[FFmpeg-devel] [PATCH 1/1] aacenc_pred: prevent UB in ff_aac_adjust_common_pred()

[Sean McGovern]

Hi Andreas,

First off all, thanks for having a look! :)

On Tue, Feb 27, 2024 at 1:37 PM Andreas Rheinhardt
<andreas.rheinhardt at outlook.com> wrote:
>
> Sean McGovern:
> > ---
> >  libavcodec/aacenc_pred.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/libavcodec/aacenc_pred.c b/libavcodec/aacenc_pred.c
> > index f87fcd5a00..d3efade85e 100644
> > --- a/libavcodec/aacenc_pred.c
> > +++ b/libavcodec/aacenc_pred.c
> > @@ -162,9 +162,11 @@ void ff_aac_adjust_common_pred(AACEncContext *s, ChannelElement *cpe)
> >          sce1->ics.window_sequence[0] == EIGHT_SHORT_SEQUENCE)
> >          return;
> >
> > +    const int num_swb = FFMIN(sce0->ics.num_swb, sizeof(sce0->ics.prediction_used));
> > +
> >      for (w = 0; w < sce0->ics.num_windows; w += sce0->ics.group_len[w]) {
> >          start = 0;
> > -        for (g = 0; g < sce0->ics.num_swb; g++) {
> > +        for (g = 0; g < num_swb; g++) {
> >              int sfb = w*16+g;
> >              int sum = sce0->ics.prediction_used[sfb] + sce1->ics.prediction_used[sfb];
> >              float ener0 = 0.0f, ener1 = 0.0f, ener01 = 0.0f;
>
> As you can see, the actual index used for accesses is w*16 + g and not
> only g. So I was surprised that your fix fixed the test (as you claim).
> Digging into the code, num_windows can be either 1 or eight and it is
> only eight if window_sequence[0] is EIGHT_SHORT_SEQUENCE (see lines
> 477-488 in aacpsy.c as well as lines 877-897 in aacenc.c). In case
> window_sequence[0] is EIGHT_SHORT_SEQUENCE, we do not even enter this
> loop in ff_aac_adjust_common_pred(). This means that the outer loop
> above is actually not a loop at all and your fix would indeed fix the
> undefined behaviour.
> But this also shows that this whole code is a mess. Someone who actually
> knows it should take a look. Or maybe the grim reaper.
> Anyway, your fix would lead to a wdeclaration-after-statement warning.
>

Ooof, OK thanks. I was wondering about that when I looked on Patchwork.

Thanks,
-- Sean McGovern