mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-27 14:43:58 +08:00
62e153c46f
Signed-off-by: Eric Engestrom <eric@engestrom.ch> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
165 lines
7.8 KiB
Plaintext
165 lines
7.8 KiB
Plaintext
|
|
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.
|