[3.13.y.z extended stable] Patch "Documentation: lzo: document part of the encoding" has been added to staging queue
Kamal Mostafa
kamal at canonical.com
Tue Oct 21 20:09:22 UTC 2014
This is a note to let you know that I have just added a patch titled
Documentation: lzo: document part of the encoding
to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree
which can be found at:
http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue
This patch is scheduled to be released in version 3.13.11.10.
If you, or anyone else, feels it should not be added to this tree, please
reply to this email.
For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable
Thanks.
-Kamal
------
>From 5daeec74bfb5a506d72662fec0660627823378c1 Mon Sep 17 00:00:00 2001
From: Willy Tarreau <w at 1wt.eu>
Date: Sat, 27 Sep 2014 12:31:35 +0200
Subject: Documentation: lzo: document part of the encoding
commit d98a0526434d27e261f622cf9d2e0028b5ff1a00 upstream.
Add a complete description of the LZO format as processed by the
decompressor. I have not found a public specification of this format
hence this analysis, which will be used to better understand the code.
Cc: Willem Pinckaers <willem at lekkertech.net>
Cc: "Don A. Bailey" <donb at securitymouse.com>
Signed-off-by: Willy Tarreau <w at 1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
Documentation/lzo.txt | 164 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 164 insertions(+)
create mode 100644 Documentation/lzo.txt
diff --git a/Documentation/lzo.txt b/Documentation/lzo.txt
new file mode 100644
index 0000000..ea45dd3
--- /dev/null
+++ b/Documentation/lzo.txt
@@ -0,0 +1,164 @@
+
+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 garanteed before parsing the instructions.
+ They just have to "refill" this credit if they consume extra bytes. This is
+ an implementation design choice independant 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 at 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.
--
1.9.1
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