[Ffmpeg-devel] Divide by zero in av_reduce - Possible Bug

[Ian McIntosh]

Hi

I had an mpeg2 (Elementary Video Stream) source that had quite a few 
errors in the stream. While decoding using the ffmpeg library, my 
program would crash with a divide by zero error. Looking at the 
backtrace I noticed that in mpeg12.c the frame_rate_index was 0 just 
before calling av_reduce. The result of this is that the 'nom' and 'den' 
parameters in av_reduce are 0 which ends up making the 'gcd' variable 
inside av_reduce (rational.c) 0, which is later used as a divisor in 
another calculation which equals core dump.

Anyway it turns out that in the mpeg2 source stream there was an error 
with the 'frame rate code' bits in one of the sequence headers (they 
were 0 for some reason). A quick hack to fix this was to check the 
frame_rate_index just before the av_reduce function call and set it to 
some valid value if it was 0. This resolved the problem, but I was 
thinking that there is probably a better way to handle this condition. 
Anyone have any thoughts on the matter or suggestions for a fix ?

Unfortunately I no longer have the source as I mistakenly deleted it 
thinking it was something else (sometimes I do some pretty dumb things). 
However regenerating such a source using a hex-editor and a valid mpeg2 
source would be quite easy.

Thanks
Ian