= 1kB distance |
| length = 2 |
| state = S (copy S literals after this block) |
| Always followed by exactly one byte : H H H H H H H H |
| distance = (H << 2) + D + 1 |
| |
| If last instruction used to copy 4 or more literals (as detected by |
| state == 4), the instruction becomes a copy of a 3-byte block from the |
| dictionary from a 2..3kB distance, and must be interpreted like this : |
| |
| 0 0 0 0 D D S S (0..15) : copy 3 bytes from 2..3 kB distance |
| length = 3 |
| state = S (copy S literals after this block) |
| Always followed by exactly one byte : H H H H H H H H |
| distance = (H << 2) + D + 2049 |
| |
| 0 0 0 1 H L L L (16..31) |
| Copy of a block within 16..48kB distance (preferably less than 10B) |
| length = 2 + (L ?: 7 + (zero_bytes * 255) + non_zero_byte) |
| Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S |
| distance = 16384 + (H << 14) + D |
| state = S (copy S literals after this block) |
| End of stream is reached if distance == 16384 |
| |
| 0 0 1 L L L L L (32..63) |
| Copy of small block within 16kB distance (preferably less than 34B) |
| length = 2 + (L ?: 31 + (zero_bytes * 255) + non_zero_byte) |
| Always followed by exactly one LE16 : D D D D D D D D : D D D D D D S S |
| distance = D + 1 |
| state = S (copy S literals after this block) |
| |
| 0 1 L D D D S S (64..127) |
| Copy 3-4 bytes from block within 2kB distance |
| state = S (copy S literals after this block) |
| length = 3 + L |
| Always followed by exactly one byte : H H H H H H H H |
| distance = (H << 3) + D + 1 |
| |
| 1 L L D D D S S (128..255) |
| Copy 5-8 bytes from block within 2kB distance |
| state = S (copy S literals after this block) |
| length = 5 + L |
| Always followed by exactly one byte : H H H H H H H H |
| distance = (H << 3) + D + 1 |
| |
| Authors |
| |
| This document was written by Willy Tarreau <w@1wt.eu> on 2014/07/19 during an |
| analysis of the decompression code available in Linux 3.16-rc5. The code is |
| tricky, it is possible that this document contains mistakes or that a few |
| corner cases were overlooked. In any case, please report any doubt, fix, or |
| proposed updates to the author(s) so that the document can be updated. |
