[FFmpeg-devel] [PATCH] avformat/amr: reduce raw amr false positive detection

[Carl Eugen Hoyos]

Am Mi., 2. Okt. 2019 um 20:19 Uhr schrieb Paul B Mahol <onemda at gmail.com>:
>
> Signed-off-by: Paul B Mahol <onemda at gmail.com>
> ---
>  libavformat/amr.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/libavformat/amr.c b/libavformat/amr.c
> index 42840a50a3..600cb1b0f0 100644
> --- a/libavformat/amr.c
> +++ b/libavformat/amr.c
> @@ -190,9 +190,12 @@ static int amrnb_probe(const AVProbeData *p)
>                  if (b[++i] != last)
>                      break;
>              }
> -            if (size > 0) {
> -                valid++;
> -                i += size;
> +            while (size--) {
> +                if (b[i])
> +                    valid++;
> +                else
> +                    invalid++;
> +                i++;
>              }
>          } else {
>              valid = 0;
> @@ -246,9 +249,12 @@ static int amrwb_probe(const AVProbeData *p)
>                  if (b[++i] != last)
>                      break;
>              }
> -            if (size > 0) {
> -                valid++;
> -                i += size;
> +            while (size--) {
> +                if (b[i])
> +                    valid++;
> +                else
> +                    invalid++;
> +                i++;
>              }
>          } else {
>              valid = 0;

The changes to amrwb are unneeded for the file you provided.
(A "PGP Secret Sub-key" according to file.)

I encoded a few amr files and "00 00" seems common and not
invalid.

The following works here but I believe the main issue is that
our amr decoder happily decodes the "PGP Secret Sub-key"
without any error messages so I wonder if the detection is
wrong at all.

diff --git a/libavformat/amr.c b/libavformat/amr.c
index 42840a50a3..2645af95c2 100644
--- a/libavformat/amr.c
+++ b/libavformat/amr.c
@@ -186,8 +186,18 @@ static int amrnb_probe(const AVProbeData *p)
         if (mode < 9 && (b[i] & 0x4) == 0x4) {
             int last = b[i];
             int size = amrnb_packed_size[mode];
+            int changes = 0, repeats = 0;
             while (size--) {
                 if (b[++i] != last)
+                    changes++;
+                else
+                    repeats++;
+                last = b[i];
+                if (repeats >= 2) {
+                    i += size;
+                    size = 0;
+                    break;
+                } else if (changes > 4)
                     break;
             }
             if (size > 0) {
@@ -200,7 +210,7 @@ static int amrnb_probe(const AVProbeData *p)
             i++;
         }
     }
-    if (valid > 100 && valid >> 4 > invalid)
+    if (valid > 120 && valid >> 4 > invalid)
         return AVPROBE_SCORE_EXTENSION / 2 + 1;
     return 0;
 }


Carl Eugen