path: root/lib/xz/xz_dec_lzma2.c

/*
 * LZMA2 decoder
 *
 * Authors: Lasse Collin <lasse.collin@tukaani.org>
 *          Igor Pavlov <https://7-zip.org/>
 *
 * This file has been put into the public domain.
 * You can do whatever you want with this file.
 */

#include "xz_private.h"
#include "xz_lzma2.h"

/*
 * Range decoder initialization eats the first five bytes of each LZMA chunk.
 */
#define RC_INIT_BYTES 5

/*
 * Minimum number of usable input buffer to safely decode one LZMA symbol.
 * The worst case is that we decode 22 bits using probabilities and 26
 * direct bits. This may decode at maximum of 20 bytes of input. However,
 * lzma_main() does an extra normalization before returning, thus we
 * need to put 21 here.
 */
#define LZMA_IN_REQUIRED 21

/*
 * Dictionary (history buffer)
 *
 * These are always true:
 *    start <= pos <= full <= end
 *    pos <= limit <= end
 *
 * In multi-call mode, also these are true:
 *    end == size
 *    size <= size_max
 *    allocated <= size
 *
 * Most of these variables are size_t to support single-call mode,
 * in which the dictionary variables address the actual output
 * buffer directly.
 */
struct dictionary {
	/* Beginning of the history buffer */
	uint8_t *buf;

	/* Old position in buf (before decoding more data) */
	size_t start;

	/* Position in buf */
	size_t pos;

	/*
	 * How full dictionary is. This is used to detect corrupt input that
	 * would read beyond the beginning of the uncompressed stream.
	 */
	size_t full;

	/* Write limit; we don't write to buf[limit] or later bytes. */
	size_t limit;

	/*
	 * End of the dictionary buffer. In multi-call mode, this is
	 * the same as the dictionary size. In single-call mode, this
	 * indicates the size of the output buffer.
	 */
	size_t end;

	/*
	 * Size of the dictionary as specified in Block Header. This is used
	 * together with "full" to detect corrupt input that would make us
	 * read beyond the beginning of the uncompressed stream.
	 */
	uint32_t size;

	/*
	 * Maximum allowed dictionary size in multi-call mode.
	 * This is ignored in single-call mode.
	 */
	uint32_t size_max;

	/*
	 * Amount of memory currently allocated for the dictionary.
	 * This is used only with XZ_DYNALLOC. (With XZ_PREALLOC,
	 * size_max is always the same as the allocated size.)
	 */
	uint32_t allocated;

	/* Operation mode */
	enum xz_mode mode;
};

/* Range decoder */
struct rc_dec {
	uint32_t range;
	uint32_t code;

	/*
	 * Number of initializing bytes remaining to be read
	 * by rc_read_init().
	 */
	uint32_t init_bytes_left;

	/*
	 * Buffer from which we read our input. It can be either
	 * temp.buf or the caller-provided input buffer.
	 */
	const uint8_t *in;
	size_t in_pos;
	size_t in_limit;
};

/* Probabilities for a length decoder. */
struct lzma_len_dec {
	/* Probability of match length being at least 10 */
	uint16_t choice;

	/* Probability of match length being at least 18 */
	uint16_t choice2;

	/* Probabilities for match lengths 2-9 */
	uint16_t low[POS_STATES_MAX][LEN_LOW_SYMBOLS];

	/* Probabilities for match lengths 10-17 */
	uint16_t mid[POS_STATES_MAX][LEN_MID_SYMBOLS];

	/* Probabilities for match lengths 18-273 */
	uint16_t high[LEN_HIGH_SYMBOLS];
};

struct lzma_dec {
	/* Distances of latest four matches */
	uint32_t rep0;
	uint32_t rep1;
	uint32_t rep2;
	uint32_t rep3;

	/* Types of the most recently seen LZMA symbols */
	enum lzma_state state;

	/*
	 * Length of a match. This is updated so that dict_repeat can
	 * be called again to finish repeating the whole match.
	 */
	uint32_t len;

	/*
	 * LZMA properties or related bit masks (number of literal
	 * context bits, a mask dervied from the number of literal
	 * position bits, and a mask dervied from the number
	 * position bits)
	 */
	uint32_t lc;
	uint32_t literal_pos_mask; /* (1 << lp) - 1 */
	uint32_t pos_mask;         /* (1 << pb) - 1 */

	/* If 1, it's a match. Otherwise it's a single 8-bit literal. */
	uint16_t is_match[STATES][POS_STATES_MAX];

	/* If 1, it's a repeated match. The distance is one of rep0 .. rep3. */
	uint16_t is_rep[STATES];

	/*
	 * If 0, distance of a repeated match is rep0.
	 * Otherwise check is_rep1.
	 */
	uint16_t is_rep0[STATES];

	/*
	 * If 0, distance of a repeated match is rep1.
	 * Otherwise check is_rep2.
	 */
	uint16_t is_rep1[STATES];

	/* If 0, distance of a repeated match is rep2. Otherwise it is rep3. */
	uint16_t is_rep2[STATES];

	/*
	 * If 1, the repeated match has length of one byte. Otherwise
	 * the length is decoded from rep_len_decoder.
	 */
	uint16_t is_rep0_long[STATES][POS_STATES_MAX];

	/*
	 * Probability tree for the highest two bits of the match
	 * distance. There is a separate probability tree for match
	 * lengths of 2 (i.e. MATCH_LEN_MIN), 3, 4, and [5, 273].
	 */
	uint16_t dist_slot[DIST_STATES][DIST_SLOTS];

	/*
	 * Probility trees for additional bits for match distance
	 * when the distance is in the range [4, 127].
	 */
	uint16_t dist_special[FULL_DISTANCES - DIST_MODEL_END];

	/*
	 * Probability tree for the lowest four bits of a match
	 * distance that is equal to or greater than 128.
	 */
	uint16_t dist_align[ALIGN_SIZE];

	/* Length of a normal match */
	struct lzma_len_dec match_len_dec;

	/* Length of a repeated match */
	struct lzma_len_dec rep_len_dec;

	/* Probabilities of literals */
	uint16_t literal[LITERAL_CODERS_MAX][LITERAL_CODER_SIZE];
};

struct lzma2_dec {
	/* Position in xz_dec_lzma2_run(). */
	enum lzma2_seq {
		SEQ_CONTROL,
		SEQ_UNCOMPRESSED_1,
		SEQ_UNCOMPRESSED_2,
		SEQ_COMPRESSED_0,
		SEQ_COMPRESSED_1,
		SEQ_PROPERTIES,
		SEQ_LZMA_PREPARE,
		SEQ_LZMA_RUN,
		SEQ_COPY
	} sequence;

	/* Next position after decoding the compressed size of the chunk. */
	enum lzma2_seq next_sequence;

	/* Uncompressed size of LZMA chunk (2 MiB at maximum) */
	uint32_t uncompressed;

	/*
	 * Compressed size of LZMA chunk or compressed/uncompressed
	 * size of uncompressed chunk (64 KiB at maximum)
	 */
	uint32_t compressed;

	/*
	 * True if dictionary reset is needed. This is false before
	 * the first chunk (LZMA or uncompressed).
	 */
	bool need_dict_reset;

	/*
	 * True if new LZMA properties are needed. This is false
	 * before the first LZMA chunk.
	 */
	bool need_props;
};

struct xz_dec_lzma2 {
	/*
	 * The order below is important on x86 to reduce code size and
	 * it shouldn't hurt on other platforms. Everything up to and
	 * including lzma.pos_mask are in the first 128 bytes on x86-32,
	 * which allows using smaller instructions to access those
	 * variables. On x86-64, fewer variables fit into the first 128
	 * bytes, but this is still the best order without sacrificing
	 * the readability by splitting the structures.
	 */
	struct rc_dec rc;
	struct dictionary dict;
	struct lzma2_dec lzma2;
	struct lzma_dec lzma;

	/*
	 * Temporary buffer which holds small number of input bytes between
	 * decoder calls. See lzma2_lzma() for details.
	 */
	struct {
		uint32_t size;
		uint8_t buf[3 * LZMA_IN_REQUIRED];
	} temp;
};

/**************
 * Dictionary *
 **************/

/*
 * Reset the dictionary state. When in single-call mode, set up the beginning
 * of the dictionary to point to the actual output buffer.
 */
static void dict_reset(struct dictionary *dict, struct xz_buf *b)
{
	if (DEC_IS_SINGLE(dict->mode)) {
		dict->buf = b->out + b->out_pos;
		dict->end = b->out_size - b->out_pos;
	}

	dict->start = 0;
	dict->pos = 0;
	dict->limit = 0;
	dict->full = 0;
}

/* Set dictionary write limit */
static void dict_limit(struct dictionary *dict, size_t out_max)
{
	if (dict->end - dict->pos <= out_max)
		dict->limit = dict->end;
	else
		dict->limit = dict->pos + out_max;
}

/* Return true if at least one byte can be written into the dictionary. */
static inline bool dict_has_space(const struct dictionary *dict)
{
	return dict->pos < dict->limit;
}

/*
 * Get a byte from the dictionary at the given distance. The distance is
 * assumed to valid, or as a special case, zero when the dictionary is
 * still empty. This special case is needed for single-call decoding to
 * avoid writing a '\0' to the end of the destination buffer.
 */
static inline uint32_t dict_get(const struct dictionary *dict, uint32_t dist)
{
	size_t offset = dict->pos - dist - 1;

	if (dist >= dict->pos)
		offset += dict->end;

	return dict->full > 0 ? dict->buf[offset] : 0;
}

/*
 * Put one byte into the dictionary. It is assumed that there is space for it.
 */
static inline void dict_put(struct dictionary *dict, uint8_t byte)
{
	dict->buf[dict->pos++] = byte;

	if (dict->full < dict->pos)
		dict->full = dict->pos;
}

/*
 * Repeat given number of bytes from the given distance. If the distance is
 * invalid, false is returned. On success, true is returned and *len is
 * updated to indicate how many bytes were left to be repeated.
 */
static bool dict_repeat(struct dictionary *dict, uint32_t *len, uint32_t dist)
{
	size_t back;
	uint32_t left;

	if (dist >= dict->full || dist >= dict->size)
		return false;

	left = min_t(size_t, dict->limit - dict->pos, *len);
	*len -= left;

	back = dict->pos - dist - 1;
	if (dist >= dict->pos)
		back += dict->end;

	do {
		dict->buf[dict->pos++] = dict->buf[back++];
		if (back == dict->end)
			back = 0;
	} while (--left > 0);

	if (dict->full < dict->pos)
		dict->full = dict->pos;

	return true;
}

/* Copy uncompressed data as is from input to dictionary and output buffers. */
static void dict_uncompressed(struct dictionary *dict, struct xz_buf *b,
			      uint32_t *left)
{
	size_t copy_size;

	while (*left > 0 && b->in_pos < b->in_size
			&& b->out_pos < b->out_size) {
		copy_size = min(b->in_size - b->in_pos,
				b->out_size - b->out_pos);
		if (copy_size > dict->end - dict->pos)
			copy_size = dict->end - dict->pos;
		if (copy_size > *left)
			copy_size = *left;

		*left -= copy_size;

		memcpy(dict->buf + dict->pos, b->in + b->in_pos, copy_size);
		dict->pos += copy_size;

		if (dict->full < dict->pos)
			dict->full = dict->pos;

		if (DEC_IS_MULTI(dict->mode)) {
			if (dict->pos == dict->end)
				dict->pos = 0;

			memcpy(b->out + b->out_pos, b->in + b->in_pos,
					copy_size);
		}

		dict->start = dict->pos;

		b->out_pos += copy_size;
		b->in_pos += copy_size;
	}
}

/*
 * Flush pending data from dictionary to b->out. It is assumed that there is
 * enough space in b->out. This is guaranteed because caller uses dict_limit()
 * before decoding data into the dictionary.
 */
static uint32_t dict_flush(struct dictionary *dict, struct xz_buf *b)
{
	size_t copy_size = dict->pos - dict->start;

	if (DEC_IS_MULTI(dict->mode)) {
		if (dict->pos == dict->end)
			dict->pos = 0;

		memcpy(b->out + b->out_pos, dict->buf + dict->start,
				copy_size);
	}

	dict->start = dict->pos;
	b->out_pos += copy_size;
	return copy_size;
}

/*****************
 * Range decoder *
 *****************/

/* Reset the range decoder. */
static void rc_reset(struct rc_dec *rc)
{
	rc->range = (uint32_t)-1;
	rc->code = 0;
	rc->init_bytes_left = RC_INIT_BYTES;
}

/*
 * Read the first five initial bytes into rc->code if they haven't been
 * read already. (Yes, the first byte gets completely ignored.)
 */
static bool rc_read_init(struct rc_dec *rc, struct xz_buf *b)
{
	while (rc->init_bytes_left > 0) {
		if (b->in_pos == b->in_size)
			return false;

		rc->code = (rc->code << 8) + b->in[b->in_pos++];
		--rc->init_bytes_left;
	}

	return true;
}

/* Return true if there may not be enough input for the next decoding loop. */
static inline bool rc_limit_exceeded(const struct rc_dec *rc)
{
	return rc->in_pos > rc->in_limit;
}

/*
 * Return true if it is possible (from point of view of range decoder) that
 * we have reached the end of the LZMA chunk.
 */
static inline bool rc_is_finished(const struct rc_dec *rc)
{
	return rc->code == 0;
}

/* Read the next input byte if needed. */
static __always_inline void rc_normalize(struct rc_dec *rc)
{
	if (rc->range < RC_TOP_VALUE) {
		rc->range <<= RC_SHIFT_BITS;
		rc->code = (rc->code << RC_SHIFT_BITS) + rc->in[rc->in_pos++];
	}
}

/*
 * Decode one bit. In some versions, this function has been splitted in three
 * functions so that the compiler is supposed to be able to more easily avoid
 * an extra branch. In this particular version of the LZMA decoder, this
 * doesn't seem to be a good idea (tested with GCC 3.3.6, 3.4.6, and 4.3.3
 * on x86). Using a non-splitted version results in nicer looking code too.
 *
 * NOTE: This must return an int. Do not make it return a bool or the speed
 * of the code generated by GCC 3.x decreases 10-15 %. (GCC 4.3 doesn't care,
 * and it generates 10-20 % faster code than GCC 3.x from this file anyway.)
 */
static __always_inline int rc_bit(struct rc_dec *rc, uint16_t *prob)
{
	uint32_t bound;
	int bit;

	rc_normalize(rc);
	bound = (rc->range >> RC_BIT_MODEL_TOTAL_BITS) * *prob;
	if (rc->code < bound) {
		rc->range = bound;
		*prob += (RC_BIT_MODEL_TOTAL - *prob) >> RC_MOVE_BITS;
		bit = 0;
	} else {
		rc->range -= bound;
		rc->code -= bound;
		*prob -= *prob >> RC_MOVE_BITS;
		bit = 1;
	}

	return bit;
}

/* Decode a bittree starting from the most significant bit. */
static __always_inline uint32_t rc_bittree(struct rc_dec *rc,
					   uint16_t *probs, uint32_t limit)
{
	uint32_t symbol = 1;

	do {
		if (rc_bit(rc, &probs[symbol]))
			symbol = (symbol << 1) + 1;
		else
			symbol <<= 1;
	} while (symbol < limit);

	return symbol;
}

/* Decode a bittree starting from the least significant bit. */
static __always_inline void rc_bittree_reverse(struct rc_dec *rc,
					       uint16_t *probs,
					       uint32_t *dest, uint32_t limit)
{
	uint32_t symbol = 1;
	uint32_t i = 0;

	do {
		if (rc_bit(rc, &probs[symbol])) {
			symbol = (symbol << 1) + 1;
			*dest += 1 << i;
		} else {
			symbol <<= 1;
		}
	} while (++i < limit);
}

/* Decode direct bits (fixed fifty-fifty probability) */
static inline void rc_direct(struct rc_dec *rc, uint32_t *dest, uint32_t limit)
{
	uint32_t mask;

	do {
		rc_normalize(rc);
		rc->range >>= 1;
		rc->code -= rc->range;
		mask = (uint32_t)0 - (rc->code >> 31);
		rc->code += rc->range & mask;
		*dest = (*dest << 1) + (mask + 1);
	} while (--limit > 0);
}

/********
 * LZMA *
 ********/

/* Get pointer to literal coder probability array. */
static uint16_t *lzma_literal_probs(struct xz_dec_lzma2 *s)
{
	uint32_t prev_byte = dict_get(&s->dict, 0);
	uint32_t low = prev_byte >> (8 - s->lzma.lc);
	uint32_t high = (s->dict.pos & s->lzma.literal_pos_mask) << s->lzma.lc;
	return s->lzma.literal[low + high];
}

/* Decode a literal (one 8-bit byte) */
static void lzma_literal(struct xz_dec_lzma2 *s)
{
	uint16_t *probs;
	uint32_t symbol;
	uint32_t match_byte;
	uint32_t match_bit;
	uint32_t offset;
	uint32_t i;

	probs = lzma_literal_probs(s);

	if (lzma_state_is_literal(s->lzma.state)) {
		symbol = rc_bittree(&s->rc, probs, 0x100);
	} else {
		symbol = 1;
		match_byte = dict_get(&s->dict, s->lzma.rep0) << 1;
		offset = 0x100;

		do {
			match_bit = match_byte & offset;
			match_byte <<= 1;
			i = offset + match_bit + symbol;

			if (rc_bit(&s->rc, &probs[i])) {
				symbol = (symbol << 1) + 1;
				offset &= match_bit;
			} else {
				symbol <<= 1;
				offset &= ~match_bit;
			}
		} while (symbol < 0x100);
	}

	dict_put(&s->dict, (uint8_t)symbol);
	lzma_state_literal(&s->lzma.state);
}

/* Decode the length of the match into s->lzma.len. */
static void lzma_len(struct xz_dec_lzma2 *s, struct lzma_len_dec *l,
		     uint32_t pos_state)
{
	uint16_t *probs;
	uint32_t limit;

	if (!rc_bit(&s->rc, &l->choice)) {
		probs = l->low[pos_state];
		limit = LEN_LOW_SYMBOLS;
		s->lzma.len = MATCH_LEN_MIN;
	} else {
		if (!rc_bit(&s->rc, &l->choice2)) {
			probs = l->mid[pos_state];
			limit = LEN_MID_SYMBOLS;
			s->lzma.len = MATCH_LEN_MIN + LEN_LOW_SYMBOLS;
		} else {
			probs = l->high;
			limit = LEN_HIGH_SYMBOLS;
			s->lzma.len = MATCH_LEN_MIN + LEN_LOW_SYMBOLS
					+ LEN_MID_SYMBOLS;
		}
	}

	s->lzma.len += rc_bittree(&s->rc, probs, limit) - limit;
}

/* Decode a match. The distance will be stored in s->lzma.rep0. */
static void lzma_match(struct xz_dec_lzma2 *s, uint32_t pos_state)
{
	uint16_t *probs;
	uint32_t dist_slot;
	uint32_t limit;

	lzma_state_match(&s->lzma.state);

	s->lzma.rep3 = s->lzma.rep2;
	s->lzma.rep2 = s->lzma.rep1;
	s->lzma.rep1 = s->lzma.rep0;

	lzma_len(s, &s->lzma.match_len_dec, pos_state);

	probs = s->lzma.dist_slot[lzma_get_dist_state(s->lzma.len)];
	dist_slot = rc_bittree(&s->rc, probs, DIST_SLOTS) - DIST_SLOTS;

	if (dist_slot < DIST_MODEL_START) {
		s->lzma.rep0 = dist_slot;
	} else {
		limit = (dist_slot >> 1) - 1;
		s->lzma.rep0 = 2 + (dist_slot & 1);

		if (dist_slot < DIST_MODEL_END) {
			s->lzma.rep0 <<= limit;
			probs = s->lzma.dist_special + s->lzma.rep0
					- dist_slot - 1;
			rc_bittree_reverse(&s->rc, probs,
					&s->lzma.rep0, limit);
		} else {
			rc_direct(&s->rc, &s->lzma.rep0, limit - ALIGN_BITS);
			s->lzma.rep0 <<= ALIGN_BITS;
			rc_bittree_reverse(&s->rc, s->lzma.dist_align,
					&s->lzma.rep0, ALIGN_BITS);
		}
	}
}

/*
 * Decode a repeated match. The distance is one of the four most recently
 * seen matches. The distance will be stored in s->lzma.rep0.
 */
static void lzma_rep_match(struct xz_dec_lzma2 *s, uint32_t pos_state)
{
	uint32_t tmp;

	if (!rc_bit(&s->rc, &s->lzma.is_rep0[s->lzma.state])) {
		if (!rc_bit(&s->rc, &s->lzma.is_rep0_long[
				s->lzma.state][pos_state])) {
			lzma_state_short_rep(&s->lzma.state);
			s->lzma.len = 1;
			return;
		}
	} else {
		if (!rc_bit(&s->rc, &s->lzma.is_rep1[s->lzma.state])) {
			tmp = s->lzma.rep1;
		} else {
			if (!rc_bit(&s->rc, &s->lzma.is_rep2[s->lzma.state])) {
				tmp = s->lzma.rep2;
			} else {
				tmp = s->lzma.rep3;
				s->lzma.rep3 = s->lzma.rep2;
			}

			s->lzma.rep2 = s->lzma.rep1;
		}

		s->lzma.rep1 = s->lzma.rep0;
		s->lzma.rep0 = tmp;
	}

	lzma_state_long_rep(&s->lzma.state);
	lzma_len(s, &s->lzma.rep_len_dec, pos_state);
}

/* LZMA decoder core */
static bool lzma_main(struct xz_dec_lzma2 *s)
{
	uint32_t pos_state;

	/*
	 * If the dictionary was reached during the previous call, try to
	 * finish the possibly pending repeat in the dictionary.
	 */
	if (dict_has_space(&s->dict) && s->lzma.len > 0)
		dict_repeat(&s->dict, &s->lzma.len, s->lzma.rep0);

	/*
	 * Decode more LZMA symbols. One iteration may consume up to
	 * LZMA_IN_REQUIRED - 1 bytes.
	 */
	while (dict_has_space(&s->dict) && !rc_limit_exceeded(&s->rc)) {
		pos_state = s->dict.pos & s->lzma.pos_mask;

		if (!rc_bit(&s->rc, &s->lzma.is_match[
				s->lzma.state][pos_state])) {
			lzma_literal(s);
		} else {
			if (rc_bit(&s->rc, &s->lzma.is_rep[s->lzma.state]))
				lzma_rep_match(s, pos_state);
			else
				lzma_match(s, pos_state);

			if (!dict_repeat(&s->dict, &s->lzma.len, s->lzma.rep0))
				return false;
		}
	}

	/*
	 * Having the range decoder always normalized when we are outside
	 * this function makes it easier to correctly handle end of the chunk.
	 */
	rc_normalize(&s->rc);

	return true;
}

/*
 * Reset the LZMA decoder and range decoder state. Dictionary is nore reset
 * here, because LZMA state may be reset without resetting the dictionary.
 */
static void lzma_reset(struct xz_dec_lzma2 *s)
{
	uint16_t *probs;
	size_t i;

	s->lzma.state = STATE_LIT_LIT;
	s->lzma.rep0 = 0;
	s->lzma.rep1 = 0;
	s->lzma.rep2 = 0;
	s->lzma.rep3 = 0;

	/*
	 * All probabilities are initialized to the same value. This hack
	 * makes the code smaller by avoiding a separate loop for each
	 * probability array.
	 *
	 * This could be optimized so that only that part of literal
	 * probabilities that are actually required. In the common case
	 * we would write 12 KiB less.
	 */
	probs = s->lzma.is_match[0];
	for (i = 0; i < PROBS_TOTAL; ++i)
		probs[i] = RC_BIT_MODEL_TOTAL / 2;

	rc_reset(&s->rc);
}

/*
 * Decode and validate LZMA properties (lc/lp/pb) and calculate the bit masks
 * from the decoded lp and pb values. On success, the LZMA decoder state is
 * reset and true is returned.
 */
static bool lzma_props(struct xz_dec_lzma2 *s, uint8_t props)
{
	if (props > (4 * 5 + 4) * 9 + 8)
		return false;

	s->lzma.pos_mask = 0;
	while (props >= 9 * 5) {
		props -= 9 * 5;
		++s->lzma.pos_mask;
	}

	s->lzma.pos_mask = (1 << s->lzma.pos_mask) - 1;

	s->lzma.literal_pos_mask = 0;
	while (props >= 9) {
		props -= 9;
		++s->lzma.literal_pos_mask;
	}

	s->lzma.lc = props;

	if (s->lzma.lc + s->lzma.literal_pos_mask > 4)
		return false;

	s->lzma.literal_pos_mask = (1 << s->lzma.literal_pos_mask) - 1;

	lzma_reset(s);

	return true;
}

/*********
 * LZMA2 *
 *********/

/*
 * The LZMA decoder assumes that if the input limit (s->rc.in_limit) hasn't
 * been exceeded, it is safe to read up to LZMA_IN_REQUIRED bytes. This
 * wrapper function takes care of making the LZMA decoder's assumption safe.
 *
 * As long as there is plenty of input left to be decoded in the current LZMA
 * chunk, we decode directly from the caller-supplied input buffer until
 * there's LZMA_IN_REQUIRED bytes left. Those remaining bytes are copied into
 * s->temp.buf, which (hopefully) gets filled on the next call to this
 * function. We decode a few bytes from the temporary buffer so that we can
 * continue decoding from the caller-supplied input buffer again.
 */
static bool lzma2_lzma(struct xz_dec_lzma2 *s, struct xz_buf *b)
{
	size_t in_avail;
	uint32_t tmp;

	in_avail = b->in_size - b->in_pos;
	if (s->temp.size > 0 || s->lzma2.compressed == 0) {
		tmp = 2 * LZMA_IN_REQUIRED - s->temp.size;
		if (tmp > s->lzma2.compressed - s->temp.size)
			tmp = s->lzma2.compressed - s->temp.size;
		if (tmp > in_avail)
			tmp = in_avail;

		memcpy(s->temp.buf + s->temp.size, b->in + b->in_pos, tmp);

		if (s->temp.size + tmp == s->lzma2.compressed) {
			memzero(s->temp.buf + s->temp.size + tmp,
					sizeof(s->temp.buf)
						- s->temp.size - tmp);
			s->rc.in_limit = s->temp.size + tmp;
		} else if (s->temp.size + tmp < LZMA_IN_REQUIRED) {
			s->temp.size += tmp;
			b->in_pos += tmp;
			return true;
		} else {
			s->rc.in_limit = s->temp.size + tmp - LZMA_IN_REQUIRED;
		}

		s->rc.in = s->temp.buf;
		s->rc.in_pos = 0;

		if (!lzma_main(s) || s->rc.in_pos > s->temp.size + tmp)
			return false;

		s->lzma2.compressed -= s->rc.in_pos;

		if (s->rc.in_pos < s->temp.size) {
			s->temp.size -= s->rc.in_pos;
			memmove(s->temp.buf, s->temp.buf + s->rc.in_pos,
					s->temp.size);
			return true;
		}

		b->in_pos += s->rc.in_pos - s->temp.size;
		s->temp.size = 0;
	}

	in_avail = b->in_size - b->in_pos;
	if (in_avail >= LZMA_IN_REQUIRED) {
		s->rc.in = b->in;
		s->rc.in_pos = b->in_pos;

		if (in_avail >= s->lzma2.compressed + LZMA_IN_REQUIRED)
			s->rc.in_limit = b->in_pos + s->lzma2.compressed;
		else
			s->rc.in_limit = b->in_size - LZMA_IN_REQUIRED;

		if (!lzma_main(s))
			return false;

		in_avail = s->rc.in_pos - b->in_pos;
		if (in_avail > s->lzma2.compressed)
			return false;

		s->lzma2.compressed -= in_avail;
		b->in_pos = s->rc.in_pos;
	}

	in_avail = b->in_size - b->in_pos;
	if (in_avail < LZMA_IN_REQUIRED) {
		if (in_avail > s->lzma2.compressed)
			in_avail = s->lzma2.compressed;

		memcpy(s->temp.buf, b->in + b->in_pos, in_avail);
		s->temp.size = in_avail;
		b->in_pos += in_avail;
	}

	return true;
}

/*
 * Take care of the LZMA2 control layer, and forward the job of actual LZMA
 * decoding or copying of uncompressed chunks to other functions.
 */
XZ_EXTERN enum xz_ret xz_dec_lzma2_run(struct xz_dec_lzma2 *s,
				       struct xz_buf *b)
{
	uint32_t tmp;

	while (b->in_pos < b->in_size || s->lzma2.sequence == SEQ_LZMA_RUN) {
		switch (s->lzma2.sequence) {
		case SEQ_CONTROL:
			/*
			 * LZMA2 control byte
			 *
			 * Exact values:
			 *   0x00   End marker
			 *   0x01   Dictionary reset followed by
			 *          an uncompressed chunk
			 *   0x02   Uncompressed chunk (no dictionary reset)
			 *
			 * Highest three bits (s->control & 0xE0):
			 *   0xE0   Dictionary reset, new properties and state
			 *          reset, followed by LZMA compressed chunk
			 *   0xC0   New properties and state reset, followed
			 *          by LZMA compressed chunk (no dictionary
			 *          reset)
			 *   0xA0   State reset using old properties,
			 *          followed by LZMA compressed chunk (no
			 *          dictionary reset)
			 *   0x80   LZMA chunk (no dictionary or state reset)
			 *
			 * For LZMA compressed chunks, the lowest five bits
			 * (s->control & 1F) are the highest bits of the
			 * uncompressed size (bits 16-20).
			 *
			 * A new LZMA2 stream must begin with a dictionary
			 * reset. The first LZMA chunk must set new
			 * properties and reset the LZMA state.
			 *
			 * Values that don't match anything described above
			 * are invalid and we return XZ_DATA_ERROR.
			 */
			tmp = b->in[b->in_pos++];

			if (tmp == 0x00)
				return XZ_STREAM_END;

			if (tmp >= 0xE0 || tmp == 0x01) {
				s->lzma2.need_props = true;
				s->lzma2.need_dict_reset = false;
				dict_reset(&s->dict, b);
			} else if (s->lzma2.need_dict_reset) {
				return XZ_DATA_ERROR;
			}

			if (tmp >= 0x80) {
				s->lzma2.uncompressed = (tmp & 0x1F) << 16;
				s->lzma2.sequence = SEQ_UNCOMPRESSED_1;

				if (tmp >= 0xC0) {
					/*
					 * When there are new properties,
					 * state reset is done at
					 * SEQ_PROPERTIES.
					 */
					s->lzma2.need_props = false;
					s->lzma2.next_sequence
							= SEQ_PROPERTIES;

				} else if (s->lzma2.need_props) {
					return XZ_DATA_ERROR;

				} else {
					s->lzma2.next_sequence
							= SEQ_LZMA_PREPARE;
					if (tmp >= 0xA0)
						lzma_reset(s);
				}
			} else {
				if (tmp > 0x02)
					return XZ_DATA_ERROR;

				s->lzma2.sequence = SEQ_COMPRESSED_0;
				s->lzma2.next_sequence = SEQ_COPY;
			}

			break;

		case SEQ_UNCOMPRESSED_1:
			s->lzma2.uncompressed
					+= (uint32_t)b->in[b->in_pos++] << 8;
			s->lzma2.sequence = SEQ_UNCOMPRESSED_2;
			break;

		case SEQ_UNCOMPRESSED_2:
			s->lzma2.uncompressed
					+= (uint32_t)b->in[b->in_pos++] + 1;
			s->lzma2.sequence = SEQ_COMPRESSED_0;
			break;

		case SEQ_COMPRESSED_0:
			s->lzma2.compressed
					= (uint32_t)b->in[b->in_pos++] << 8;
			s->lzma2.sequence = SEQ_COMPRESSED_1;
			break;

		case SEQ_COMPRESSED_1:
			s->lzma2.compressed
					+= (uint32_t)b->in[b->in_pos++] + 1;
			s->lzma2.sequence = s->lzma2.next_sequence;
			break;

		case SEQ_PROPERTIES:
			if (!lzma_props(s, b->in[b->in_pos++]))
				return XZ_DATA_ERROR;

			s->lzma2.sequence = SEQ_LZMA_PREPARE;

			/* fall through */

		case SEQ_LZMA_PREPARE:
			if (s->lzma2.compressed < RC_INIT_BYTES)
				return XZ_DATA_ERROR;

			if (!rc_read_init(&s->rc, b))
				return XZ_OK;

			s->lzma2.compressed -= RC_INIT_BYTES;
			s->lzma2.sequence = SEQ_LZMA_RUN;

			/* fall through */

		case SEQ_LZMA_RUN:
			/*
			 * Set dictionary limit to indicate how much we want
			 * to be encoded at maximum. Decode new data into the
			 * dictionary. Flush the new data from dictionary to
			 * b->out. Check if we finished decoding this chunk.
			 * In case the dictionary got full but we didn't fill
			 * the output buffer yet, we may run this loop
			 * multiple times without changing s->lzma2.sequence.
			 */
			dict_limit(&s->dict, min_t(size_t,
					b->out_size - b->out_pos,
					s->lzma2.uncompressed));
			if (!lzma2_lzma(s, b))
				return XZ_DATA_ERROR;

			s->lzma2.uncompressed -= dict_flush(&s->dict, b);

			if (s->lzma2.uncompressed == 0) {
				if (s->lzma2.compressed > 0 || s->lzma.len > 0
						|| !rc_is_finished(&s->rc))
					return XZ_DATA_ERROR;

				rc_reset(&s->rc);
				s->lzma2.sequence = SEQ_CONTROL;

			} else if (b->out_pos == b->out_size
					|| (b->in_pos == b->in_size
						&& s->temp.size
						< s->lzma2.compressed)) {
				return XZ_OK;
			}

			break;

		case SEQ_COPY:
			dict_uncompressed(&s->dict, b, &s->lzma2.compressed);
			if (s->lzma2.compressed > 0)
				return XZ_OK;

			s->lzma2.sequence = SEQ_CONTROL;
			break;
		}
	}

	return XZ_OK;
}

XZ_EXTERN struct xz_dec_lzma2 *xz_dec_lzma2_create(enum xz_mode mode,
						   uint32_t dict_max)
{
	struct xz_dec_lzma2 *s = kmalloc(sizeof(*s), GFP_KERNEL);
	if (s == NULL)
		return NULL;

	s->dict.mode = mode;
	s->dict.size_max = dict_max;

	if (DEC_IS_PREALLOC(mode)) {
		s->dict.buf = vmalloc(dict_max);
		if (s->dict.buf == NULL) {
			kfree(s);
			return NULL;
		}
	} else if (DEC_IS_DYNALLOC(mode)) {
		s->dict.buf = NULL;
		s->dict.allocated = 0;
	}

	return s;
}

XZ_EXTERN enum xz_ret xz_dec_lzma2_reset(struct xz_dec_lzma2 *s, uint8_t props)
{
	/* This limits dictionary size to 3 GiB to keep parsing simpler. */
	if (props > 39)
		return XZ_OPTIONS_ERROR;

	s->dict.size = 2 + (props & 1);
	s->dict.size <<= (props >> 1) + 11;

	if (DEC_IS_MULTI(s->dict.mode)) {
		if (s->dict.size > s->dict.size_max)
			return XZ_MEMLIMIT_ERROR;

		s->dict.end = s->dict.size;

		if (DEC_IS_DYNALLOC(s->dict.mode)) {
			if (s->dict.allocated < s->dict.size) {
				s->dict.allocated = s->dict.size;
				vfree(s->dict.buf);
				s->dict.buf = vmalloc(s->dict.size);
				if (s->dict.buf == NULL) {
					s->dict.allocated = 0;
					return XZ_MEM_ERROR;
				}
			}
		}
	}

	s->lzma.len = 0;

	s->lzma2.sequence = SEQ_CONTROL;
	s->lzma2.need_dict_reset = true;

	s->temp.size = 0;

	return XZ_OK;
}

XZ_EXTERN void xz_dec_lzma2_end(struct xz_dec_lzma2 *s)
{
	if (DEC_IS_MULTI(s->dict.mode))
		vfree(s->dict.buf);

	kfree(s);
}