[FFmpeg-devel] [PATCH 1/2] avformat/icodec: ico probe with unknown data

[Mark Harris]

>>     Header:  OK, 2 frames
>>     Frame 0: Unknown (offset points beyond end of probe buffer)
>>     Frame 1: Invalid
>> Previously this example had a score of 25, even though the score would
>> be 1 if the unknown frame was known to be valid or 0 if it was known
>> to be invalid.  For this example the score is now 1.
>
> If the header is ok and the offset points beyond the end
> of the probe buffer, the score was always 25 (before I
> extended the check). Why should this be changed?
> 25 is not very high after checking ~40 bit.

If you think that 25 is better when there is 1 unknown frame and 1
invalid frame, I don't see an issue with that.  However in that case,
when the unknown frame is known to be valid it should have a score
that is at least the same value.  It doesn't make sense for the score
with an unknown value to be higher than the highest score that could
possibly be produced if the unknown value was known.

For this patch I just wanted to keep the behavior exactly the same in
the common case where all of the data is in the buffer.

For the case of unknown data, the existing code can do strange things.
For example if the probe buffer ends in the middle of a size or offset
field then it can end up reading a partial value, which can cause it
to check bytes at the wrong offset or produce false positives or false
negatives.  There were also inconsistencies if there was a mixture of
unknown and invalid data.  It seemed only logical that with unknown
data the score should never be higher than what it would be if the
data was known to be valid, and never be lower than what it would be
if the data was known to be invalid.  So I preserved the existing
behavior for the cases of known data, and adjusted the behavior for
unknown data so that the scores would be within these constraints and
so that there was no reliance on the zero padding.

 - Mark