Commit 9594370e authored by John Koleszar's avatar John Koleszar
Browse files

Update VP8DX_BOOL_DECODER_FILL to better detect EOS

Allow more reliable detection of truncated bitstreams by being more
precise with the count of "virtual" bits in the value buffer.
Specifically, the VP8_LOTS_OF_BITS value is accumulated into count,
rather than being assigned, which was losing the prior value,
increasing the required tolerance when testing for the error condition.

Change-Id: Ib5172eaa57323b939c439fff8a8ab5fa38da9b69
parent d5c46bdf
......@@ -55,7 +55,7 @@ void vp8dx_bool_decoder_fill(BOOL_DECODER *br);
for(shift = VP8_BD_VALUE_SIZE - 8 - ((_count) + 8); shift >= 0; ) \
{ \
if((_bufptr) >= (_bufend)) { \
(_count) = VP8_LOTS_OF_BITS; \
(_count) += VP8_LOTS_OF_BITS; \
break; \
} \
(_count) += 8; \
......@@ -119,18 +119,19 @@ static int vp8_decode_value(BOOL_DECODER *br, int bits)
static int vp8dx_bool_error(BOOL_DECODER *br)
{
/* Check if we have reached the end of the buffer.
*
* Variable 'count' stores the number of bits in the 'value' buffer,
* minus 8. So if count == 8, there are 16 bits available to be read.
* Normally, count is filled with 8 and one byte is filled into the
* value buffer. When we reach the end of the buffer, count is instead
* filled with VP8_LOTS_OF_BITS, 8 of which represent the last 8 real
* bits from the bitstream. So the last bit in the bitstream will be
* represented by count == VP8_LOTS_OF_BITS - 16.
*/
if ((br->count > VP8_BD_VALUE_SIZE)
&& (br->count <= VP8_LOTS_OF_BITS - 16))
/* Check if we have reached the end of the buffer.
*
* Variable 'count' stores the number of bits in the 'value' buffer, minus
* 8. The top byte is part of the algorithm, and the remainder is buffered
* to be shifted into it. So if count == 8, the top 16 bits of 'value' are
* occupied, 8 for the algorithm and 8 in the buffer.
*
* When reading a byte from the user's buffer, count is filled with 8 and
* one byte is filled into the value buffer. When we reach the end of the
* data, count is additionally filled with VP8_LOTS_OF_BITS. So when
* count == VP8_LOTS_OF_BITS - 1, the user's data has been exhausted.
*/
if ((br->count > VP8_BD_VALUE_SIZE) && (br->count < VP8_LOTS_OF_BITS))
{
/* We have tried to decode bits after the end of
* stream was encountered.
......
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