1  ///////////////////////////////////////////////////////////////////////////////
       2  //
       3  /// \file       vli_decoder.c
       4  /// \brief      Decodes variable-length integers
       5  //
       6  //  Author:     Lasse Collin
       7  //
       8  //  This file has been put into the public domain.
       9  //  You can do whatever you want with this file.
      10  //
      11  ///////////////////////////////////////////////////////////////////////////////
      12  
      13  #include "common.h"
      14  
      15  
      16  extern LZMA_API(lzma_ret)
      17  lzma_vli_decode(lzma_vli *restrict vli, size_t *vli_pos,
      18  		const uint8_t *restrict in, size_t *restrict in_pos,
      19  		size_t in_size)
      20  {
      21  	// If we haven't been given vli_pos, work in single-call mode.
      22  	size_t vli_pos_internal = 0;
      23  	if (vli_pos == NULL) {
      24  		vli_pos = &vli_pos_internal;
      25  		*vli = 0;
      26  
      27  		// If there's no input, use LZMA_DATA_ERROR. This way it is
      28  		// easy to decode VLIs from buffers that have known size,
      29  		// and get the correct error code in case the buffer is
      30  		// too short.
      31  		if (*in_pos >= in_size)
      32  			return LZMA_DATA_ERROR;
      33  
      34  	} else {
      35  		// Initialize *vli when starting to decode a new integer.
      36  		if (*vli_pos == 0)
      37  			*vli = 0;
      38  
      39  		// Validate the arguments.
      40  		if (*vli_pos >= LZMA_VLI_BYTES_MAX
      41  				|| (*vli >> (*vli_pos * 7)) != 0)
      42  			return LZMA_PROG_ERROR;;
      43  
      44  		if (*in_pos >= in_size)
      45  			return LZMA_BUF_ERROR;
      46  	}
      47  
      48  	do {
      49  		// Read the next byte. Use a temporary variable so that we
      50  		// can update *in_pos immediately.
      51  		const uint8_t byte = in[*in_pos];
      52  		++*in_pos;
      53  
      54  		// Add the newly read byte to *vli.
      55  		*vli += (lzma_vli)(byte & 0x7F) << (*vli_pos * 7);
      56  		++*vli_pos;
      57  
      58  		// Check if this is the last byte of a multibyte integer.
      59  		if ((byte & 0x80) == 0) {
      60  			// We don't allow using variable-length integers as
      61  			// padding i.e. the encoding must use the most the
      62  			// compact form.
      63  			if (byte == 0x00 && *vli_pos > 1)
      64  				return LZMA_DATA_ERROR;
      65  
      66  			return vli_pos == &vli_pos_internal
      67  					? LZMA_OK : LZMA_STREAM_END;
      68  		}
      69  
      70  		// There is at least one more byte coming. If we have already
      71  		// read maximum number of bytes, the integer is considered
      72  		// corrupt.
      73  		//
      74  		// If we need bigger integers in future, old versions liblzma
      75  		// will confusingly indicate the file being corrupt instead of
      76  		// unsupported. I suppose it's still better this way, because
      77  		// in the foreseeable future (writing this in 2008) the only
      78  		// reason why files would appear having over 63-bit integers
      79  		// is that the files are simply corrupt.
      80  		if (*vli_pos == LZMA_VLI_BYTES_MAX)
      81  			return LZMA_DATA_ERROR;
      82  
      83  	} while (*in_pos < in_size);
      84  
      85  	return vli_pos == &vli_pos_internal ? LZMA_DATA_ERROR : LZMA_OK;
      86  }