[FFmpeg-devel] [PATCH] avcodec/aacenc: Avoid 0 lambda

[Michael Niedermayer]

On Mon, May 31, 2021 at 09:22:09AM +0200, Anton Khirnov wrote:
> Quoting Michael Niedermayer (2021-05-28 22:15:51)
> > Fixes: Ticket8003
> > Fixes: CVE-2020-20453
> > 
> > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > ---
> >  libavcodec/aacenc.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/libavcodec/aacenc.c b/libavcodec/aacenc.c
> > index aa223cf25f..e80591ba86 100644
> > --- a/libavcodec/aacenc.c
> > +++ b/libavcodec/aacenc.c
> > @@ -28,6 +28,7 @@
> >   *              TODOs:
> >   * add sane pulse detection
> >   ***********************************/
> > +#include <float.h>
> >  
> >  #include "libavutil/libm.h"
> >  #include "libavutil/float_dsp.h"
> > @@ -852,7 +853,7 @@ static int aac_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
> >                  /* Not so fast though */
> >                  ratio = sqrtf(ratio);
> >              }
> > -            s->lambda = FFMIN(s->lambda * ratio, 65536.f);
> > +            s->lambda = av_clipf(s->lambda * ratio, FLT_MIN, 65536.f);
> 
> Would FLT_EPSILON not be more appropriate? IIUC FLT_MIN is still
> effectively zero.

yes, i was just trying to eliminate the x/0.
In theory, lambda = 0 or lambda = infinity is not semantically wrong, one
would mean smallest file disregarding distortion the other lowest distortion
disregarding size.
I do not know what the intend of the author of the original code was
But the places where divisions by lambda seem to happen semm all followed
by cliping so i guess the minimum doesnt matter and all reasonable
options will give the same result more or less.
ill change it to FLT_EPSILON

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Rewriting code that is poorly written but fully understood is good.
Rewriting code that one doesnt understand is a sign that one is less smart
than the original author, trying to rewrite it will not make it better.
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