[FFmpeg-devel] [PATCH] avcodec/ac3dec: Check expacc

[Andreas Cadhalpun]

On 03.11.2016 11:30, Michael Niedermayer wrote:
> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> ---
>  libavcodec/ac3dec.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
> index a95c204..499971a 100644
> --- a/libavcodec/ac3dec.c
> +++ b/libavcodec/ac3dec.c
> @@ -426,6 +426,10 @@ static int decode_exponents(AC3DecodeContext *s,
>      group_size = exp_strategy + (exp_strategy == EXP_D45);
>      for (grp = 0, i = 0; grp < ngrps; grp++) {
>          expacc = get_bits(gbc, 7);
> +        if (expacc >= 125) {

This is somewhat a magic number, which can be understood from reading section
"7.1.2 Exponent Strategy" of the ac3 specification, in short:
Three exponents each represented as number 0-4 are grouped together and
base-5 encoded, so the maximal correct value is 25*4 + 5*4 + 4 = 124.

Maybe mention this in the commit message.

> +            av_log(s->avctx, AV_LOG_ERROR, "expacc %d is out-of-range\n", expacc);
> +            return AVERROR_INVALIDDATA;
> +        }
>          dexp[i++] = ungroup_3_in_7_bits_tab[expacc][0];
>          dexp[i++] = ungroup_3_in_7_bits_tab[expacc][1];
>          dexp[i++] = ungroup_3_in_7_bits_tab[expacc][2];
> 

Anyway, the change itself looks good to me.

Best regards,
Andreas