[FFmpeg-devel] [PATCH] fix possible crash if vpre before vcodec

[Robert Swain]

2009/2/27 Michael Niedermayer <michaelni at gmx.at>:
> On Fri, Feb 27, 2009 at 03:10:13PM +0100, Michael Niedermayer wrote:
>> On Fri, Feb 27, 2009 at 02:54:58PM +0100, Reimar D?ffinger wrote:
>> > On Fri, Feb 27, 2009 at 01:33:02PM +0000, Robert Swain wrote:
>> > > 2009/2/27 Reimar D?ffinger <Reimar.Doeffinger at stud.uni-karlsruhe.de>:
>> > > > On Fri, Feb 27, 2009 at 12:36:27PM +0000, Robert Swain wrote:
>> > > >> 2009/2/27 Michael Niedermayer <michaelni at gmx.at>:
>> > > >> > On Fri, Feb 27, 2009 at 11:25:26AM +0100, Reimar D?ffinger wrote:
>> > > >> >> I recently tried the following command
>> > > >> >> ffmpeg -i baldursgate-logo.mve -vpre hq -vcodec libx264 -b 500k -an out.mp4
>> > > >> >> and wondered why it would not work.
>> > > >> >> strace showed the following:
>> > > >> >> open("/home/reimar/.ffmpeg/hq.ffpreset", O_RDONLY) = -1 ENOENT (No such file or directory)
>> > > >> >> open("/home/reimar/.ffmpeg/(null)-hq.ffpreset", O_RDONLY) = -1 ENOENT (No such file or directory)
>> > > >> >> open("/usr/local/share/ffmpeg/hq.ffpreset", O_RDONLY) = -1 ENOENT (No such file or directory)
>> > > >> >> open("/usr/local/share/ffmpeg/(null)-hq.ffpreset", O_RDONLY) = -1 ENOENT (No such file or directory)
>> > > >> >> obviously ffmpeg tries to open the file already during option parsing (this could probably be considered
>> > > >> >> the real bug). When -vcodec is set only afterwards it passes NULL to
>> > > >> >> snprintf %s argument which may (and will) crash with some
>> > > >> >> implementations.
>> > > >> >> I propose a patch that at least avoids the crash, but e.g. this:
>> > > >> >> -vcodec blub -vpre hq -vcodec libx264
>> > > >> >> will still have the counter-intuitive behaviour of reading
>> > > >> >> blub-hq.ffpreset but encoding with libx264.
>> > > >> >
>> > > >> > would it not make more sense to warn the user that the codec is not set
>> > > >> > before -*pre instead of silently skiping searching for the file?
>> > > >>
>> > > >> I agree. I prefer command lines to error out on options that don't
>> > > >> work as intended than to continue on. But a warning at least is
>> > > >> absolutely necessary.
>> > > >>
>> > > >> Also, it should be documented that the preset options should come
>> > > >> after the codec name.
>> > > >
>> > > > Well, the warning still would not work right, as the example
>> > > > -vcodec blub -vpre hq -vcodec libx264
>> > > > above shows, that's why I didn't consider it particularly useful.
>> > >
>> > > Maybe the non-explicit codec idea wasn't so good after all. Maybe we
>> > > should suggest people use -vpre libx264-hq?
>> >
>> > I think it was a good idea, just the implementation is not optimal.
>> > I don't know FFmpeg command line parsing well, but in MPlayer I'd just
>> > store the -?pre string somewhere and open the file after parsing all the
>> > other options...
>>
>> can be done, add a static char, store it there
>> then call the parsing function from new_video_stream() and others
>> but beware of chains of bitstream filters ...
>>
>> [i hope ive not missed some detail ...]
>
> one ive missed ...
>
> currently -vpre balh -some thing
> allows things to be overridden from vpre but if vpre is set later
> this would no longer work

Well-spotted. This functionality is very useful for applying
constraints to quality presets and for overriding things for specific
circumstances. Back to the drawing board.

Regards,
Rob