[FFmpeg-devel] [PATCH 1/8] avcodec/cinepak: Require 1 bit per 4x4 block as minimum input

[Tomas Härdin]

sön 2019-08-18 klockan 19:04 +0200 skrev Michael Niedermayer:
> On Sun, Aug 18, 2019 at 02:49:39PM +0200, Tomas Härdin wrote:
> > sön 2019-08-18 klockan 14:18 +0200 skrev Michael Niedermayer:
> > > On Sun, Aug 18, 2019 at 01:40:01PM +0200, Tomas Härdin wrote:
> > > > sön 2019-08-18 klockan 12:19 +0200 skrev Michael Niedermayer:
> > > > > On Sun, Aug 18, 2019 at 12:00:45PM +0200, Paul B Mahol wrote:
> > > > > > On Sun, Aug 18, 2019 at 11:44 AM Michael Niedermayer <michael at niedermayer.cc>
> > > > > > > and yes i too wish there was a magic fix but i think most things that
> > > > > > > look like magic fixes have a fatal flaw. But maybe iam missing something
> > > > > > > in fact i hope that iam missing something and that there is a magic fix
> > > > > > > 
> > > > > > 
> > > > > > Magic fix is enabling reference counted frames in fuzzer.
> > > > > 
> > > > > That is covered by the part below which you maybe did not read
> > > > > 
> > > > > > > PS: if you think of changing the API, i dont think its the API.
> > > > > > > I mean every user application will read the frames it receives, so
> > > > > > > even if inside the decoder we just return the same frame with 2 pixels
> > > > > > > different the user doesnt know this and has to read the whole frame.
> > > > > > > The problem is moved around but its still there.
> > > > 
> > > > Copying is still slower than not copying. Enabling refcounting fixes
> > > > the timeout issue here, and will likely silence a whole bunch of false
> > > > positives for this class of files.
> > > 
> > > it makes probably sense to enable ref counting but we should
> > > immedeatly in the next or a previous commit make the fuzzer read the frames
> > > from the  decoder. Thats what basically every user app would do.
> > > Otherwise we would have a bunch of issues closed and then reopened
> > > later.
> > 
> > Why should we care how much work the user does? We're fuzzing lavc
> > here, not user programs. Certain use cases are decode-only (ffprobe
> > -show_frames for example)
> 
> thats a valid point of view
> 
> The user though has few options if she gets many frames, she can just
> continue processing or stop, she doesnt know how (in)valid the stream is

This is way too abstract. Who is this user, specifically? If we were
talking about ffmpeg, handbrake, peertube or something else then we
could actually help them if they're using the API incorrectly. Worrying
about hypotheticals is a waste of time. You can't test them.

> OTOH libavcodec knows the codec bitstream, and can check various things
> 
> so just moving the responsibility to the user app would move it where
> its much harder to fix well
> That is currently, we could export all kinds of metadata about the
> amount of errors. Then a user app could decide what to do.
> It would become duplicated code between user apps though...

No one outside of forensics or satellite TV is going to care about
that, and if they do they can pay for it, and we can maintain tests for
it. It's not hard for a user to add checks like "if resolution > 3840 ×
 2160 then reject the file", which makes a lot of these fuzzy "anti-
DoS" patches pointless busywork. It just causes developer fatigue.

/Tomas