[FFmpeg-devel] [PATCH] Fix potential av_find_opt() crash if context is NULL

[Stefano Sabatini]

On date Sunday 2008-11-09 13:36:39 +0100, Michael Niedermayer encoded:
> On Sun, Nov 09, 2008 at 12:48:37PM +0100, Stefano Sabatini wrote:
> > On date Thursday 2008-10-30 23:07:29 +0100, Michael Niedermayer encoded:
> > > On Thu, Oct 30, 2008 at 09:05:32PM +0100, Stefano Sabatini wrote:
> > [...]
> > > > OK, but then we should check for its nullness every time we access to
> > > > it in cmdline.c, since that's generic code and from there we can't
> > > > know if sws_opts has been initialized or not, same consideration for
> > > > avformat_opts and avcodec_opts.
> > > > 
> > > > *Or*, we can change the semantics of av_find_opt() (micro bump) to
> > > > make it accept a NULL, for which solution I have a slight preference.
> > > 
> > > i disagree, its all fine as it is, NULL is not allowed and that should be
> > > clear from the docs,
> > > the avfilter patch is broken its the onle thing that need fixing.
> > 
> > I'll try to explain myself better...
> > 
> > av*_opts and swscale_opts are declared and defined (as NULL) in
> > cmdutils.c, applications linking to it have to define them to
> > something different from NULL or they will crash when calling
> > opt_default().
> > 
> > Patching libavfilter-soc would mean in this case something like the
> > following in cmdutils.c:
> > 
> > #ifndef CONFIG_AVFILTER
> >     if(!o)
> >         o = av_set_string2(sws_opts, opt, arg, 1);
> > #endif
> > 
> > which I think it's just plain wrong, since cmdutils.o should be
> > application agnostic, and there is no reason for which we shouldn't
> > look in sws_opts simply because CONFIG_AVFILTER is set.
> 
> there is NO sws_opts when avfilter is in use! The variable does NOT exist
> semantically.

Currently sws_opts is declared as extern in cmdutils.h and defined in
cmdutils.c, then used in the ff*.c tools, so the variable *does* exist
semantically for every application, with or without
--enable-libavfilter, what changes is that for example in
libavfilter-soc ffplay.c its value is set only if libavfilter is not
enabled.

#if !ENABLE_AVFILTER
    sws_opts = sws_getContext(16,16,0, 16,16,0, sws_flags, NULL,NULL,NULL);
#endif

I'm not sure, but I remember vaguely that in some older
libavfilter-soc version sws_opts declaration and use was under an #if
!ENABLE_AVFILTER.

After your patch which moved opt_default() and *_opts to
cmdutils.[ch], in oder to get the same effect we should patch
cmdutils.[ch], which as I already wrote I don't think is a good idea.

> Thats simply because the corresponding command line option does NOT exist
> when avfilter is enabled.

Which commandline option? Do you mean sws_flags?

Sorry if I'm misunderstanding something, regards.
-- 
FFmpeg = Friendly and Fantastic MultiPurpose Educated Glue