[FFmpeg-devel] [PATCH] avfilter/af_alimiter: fix misbehavior when nb_channels != 2

[David Flater]

Some code in alimiter assumes that there are 2 channels, resulting in
clipping if the loudest channel is 3 or above and an out-of-bounds read if
the input is monophonic.  Fix that in 2 places.

Signed-off-by: David Flater 
---
 libavfilter/af_alimiter.c | 23 +--
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/libavfilter/af_alimiter.c b/libavfilter/af_alimiter.c
index 622dc66324..c683c4bcf4 100644
--- a/libavfilter/af_alimiter.c
+++ b/libavfilter/af_alimiter.c
@@ -194,10 +194,11 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
 } else {
 for (i = s->nextiter; i < s->nextiter + s->nextlen; i++) {
 int j = i % buffer_size;
-double ppeak, pdelta;
+double ppeak = 0, pdelta;
 
-ppeak = fabs(buffer[nextpos[j]]) > fabs(buffer[nextpos[j] 
+ 1]) ?
-fabs(buffer[nextpos[j]]) : fabs(buffer[nextpos[j] 
+ 1]);
+for (c = 0; c < channels; c++) {
+ppeak = FFMAX(ppeak, fabs(buffer[nextpos[j] + c]));
+}
 pdelta = (limit / peak - limit / ppeak) / (((buffer_size - 
nextpos[j] + s->pos) % buffer_size) / channels);
 if (pdelta < nextdelta[j]) {
 nextdelta[j] = pdelta;
@@ -241,14 +242,16 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
 s->delta = get_rdelta(s, release, inlink->sample_rate,
   peak, limit, s->att, 1);
 if (s->nextlen > 1) {
+double ppeak = 0, pdelta;
 int pnextpos = nextpos[(s->nextiter + 1) % buffer_size];
-double ppeak = fabs(buffer[pnextpos]) > 
fabs(buffer[pnextpos + 1]) ?
-
fabs(buffer[pnextpos]) :
-
fabs(buffer[pnextpos + 1]);
-double pdelta = (limit / ppeak - s->att) /
-(((buffer_size + pnextpos -
-((s->pos + channels) % buffer_size)) %
-buffer_size) / channels);
+
+for (c = 0; c < channels; c++) {
+ppeak = FFMAX(ppeak, fabs(buffer[pnextpos + c]));
+}
+pdelta = (limit / ppeak - s->att) /
+ (((buffer_size + pnextpos -
+ ((s->pos + channels) % buffer_size)) %
+ buffer_size) / channels);
 if (pdelta < s->delta)
 s->delta = pdelta;
 }
-- 
2.35.3

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Re: [FFmpeg-devel] [PATCH] avfilter/af_alimiter: fix misbehavior when nb_channels != 2

[David Flater]

This is a friendly reminder.

Cc: Paul B Mahol 

On 2022-08-14 21:53, David Flater wrote:
> Some code in alimiter assumes that there are 2 channels, resulting in
> clipping if the loudest channel is 3 or above and an out-of-bounds read if
> the input is monophonic.  Fix that in 2 places.
> 
> Signed-off-by: David Flater 
> ---
>  libavfilter/af_alimiter.c | 23 +--
>  1 file changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/libavfilter/af_alimiter.c b/libavfilter/af_alimiter.c
> index 622dc66324..c683c4bcf4 100644
> --- a/libavfilter/af_alimiter.c
> +++ b/libavfilter/af_alimiter.c
> @@ -194,10 +194,11 @@ static int filter_frame(AVFilterLink *inlink, AVFrame 
> *in)
>  } else {
>  for (i = s->nextiter; i < s->nextiter + s->nextlen; i++) {
>  int j = i % buffer_size;
> -double ppeak, pdelta;
> +double ppeak = 0, pdelta;
>  
> -ppeak = fabs(buffer[nextpos[j]]) > 
> fabs(buffer[nextpos[j] + 1]) ?
> -fabs(buffer[nextpos[j]]) : 
> fabs(buffer[nextpos[j] + 1]);
> +for (c = 0; c < channels; c++) {
> +ppeak = FFMAX(ppeak, fabs(buffer[nextpos[j] + c]));
> +}
>  pdelta = (limit / peak - limit / ppeak) / (((buffer_size 
> - nextpos[j] + s->pos) % buffer_size) / channels);
>  if (pdelta < nextdelta[j]) {
>  nextdelta[j] = pdelta;
> @@ -241,14 +242,16 @@ static int filter_frame(AVFilterLink *inlink, AVFrame 
> *in)
>  s->delta = get_rdelta(s, release, inlink->sample_rate,
>peak, limit, s->att, 1);
>  if (s->nextlen > 1) {
> +double ppeak = 0, pdelta;
>  int pnextpos = nextpos[(s->nextiter + 1) % buffer_size];
> -double ppeak = fabs(buffer[pnextpos]) > 
> fabs(buffer[pnextpos + 1]) ?
> -
> fabs(buffer[pnextpos]) :
> -
> fabs(buffer[pnextpos + 1]);
> -double pdelta = (limit / ppeak - s->att) /
> -(((buffer_size + pnextpos -
> -((s->pos + channels) % buffer_size)) %
> -buffer_size) / channels);
> +
> +for (c = 0; c < channels; c++) {
> +ppeak = FFMAX(ppeak, fabs(buffer[pnextpos + c]));
> +}
> +pdelta = (limit / ppeak - s->att) /
> + (((buffer_size + pnextpos -
> + ((s->pos + channels) % buffer_size)) %
> + buffer_size) / channels);
>  if (pdelta < s->delta)
>  s->delta = pdelta;
>  }

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