Fix the issue of undefined-shift in VP8GetBit.

webrtc/modules/video_coding/utility/vp8_header_parser.cc

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 #include "webrtc/modules/video_coding/utility/vp8_header_parser.h"
@@ skipping 48 matching lines @@
 
 static void VP8InitBitReader(VP8BitReader* const br,
                              const uint8_t* const start,
                              const uint8_t* const end) {
   br->range_ = 255 - 1;
   br->buf_ = start;
   br->buf_end_ = end;
   br->value_ = 0;
   br->bits_ = -8;  // To load the very first 8bits.
   br->eof_ = 0;
+  br->error_ = false;

[pbos-webrtc, 2016/04/18 19:25:33]

Is this already covered by the eof_ flag? If so we don't need error_.

[jackychen_, 2016/04/18 20:49:47]

Not really, the error here might not be the same with what eof_ covers,
but do you want me to reuse eof_ to also cover this error?

[pbos-webrtc, 2016/04/18 20:52:02]

Have you diagnosed the actual error that happens?

   VP8LoadNewBytes(br);
 }
 
 // Read a bit with proba 'prob'.
 static int VP8GetBit(VP8BitReader* const br, int prob) {
   uint8_t range = br->range_;
   if (br->bits_ < 0) {
     VP8LoadNewBytes(br);
   }
-
+  if (br->bits_ < 0) {

[pbos-webrtc, 2016/04/18 19:25:33]

Shouldn't this be set inside VP8LoadNewBytes when it fails?

[jackychen_, 2016/04/18 20:49:47]

I want to stop the operation on br->bits here since otherwise I need to change
the return value of VP8LoadNewBytes and also add the judge here. Besides, the
error may also happens in VP8LoadFinalBytes. So I thought it might be better to
put it here.

+    br->error_ = true;
+    return 0;
+  }
   const int pos = br->bits_;
   const uint8_t split = (range * prob) >> 8;
   const uint8_t value = static_cast<uint8_t>(br->value_ >> pos);
   int bit;
   if (value > split) {
     range -= split + 1;
     br->value_ -= static_cast<uint32_t>(split + 1) << pos;
     bit = 1;
   } else {
     range = split;
@@ skipping 99 matching lines @@
   ParseSegmentHeader(&br);
   ParseFilterHeader(&br);
   // Number of coefficient data partitions.
   VP8GetValue(&br, 2);
   // Base QP.
   const int base_q0 = VP8GetValue(&br, 7);
   if (br.eof_ == 1) {
     LOG(LS_WARNING) << "Failed to get QP, end of file reached.";
     return false;
   }
+  if (br.error_) {
+    LOG(LS_WARNING) << "Failed to get QP, invalid bitstream.";

[pbos-webrtc, 2016/04/18 20:56:46]

Are you sure that this isn't because the stream is cut off (e.g. should go under
eof_?)?

+    return false;
+  }
   *qp = base_q0;
   return true;
 }
 
 }  // namespace vp8
 
 }  // namespace webrtc