Merge tag 'compress-3.18-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core

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@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.

#include <linux/lzo.h>

#include "lzodefs.h"

#define HAVE_IP(t, x)					\

	(((size_t)(ip_end - ip) >= (size_t)(t + x)) &&	\

	 (((t + x) >= t) && ((t + x) >= x)))

#define HAVE_IP(x)      ((size_t)(ip_end - ip) >= (size_t)(x))

#define HAVE_OP(x)      ((size_t)(op_end - op) >= (size_t)(x))

#define NEED_IP(x)      if (!HAVE_IP(x)) goto input_overrun

#define NEED_OP(x)      if (!HAVE_OP(x)) goto output_overrun

#define TEST_LB(m_pos)  if ((m_pos) < out) goto lookbehind_overrun

#define HAVE_OP(t, x)					\

	(((size_t)(op_end - op) >= (size_t)(t + x)) &&	\

	 (((t + x) >= t) && ((t + x) >= x)))

#define NEED_IP(t, x)					\

	do {						\

		if (!HAVE_IP(t, x))			\

			goto input_overrun;		\

	} while (0)

#define NEED_OP(t, x)					\

	do {						\

		if (!HAVE_OP(t, x))			\

			goto output_overrun;		\

	} while (0)

#define TEST_LB(m_pos)					\

	do {						\

		if ((m_pos) < out)			\

			goto lookbehind_overrun;	\

	} while (0)

/* This MAX_255_COUNT is the maximum number of times we can add 255 to a base

 * count without overflowing an integer. The multiply will overflow when

 * multiplying 255 by more than MAXINT/255. The sum will overflow earlier

 * depending on the base count. Since the base count is taken from a u8

 * and a few bits, it is safe to assume that it will always be lower than

 * or equal to 2*255, thus we can always prevent any overflow by accepting

 * two less 255 steps. See Documentation/lzo.txt for more information.

 */

#define MAX_255_COUNT      ((((size_t)~0) / 255) - 2)

int lzo1x_decompress_safe(const unsigned char *in, size_t in_len,

			  unsigned char *out, size_t *out_len)

		if (t < 16) {

			if (likely(state == 0)) {

				if (unlikely(t == 0)) {

					size_t offset;

					const unsigned char *ip_last = ip;

					while (unlikely(*ip == 0)) {

						t += 255;

						ip++;

						NEED_IP(1, 0);

						NEED_IP(1);

					}

					t += 15 + *ip++;

					offset = ip - ip_last;

					if (unlikely(offset > MAX_255_COUNT))

						return LZO_E_ERROR;

					offset = (offset << 8) - offset;

					t += offset + 15 + *ip++;

				}

				t += 3;

copy_literal_run:

#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)

				if (likely(HAVE_IP(t, 15) && HAVE_OP(t, 15))) {

				if (likely(HAVE_IP(t + 15) && HAVE_OP(t + 15))) {

					const unsigned char *ie = ip + t;

					unsigned char *oe = op + t;

					do {

				} else

#endif

				{

					NEED_OP(t, 0);

					NEED_IP(t, 3);

					NEED_OP(t);

					NEED_IP(t + 3);

					do {

						*op++ = *ip++;

					} while (--t > 0);

				m_pos -= t >> 2;

				m_pos -= *ip++ << 2;

				TEST_LB(m_pos);

				NEED_OP(2, 0);

				NEED_OP(2);

				op[0] = m_pos[0];

				op[1] = m_pos[1];

				op += 2;

		} else if (t >= 32) {

			t = (t & 31) + (3 - 1);

			if (unlikely(t == 2)) {

				size_t offset;

				const unsigned char *ip_last = ip;

				while (unlikely(*ip == 0)) {

					t += 255;

					ip++;

					NEED_IP(1, 0);

					NEED_IP(1);

				}

				t += 31 + *ip++;

				NEED_IP(2, 0);

				offset = ip - ip_last;

				if (unlikely(offset > MAX_255_COUNT))

					return LZO_E_ERROR;

				offset = (offset << 8) - offset;

				t += offset + 31 + *ip++;

				NEED_IP(2);

			}

			m_pos = op - 1;

			next = get_unaligned_le16(ip);

			m_pos -= (t & 8) << 11;

			t = (t & 7) + (3 - 1);

			if (unlikely(t == 2)) {

				size_t offset;

				const unsigned char *ip_last = ip;

				while (unlikely(*ip == 0)) {

					t += 255;

					ip++;

					NEED_IP(1, 0);

					NEED_IP(1);

				}

				t += 7 + *ip++;

				NEED_IP(2, 0);

				offset = ip - ip_last;

				if (unlikely(offset > MAX_255_COUNT))

					return LZO_E_ERROR;

				offset = (offset << 8) - offset;

				t += offset + 7 + *ip++;

				NEED_IP(2);

			}

			next = get_unaligned_le16(ip);

			ip += 2;

#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)

		if (op - m_pos >= 8) {

			unsigned char *oe = op + t;

			if (likely(HAVE_OP(t, 15))) {

			if (likely(HAVE_OP(t + 15))) {

				do {

					COPY8(op, m_pos);

					op += 8;

					m_pos += 8;

				} while (op < oe);

				op = oe;

				if (HAVE_IP(6, 0)) {

				if (HAVE_IP(6)) {

					state = next;

					COPY4(op, ip);

					op += next;

					continue;

				}

			} else {

				NEED_OP(t, 0);

				NEED_OP(t);

				do {

					*op++ = *m_pos++;

				} while (op < oe);

#endif

		{

			unsigned char *oe = op + t;

			NEED_OP(t, 0);

			NEED_OP(t);

			op[0] = m_pos[0];

			op[1] = m_pos[1];

			op += 2;

		state = next;

		t = next;

#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)

		if (likely(HAVE_IP(6, 0) && HAVE_OP(4, 0))) {

		if (likely(HAVE_IP(6) && HAVE_OP(4))) {

			COPY4(op, ip);

			op += t;

			ip += t;

		} else

#endif

		{

			NEED_IP(t, 3);

			NEED_OP(t, 0);

			NEED_IP(t + 3);

			NEED_OP(t);

			while (t > 0) {

				*op++ = *ip++;

				t--;