[XviD-devel] [BUG] Segfault with large fincr/fbase values

[Christoph Lampert]

Hi Adam,

as far as I know, the standard allows only 16bit values for fincr and 
fbase. So it's not a bug that XviD can't handle your values, but it's a 
bug that xvid crashes instead of failing in initialization with an 
"illegal values" message. 

Btw, your values should be swapped, right? fbase is supposed to be larger 
than fincr, because fps = fbase/fincr. 

gruel

On Tue, 13 Jul 2004, Adam Thayer wrote:
> libxvidcore is segfaulting in WriteVopHeader() when fincr and fbase are 
> set to unreasonably high values. While these values are not realistic, 
> the lack of bounds checking in the Bitstream<Op>() functions and the 
> lack of setting bounds when entering xvid_encore/encode() causes it to 
> write gobs of memory.
> 
> The values in question that I have seen cause a segfault are: 
> fincr=2000000, fbase=83411. This yields an fps of roughly 23.98 fps, 
> and unless some sort of framerate estimation algorithm is used before 
> setting these values, 57 seconds into the encode, WriteVopHeader() 
> attempts to write about 20+MB of bits and eventually leaves the bounds 
> of the buffer (which is 20MB in this case), segfaulting the process.
> 
> The exact situation in question is after I wrote some code to allow 
> ffmpeg to use libxvidcore as an xvid/mpeg4 encoder on Linux. An anime 
> fansub (which was encoded in XviD) got slightly corrupted after 
> encoding (one bit was off I believe, not XviD's fault) and ffmpeg 
> calculated the framerate as shown above. If I use an estimation 
> algorithm, it simplifies to: fincr=24000, fbase=1001, and encodes just 
> fine.