xz_dec_stream.c

/*
 * .xz Stream decoder
 *
 * Author: Lasse Collin <[email protected]>
 *
 * This file has been put into the public domain.
 * You can do whatever you want with this file.
 */

#include "xz_private.h"
#include "xz_stream.h"

#ifdef XZ_USE_CRC64
#	define IS_CRC64(check_type) ((check_type) == XZ_CHECK_CRC64)
#else
#	define IS_CRC64(check_type) false
#endif

/* Hash used to validate the Index field */
struct xz_dec_hash {
	vli_type unpadded;
	vli_type uncompressed;
	uint32_t crc32;
};

struct xz_dec {
	/* Position in dec_main() */
	enum {
		SEQ_STREAM_HEADER,
		SEQ_BLOCK_START,
		SEQ_BLOCK_HEADER,
		SEQ_BLOCK_UNCOMPRESS,
		SEQ_BLOCK_PADDING,
		SEQ_BLOCK_CHECK,
		SEQ_INDEX,
		SEQ_INDEX_PADDING,
		SEQ_INDEX_CRC32,
		SEQ_STREAM_FOOTER,
		SEQ_STREAM_PADDING
	} sequence;

	/* Position in variable-length integers and Check fields */
	uint32_t pos;

	/* Variable-length integer decoded by dec_vli() */
	vli_type vli;

	/* Saved in_pos and out_pos */
	size_t in_start;
	size_t out_start;

#ifdef XZ_USE_CRC64
	/* CRC32 or CRC64 value in Block or CRC32 value in Index */
	uint64_t crc;
#else
	/* CRC32 value in Block or Index */
	uint32_t crc;
#endif

	/* Type of the integrity check calculated from uncompressed data */
	enum xz_check check_type;

	/* Operation mode */
	enum xz_mode mode;

	/*
	 * True if the next call to xz_dec_run() is allowed to return
	 * XZ_BUF_ERROR.
	 */
	bool allow_buf_error;

	/* Information stored in Block Header */
	struct {
		/*
		 * Value stored in the Compressed Size field, or
		 * VLI_UNKNOWN if Compressed Size is not present.
		 */
		vli_type compressed;

		/*
		 * Value stored in the Uncompressed Size field, or
		 * VLI_UNKNOWN if Uncompressed Size is not present.
		 */
		vli_type uncompressed;

		/* Size of the Block Header field */
		uint32_t size;
	} block_header;

	/* Information collected when decoding Blocks */
	struct {
		/* Observed compressed size of the current Block */
		vli_type compressed;

		/* Observed uncompressed size of the current Block */
		vli_type uncompressed;

		/* Number of Blocks decoded so far */
		vli_type count;

		/*
		 * Hash calculated from the Block sizes. This is used to
		 * validate the Index field.
		 */
		struct xz_dec_hash hash;
	} block;

	/* Variables needed when verifying the Index field */
	struct {
		/* Position in dec_index() */
		enum {
			SEQ_INDEX_COUNT,
			SEQ_INDEX_UNPADDED,
			SEQ_INDEX_UNCOMPRESSED
		} sequence;

		/* Size of the Index in bytes */
		vli_type size;

		/* Number of Records (matches block.count in valid files) */
		vli_type count;

		/*
		 * Hash calculated from the Records (matches block.hash in
		 * valid files).
		 */
		struct xz_dec_hash hash;
	} index;

	/*
	 * Temporary buffer needed to hold Stream Header, Block Header,
	 * and Stream Footer. The Block Header is the biggest (1 KiB)
	 * so we reserve space according to that. buf[] has to be aligned
	 * to a multiple of four bytes; the size_t variables before it
	 * should guarantee this.
	 */
	struct {
		size_t pos;
		size_t size;
		uint8_t buf[1024];
	} temp;

	struct xz_dec_lzma2 *lzma2;

#ifdef XZ_DEC_BCJ
	struct xz_dec_bcj *bcj;
	bool bcj_active;
#endif
};

#ifdef XZ_DEC_ANY_CHECK
/* Sizes of the Check field with different Check IDs */
static const uint8_t check_sizes[16] = {
	0,
	4, 4, 4,
	8, 8, 8,
	16, 16, 16,
	32, 32, 32,
	64, 64, 64
};
#endif

/*
 * Fill s->temp by copying data starting from b->in[b->in_pos]. Caller
 * must have set s->temp.pos to indicate how much data we are supposed
 * to copy into s->temp.buf. Return true once s->temp.pos has reached
 * s->temp.size.
 */
static bool fill_temp(struct xz_dec *s, struct xz_buf *b)
{
	size_t copy_size = min_t(size_t,
			b->in_size - b->in_pos, s->temp.size - s->temp.pos);

	memcpy(s->temp.buf + s->temp.pos, b->in + b->in_pos, copy_size);
	b->in_pos += copy_size;
	s->temp.pos += copy_size;

	if (s->temp.pos == s->temp.size) {
		s->temp.pos = 0;
		return true;
	}

	return false;
}

/* Decode a variable-length integer (little-endian base-128 encoding) */
static enum xz_ret dec_vli(struct xz_dec *s, const uint8_t *in,
			   size_t *in_pos, size_t in_size)
{
	uint8_t byte;

	if (s->pos == 0)
		s->vli = 0;

	while (*in_pos < in_size) {
		byte = in[*in_pos];
		++*in_pos;

		s->vli |= (vli_type)(byte & 0x7F) << s->pos;

		if ((byte & 0x80) == 0) {
			/* Don't allow non-minimal encodings. */
			if (byte == 0 && s->pos != 0)
				return XZ_DATA_ERROR;

			s->pos = 0;
			return XZ_STREAM_END;
		}

		s->pos += 7;
		if (s->pos == 7 * VLI_BYTES_MAX)
			return XZ_DATA_ERROR;
	}

	return XZ_OK;
}

/*
 * Decode the Compressed Data field from a Block. Update and validate
 * the observed compressed and uncompressed sizes of the Block so that
 * they don't exceed the values possibly stored in the Block Header
 * (validation assumes that no integer overflow occurs, since vli_type
 * is normally uint64_t). Update the CRC32 or CRC64 value if presence of
 * the CRC32 or CRC64 field was indicated in Stream Header.
 *
 * Once the decoding is finished, validate that the observed sizes match
 * the sizes possibly stored in the Block Header. Update the hash and
 * Block count, which are later used to validate the Index field.
 */
static enum xz_ret dec_block(struct xz_dec *s, struct xz_buf *b)
{
	enum xz_ret ret;

	s->in_start = b->in_pos;
	s->out_start = b->out_pos;

#ifdef XZ_DEC_BCJ
	if (s->bcj_active)
		ret = xz_dec_bcj_run(s->bcj, s->lzma2, b);
	else
#endif
		ret = xz_dec_lzma2_run(s->lzma2, b);

	s->block.compressed += b->in_pos - s->in_start;
	s->block.uncompressed += b->out_pos - s->out_start;

	/*
	 * There is no need to separately check for VLI_UNKNOWN, since
	 * the observed sizes are always smaller than VLI_UNKNOWN.
	 */
	if (s->block.compressed > s->block_header.compressed
			|| s->block.uncompressed
				> s->block_header.uncompressed)
		return XZ_DATA_ERROR;

	if (s->check_type == XZ_CHECK_CRC32)
		s->crc = xz_crc32(b->out + s->out_start,
				b->out_pos - s->out_start, s->crc);
#ifdef XZ_USE_CRC64
	else if (s->check_type == XZ_CHECK_CRC64)
		s->crc = xz_crc64(b->out + s->out_start,
				b->out_pos - s->out_start, s->crc);
#endif

	if (ret == XZ_STREAM_END) {
		if (s->block_header.compressed != VLI_UNKNOWN
				&& s->block_header.compressed
					!= s->block.compressed)
			return XZ_DATA_ERROR;

		if (s->block_header.uncompressed != VLI_UNKNOWN
				&& s->block_header.uncompressed
					!= s->block.uncompressed)
			return XZ_DATA_ERROR;

		s->block.hash.unpadded += s->block_header.size
				+ s->block.compressed;

#ifdef XZ_DEC_ANY_CHECK
		s->block.hash.unpadded += check_sizes[s->check_type];
#else
		if (s->check_type == XZ_CHECK_CRC32)
			s->block.hash.unpadded += 4;
		else if (IS_CRC64(s->check_type))
			s->block.hash.unpadded += 8;
#endif

		s->block.hash.uncompressed += s->block.uncompressed;
		s->block.hash.crc32 = xz_crc32(
				(const uint8_t *)&s->block.hash,
				sizeof(s->block.hash), s->block.hash.crc32);

		++s->block.count;
	}

	return ret;
}

/* Update the Index size and the CRC32 value. */
static void index_update(struct xz_dec *s, const struct xz_buf *b)
{
	size_t in_used = b->in_pos - s->in_start;
	s->index.size += in_used;
	s->crc = xz_crc32(b->in + s->in_start, in_used, s->crc);
}

/*
 * Decode the Number of Records, Unpadded Size, and Uncompressed Size
 * fields from the Index field. That is, Index Padding and CRC32 are not
 * decoded by this function.
 *
 * This can return XZ_OK (more input needed), XZ_STREAM_END (everything
 * successfully decoded), or XZ_DATA_ERROR (input is corrupt).
 */
static enum xz_ret dec_index(struct xz_dec *s, struct xz_buf *b)
{
	enum xz_ret ret;

	do {
		ret = dec_vli(s, b->in, &b->in_pos, b->in_size);
		if (ret != XZ_STREAM_END) {
			index_update(s, b);
			return ret;
		}

		switch (s->index.sequence) {
		case SEQ_INDEX_COUNT:
			s->index.count = s->vli;

			/*
			 * Validate that the Number of Records field
			 * indicates the same number of Records as
			 * there were Blocks in the Stream.
			 */
			if (s->index.count != s->block.count)
				return XZ_DATA_ERROR;

			s->index.sequence = SEQ_INDEX_UNPADDED;
			break;

		case SEQ_INDEX_UNPADDED:
			s->index.hash.unpadded += s->vli;
			s->index.sequence = SEQ_INDEX_UNCOMPRESSED;
			break;

		case SEQ_INDEX_UNCOMPRESSED:
			s->index.hash.uncompressed += s->vli;
			s->index.hash.crc32 = xz_crc32(
					(const uint8_t *)&s->index.hash,
					sizeof(s->index.hash),
					s->index.hash.crc32);
			--s->index.count;
			s->index.sequence = SEQ_INDEX_UNPADDED;
			break;
		}
	} while (s->index.count > 0);

	return XZ_STREAM_END;
}

/*
 * Validate that the next four or eight input bytes match the value
 * of s->crc. s->pos must be zero when starting to validate the first byte.
 * The "bits" argument allows using the same code for both CRC32 and CRC64.
 */
static enum xz_ret crc_validate(struct xz_dec *s, struct xz_buf *b,
				uint32_t bits)
{
	do {
		if (b->in_pos == b->in_size)
			return XZ_OK;

		if (((s->crc >> s->pos) & 0xFF) != b->in[b->in_pos++])
			return XZ_DATA_ERROR;

		s->pos += 8;

	} while (s->pos < bits);

	s->crc = 0;
	s->pos = 0;

	return XZ_STREAM_END;
}

#ifdef XZ_DEC_ANY_CHECK
/*
 * Skip over the Check field when the Check ID is not supported.
 * Returns true once the whole Check field has been skipped over.
 */
static bool check_skip(struct xz_dec *s, struct xz_buf *b)
{
	while (s->pos < check_sizes[s->check_type]) {
		if (b->in_pos == b->in_size)
			return false;

		++b->in_pos;
		++s->pos;
	}

	s->pos = 0;

	return true;
}
#endif

/* Decode the Stream Header field (the first 12 bytes of the .xz Stream). */
static enum xz_ret dec_stream_header(struct xz_dec *s)
{
	if (!memeq(s->temp.buf, HEADER_MAGIC, HEADER_MAGIC_SIZE))
		return XZ_FORMAT_ERROR;

	if (xz_crc32(s->temp.buf + HEADER_MAGIC_SIZE, 2, 0)
			!= get_le32(s->temp.buf + HEADER_MAGIC_SIZE + 2))
		return XZ_DATA_ERROR;

	if (s->temp.buf[HEADER_MAGIC_SIZE] != 0)
		return XZ_OPTIONS_ERROR;

	/*
	 * Of integrity checks, we support none (Check ID = 0),
	 * CRC32 (Check ID = 1), and optionally CRC64 (Check ID = 4).
	 * However, if XZ_DEC_ANY_CHECK is defined, we will accept other
	 * check types too, but then the check won't be verified and
	 * a warning (XZ_UNSUPPORTED_CHECK) will be given.
	 */
	if (s->temp.buf[HEADER_MAGIC_SIZE + 1] > XZ_CHECK_MAX)
		return XZ_OPTIONS_ERROR;

	s->check_type = s->temp.buf[HEADER_MAGIC_SIZE + 1];

#ifdef XZ_DEC_ANY_CHECK
	if (s->check_type > XZ_CHECK_CRC32 && !IS_CRC64(s->check_type))
		return XZ_UNSUPPORTED_CHECK;
#else
	if (s->check_type > XZ_CHECK_CRC32 && !IS_CRC64(s->check_type))
		return XZ_OPTIONS_ERROR;
#endif

	return XZ_OK;
}

/* Decode the Stream Footer field (the last 12 bytes of the .xz Stream) */
static enum xz_ret dec_stream_footer(struct xz_dec *s)
{
	if (!memeq(s->temp.buf + 10, FOOTER_MAGIC, FOOTER_MAGIC_SIZE))
		return XZ_DATA_ERROR;

	if (xz_crc32(s->temp.buf + 4, 6, 0) != get_le32(s->temp.buf))
		return XZ_DATA_ERROR;

	/*
	 * Validate Backward Size. Note that we never added the size of the
	 * Index CRC32 field to s->index.size, thus we use s->index.size / 4
	 * instead of s->index.size / 4 - 1.
	 */
	if ((s->index.size >> 2) != get_le32(s->temp.buf + 4))
		return XZ_DATA_ERROR;

	if (s->temp.buf[8] != 0 || s->temp.buf[9] != s->check_type)
		return XZ_DATA_ERROR;

	/*
	 * Use XZ_STREAM_END instead of XZ_OK to be more convenient
	 * for the caller.
	 */
	return XZ_STREAM_END;
}

/* Decode the Block Header and initialize the filter chain. */
static enum xz_ret dec_block_header(struct xz_dec *s)
{
	enum xz_ret ret;

	/*
	 * Validate the CRC32. We know that the temp buffer is at least
	 * eight bytes so this is safe.
	 */
	s->temp.size -= 4;
	if (xz_crc32(s->temp.buf, s->temp.size, 0)
			!= get_le32(s->temp.buf + s->temp.size))
		return XZ_DATA_ERROR;

	s->temp.pos = 2;

	/*
	 * Catch unsupported Block Flags. We support only one or two filters
	 * in the chain, so we catch that with the same test.
	 */
#ifdef XZ_DEC_BCJ
	if (s->temp.buf[1] & 0x3E)
#else
	if (s->temp.buf[1] & 0x3F)
#endif
		return XZ_OPTIONS_ERROR;

	/* Compressed Size */
	if (s->temp.buf[1] & 0x40) {
		if (dec_vli(s, s->temp.buf, &s->temp.pos, s->temp.size)
					!= XZ_STREAM_END)
			return XZ_DATA_ERROR;

		s->block_header.compressed = s->vli;
	} else {
		s->block_header.compressed = VLI_UNKNOWN;
	}

	/* Uncompressed Size */
	if (s->temp.buf[1] & 0x80) {
		if (dec_vli(s, s->temp.buf, &s->temp.pos, s->temp.size)
				!= XZ_STREAM_END)
			return XZ_DATA_ERROR;

		s->block_header.uncompressed = s->vli;
	} else {
		s->block_header.uncompressed = VLI_UNKNOWN;
	}

#ifdef XZ_DEC_BCJ
	/* If there are two filters, the first one must be a BCJ filter. */
	s->bcj_active = s->temp.buf[1] & 0x01;
	if (s->bcj_active) {
		if (s->temp.size - s->temp.pos < 2)
			return XZ_OPTIONS_ERROR;

		ret = xz_dec_bcj_reset(s->bcj, s->temp.buf[s->temp.pos++]);
		if (ret != XZ_OK)
			return ret;

		/*
		 * We don't support custom start offset,
		 * so Size of Properties must be zero.
		 */
		if (s->temp.buf[s->temp.pos++] != 0x00)
			return XZ_OPTIONS_ERROR;
	}
#endif

	/* Valid Filter Flags always take at least two bytes. */
	if (s->temp.size - s->temp.pos < 2)
		return XZ_DATA_ERROR;

	/* Filter ID = LZMA2 */
	if (s->temp.buf[s->temp.pos++] != 0x21)
		return XZ_OPTIONS_ERROR;

	/* Size of Properties = 1-byte Filter Properties */
	if (s->temp.buf[s->temp.pos++] != 0x01)
		return XZ_OPTIONS_ERROR;

	/* Filter Properties contains LZMA2 dictionary size. */
	if (s->temp.size - s->temp.pos < 1)
		return XZ_DATA_ERROR;

	ret = xz_dec_lzma2_reset(s->lzma2, s->temp.buf[s->temp.pos++]);
	if (ret != XZ_OK)
		return ret;

	/* The rest must be Header Padding. */
	while (s->temp.pos < s->temp.size)
		if (s->temp.buf[s->temp.pos++] != 0x00)
			return XZ_OPTIONS_ERROR;

	s->temp.pos = 0;
	s->block.compressed = 0;
	s->block.uncompressed = 0;

	return XZ_OK;
}

static enum xz_ret dec_main(struct xz_dec *s, struct xz_buf *b)
{
	enum xz_ret ret;

	/*
	 * Store the start position for the case when we are in the middle
	 * of the Index field.
	 */
	s->in_start = b->in_pos;

	while (true) {
		switch (s->sequence) {
		case SEQ_STREAM_HEADER:
			/*
			 * Stream Header is copied to s->temp, and then
			 * decoded from there. This way if the caller
			 * gives us only little input at a time, we can
			 * still keep the Stream Header decoding code
			 * simple. Similar approach is used in many places
			 * in this file.
			 */
			if (!fill_temp(s, b))
				return XZ_OK;

			/*
			 * If dec_stream_header() returns
			 * XZ_UNSUPPORTED_CHECK, it is still possible
			 * to continue decoding if working in multi-call
			 * mode. Thus, update s->sequence before calling
			 * dec_stream_header().
			 */
			s->sequence = SEQ_BLOCK_START;

			ret = dec_stream_header(s);
			if (ret != XZ_OK)
				return ret;

		/* Fall through */

		case SEQ_BLOCK_START:
			/* We need one byte of input to continue. */
			if (b->in_pos == b->in_size)
				return XZ_OK;

			/* See if this is the beginning of the Index field. */
			if (b->in[b->in_pos] == 0) {
				s->in_start = b->in_pos++;
				s->sequence = SEQ_INDEX;
				break;
			}

			/*
			 * Calculate the size of the Block Header and
			 * prepare to decode it.
			 */
			s->block_header.size
				= ((uint32_t)b->in[b->in_pos] + 1) * 4;

			s->temp.size = s->block_header.size;
			s->temp.pos = 0;
			s->sequence = SEQ_BLOCK_HEADER;

		/* Fall through */

		case SEQ_BLOCK_HEADER:
			if (!fill_temp(s, b))
				return XZ_OK;

			ret = dec_block_header(s);
			if (ret != XZ_OK)
				return ret;

			s->sequence = SEQ_BLOCK_UNCOMPRESS;

		/* Fall through */

		case SEQ_BLOCK_UNCOMPRESS:
			ret = dec_block(s, b);
			if (ret != XZ_STREAM_END)
				return ret;

			s->sequence = SEQ_BLOCK_PADDING;

		/* Fall through */

		case SEQ_BLOCK_PADDING:
			/*
			 * Size of Compressed Data + Block Padding
			 * must be a multiple of four. We don't need
			 * s->block.compressed for anything else
			 * anymore, so we use it here to test the size
			 * of the Block Padding field.
			 */
			while (s->block.compressed & 3) {
				if (b->in_pos == b->in_size)
					return XZ_OK;

				if (b->in[b->in_pos++] != 0)
					return XZ_DATA_ERROR;

				++s->block.compressed;
			}

			s->sequence = SEQ_BLOCK_CHECK;

		/* Fall through */

		case SEQ_BLOCK_CHECK:
			if (s->check_type == XZ_CHECK_CRC32) {
				ret = crc_validate(s, b, 32);
				if (ret != XZ_STREAM_END)
					return ret;
			}
			else if (IS_CRC64(s->check_type)) {
				ret = crc_validate(s, b, 64);
				if (ret != XZ_STREAM_END)
					return ret;
			}
#ifdef XZ_DEC_ANY_CHECK
			else if (!check_skip(s, b)) {
				return XZ_OK;
			}
#endif

			s->sequence = SEQ_BLOCK_START;
			break;

		case SEQ_INDEX:
			ret = dec_index(s, b);
			if (ret != XZ_STREAM_END)
				return ret;

			s->sequence = SEQ_INDEX_PADDING;

		/* Fall through */

		case SEQ_INDEX_PADDING:
			while ((s->index.size + (b->in_pos - s->in_start))
					& 3) {
				if (b->in_pos == b->in_size) {
					index_update(s, b);
					return XZ_OK;
				}

				if (b->in[b->in_pos++] != 0)
					return XZ_DATA_ERROR;
			}

			/* Finish the CRC32 value and Index size. */
			index_update(s, b);

			/* Compare the hashes to validate the Index field. */
			if (!memeq(&s->block.hash, &s->index.hash,
					sizeof(s->block.hash)))
				return XZ_DATA_ERROR;

			s->sequence = SEQ_INDEX_CRC32;

		/* Fall through */

		case SEQ_INDEX_CRC32:
			ret = crc_validate(s, b, 32);
			if (ret != XZ_STREAM_END)
				return ret;

			s->temp.size = STREAM_HEADER_SIZE;
			s->sequence = SEQ_STREAM_FOOTER;

		/* Fall through */

		case SEQ_STREAM_FOOTER:
			if (!fill_temp(s, b))
				return XZ_OK;

			return dec_stream_footer(s);

		case SEQ_STREAM_PADDING:
			/* Never reached, only silencing a warning */
			break;
		}
	}

	/* Never reached */
}

/*
 * xz_dec_run() is a wrapper for dec_main() to handle some special cases in
 * multi-call and single-call decoding.
 *
 * In multi-call mode, we must return XZ_BUF_ERROR when it seems clear that we
 * are not going to make any progress anymore. This is to prevent the caller
 * from calling us infinitely when the input file is truncated or otherwise
 * corrupt. Since zlib-style API allows that the caller fills the input buffer
 * only when the decoder doesn't produce any new output, we have to be careful
 * to avoid returning XZ_BUF_ERROR too easily: XZ_BUF_ERROR is returned only
 * after the second consecutive call to xz_dec_run() that makes no progress.
 *
 * In single-call mode, if we couldn't decode everything and no error
 * occurred, either the input is truncated or the output buffer is too small.
 * Since we know that the last input byte never produces any output, we know
 * that if all the input was consumed and decoding wasn't finished, the file
 * must be corrupt. Otherwise the output buffer has to be too small or the
 * file is corrupt in a way that decoding it produces too big output.
 *
 * If single-call decoding fails, we reset b->in_pos and b->out_pos back to
 * their original values. This is because with some filter chains there won't
 * be any valid uncompressed data in the output buffer unless the decoding
 * actually succeeds (that's the price to pay of using the output buffer as
 * the workspace).
 */
XZ_EXTERN enum xz_ret xz_dec_run(struct xz_dec *s, struct xz_buf *b)
{
	size_t in_start;
	size_t out_start;
	enum xz_ret ret;

	if (DEC_IS_SINGLE(s->mode))
		xz_dec_reset(s);

	in_start = b->in_pos;
	out_start = b->out_pos;
	ret = dec_main(s, b);

	if (DEC_IS_SINGLE(s->mode)) {
		if (ret == XZ_OK)
			ret = b->in_pos == b->in_size
					? XZ_DATA_ERROR : XZ_BUF_ERROR;

		if (ret != XZ_STREAM_END) {
			b->in_pos = in_start;
			b->out_pos = out_start;
		}

	} else if (ret == XZ_OK && in_start == b->in_pos
			&& out_start == b->out_pos) {
		if (s->allow_buf_error)
			ret = XZ_BUF_ERROR;

		s->allow_buf_error = true;
	} else {
		s->allow_buf_error = false;
	}

	return ret;
}

#ifdef XZ_DEC_CONCATENATED
XZ_EXTERN enum xz_ret xz_dec_catrun(struct xz_dec *s, struct xz_buf *b,
				    int finish)
{
	enum xz_ret ret;

	if (DEC_IS_SINGLE(s->mode)) {
		xz_dec_reset(s);
		finish = true;
	}

	while (true) {
		if (s->sequence == SEQ_STREAM_PADDING) {
			/*
			 * Skip Stream Padding. Its size must be a multiple
			 * of four bytes which is tracked with s->pos.
			 */
			while (true) {
				if (b->in_pos == b->in_size) {
					/*
					 * Note that if we are repeatedly
					 * given no input and finish is false,
					 * we will keep returning XZ_OK even
					 * though no progress is being made.
					 * The lack of XZ_BUF_ERROR support
					 * isn't a problem here because a
					 * reasonable caller will eventually
					 * provide more input or set finish
					 * to true.
					 */
					if (!finish)
						return XZ_OK;

					if (s->pos != 0)
						return XZ_DATA_ERROR;

					return XZ_STREAM_END;
				}

				if (b->in[b->in_pos] != 0x00) {
					if (s->pos != 0)
						return XZ_DATA_ERROR;

					break;
				}

				++b->in_pos;
				s->pos = (s->pos + 1) & 3;
			}

			/*
			 * More input remains. It should be a new Stream.
			 *
			 * In single-call mode xz_dec_run() will always call
			 * xz_dec_reset(). Thus, we need to do it here only
			 * in multi-call mode.
			 */
			if (DEC_IS_MULTI(s->mode))
				xz_dec_reset(s);
		}

		ret = xz_dec_run(s, b);

		if (ret != XZ_STREAM_END)
			break;

		s->sequence = SEQ_STREAM_PADDING;
	}

	return ret;
}
#endif

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

	s->mode = mode;

#ifdef XZ_DEC_BCJ
	s->bcj = xz_dec_bcj_create(DEC_IS_SINGLE(mode));
	if (s->bcj == NULL)
		goto error_bcj;
#endif

	s->lzma2 = xz_dec_lzma2_create(mode, dict_max);
	if (s->lzma2 == NULL)
		goto error_lzma2;

	xz_dec_reset(s);
	return s;

error_lzma2:
#ifdef XZ_DEC_BCJ
	xz_dec_bcj_end(s->bcj);
error_bcj:
#endif
	kfree(s);
	return NULL;
}

XZ_EXTERN void xz_dec_reset(struct xz_dec *s)
{
	s->sequence = SEQ_STREAM_HEADER;
	s->allow_buf_error = false;
	s->pos = 0;
	s->crc = 0;
	memzero(&s->block, sizeof(s->block));
	memzero(&s->index, sizeof(s->index));
	s->temp.pos = 0;
	s->temp.size = STREAM_HEADER_SIZE;
}

XZ_EXTERN void xz_dec_end(struct xz_dec *s)
{
	if (s != NULL) {
		xz_dec_lzma2_end(s->lzma2);
#ifdef XZ_DEC_BCJ
		xz_dec_bcj_end(s->bcj);
#endif
		kfree(s);
	}
}