Re: [FFmpeg-devel] [PATCH] avformat/iamf_parse: increase PutBytes buffer when writing AAC extradata

[Michael Niedermayer <michael@niedermayer.cc>]

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On Tue, May 13, 2025 at 08:45:00PM -0300, James Almer wrote:
> We may write up to 43 bits, so 5 bytes is not enough.
> 
> Fixes: Assertion n>=0 && n<=32 failed at ./libavcodec/get_bits.h:406
> Fixes: 398527871/clusterfuzz-testcase-minimized-ffmpeg_dem_IAMF_fuzzer-6602025714647040
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: James Almer <jamrial@gmail.com>
> ---
>  libavformat/iamf_parse.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/libavformat/iamf_parse.c b/libavformat/iamf_parse.c
> index abedfdb066..11c27ebe98 100644
> --- a/libavformat/iamf_parse.c
> +++ b/libavformat/iamf_parse.c
> @@ -285,7 +285,7 @@ static int update_extradata(AVCodecParameters *codecpar)
>          AV_WL16A(codecpar->extradata + 16, AV_RB16A(codecpar->extradata + 16)); // Byte swap Output Gain
>          break;
>      case AV_CODEC_ID_AAC: {
> -        uint8_t buf[5];
> +        uint8_t buf[6];
>  
>          init_put_bits(&pb, buf, sizeof(buf));
>          ret = init_get_bits8(&gb, codecpar->extradata, codecpar->extradata_size);
> @@ -304,6 +304,10 @@ static int update_extradata(AVCodecParameters *codecpar)
>          skip_bits(&gb, 4);
>          put_bits(&pb, 4, codecpar->ch_layout.nb_channels); // set channel config
>          ret = put_bits_left(&pb);
> +        while (ret >= 32) {
> +           put_bits32(&pb, get_bits_long(&gb, 32));
> +           ret -= 32;
> +        }
>          put_bits(&pb, ret, get_bits_long(&gb, ret));
>          flush_put_bits(&pb);

bit_copy() from libavcodec/dvdec.c seems doing the same
maybe this can be factored somewhere before or after this patch

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Into a blind darkness they enter who follow after the Ignorance,
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