Re: [FFmpeg-devel] [PATCH] avformat/mov: (v4) fix get_eia608_packet

[Pavel Koshevoy <pkoshevoy@gmail.com>]

If there are no further constructive review comments today and tomorrow
then I will commit and push this change on Sunday (if I don't forget).

Pavel.

On Thu, Feb 13, 2025 at 2:22 PM Pavel Koshevoy <pkoshevoy@gmail.com> wrote:

> The problem is reproducible with "Test for Quicktime 608 CC file.mov"
> from https://samples.ffmpeg.org/MPEG2/subcc/
>
> ffmpeg -i "Test for Quicktime 608 CC file.mov" -map 0 -c copy -y
> remuxed.mov
>
> Prior to the fix QuickTime Player playback of remuxed.mov would
> render garbage text for "English CC" subtitles.
> ---
>  libavformat/mov.c | 70 +++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 59 insertions(+), 11 deletions(-)
>
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index 85aef33b19..5a91ef5b8c 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -10788,25 +10788,73 @@ static int mov_change_extradata(AVStream *st,
> AVPacket *pkt)
>      return 0;
>  }
>
> -static int get_eia608_packet(AVIOContext *pb, AVPacket *pkt, int size)
> +static int get_eia608_packet(AVIOContext *pb, AVPacket *pkt, int src_size)
>  {
> -    int new_size, ret;
> +    /* We can't make assumptions about the structure of the payload,
> +       because it may include multiple cdat and cdt2 samples. */
> +    const uint32_t cdat = AV_RB32("cdat");
> +    const uint32_t cdt2 = AV_RB32("cdt2");
> +    int ret, out_size = 0;
>
> -    if (size <= 8)
> +    /* a valid payload must have size, 4cc, and at least 1 byte pair: */
> +    if (src_size < 10)
>          return AVERROR_INVALIDDATA;
> -    new_size = ((size - 8) / 2) * 3;
> -    ret = av_new_packet(pkt, new_size);
> +
> +    /* avoid an int overflow: */
> +    if ((src_size - 8) / 2 >= INT_MAX / 3)
> +        return AVERROR_INVALIDDATA;
> +
> +    ret = av_new_packet(pkt, ((src_size - 8) / 2) * 3);
>      if (ret < 0)
>          return ret;
>
> -    avio_skip(pb, 8);
> -    for (int j = 0; j < new_size; j += 3) {
> -        pkt->data[j] = 0xFC;
> -        pkt->data[j+1] = avio_r8(pb);
> -        pkt->data[j+2] = avio_r8(pb);
> +    /* parse and re-format the c608 payload in one pass. */
> +    while (src_size >= 10) {
> +        const uint32_t atom_size = avio_rb32(pb);
> +        const uint32_t atom_type = avio_rb32(pb);
> +        const uint32_t data_size = atom_size - 8;
> +        const uint8_t cc_field =
> +            atom_type == cdat ? 1 :
> +            atom_type == cdt2 ? 2 :
> +            0;
> +
> +        /* account for bytes consumed for atom size and type. */
> +        src_size -= 8;
> +
> +        /* make sure the data size stays within the buffer boundaries. */
> +        if (data_size < 2 || data_size > src_size) {
> +            ret = AVERROR_INVALIDDATA;
> +            break;
> +        }
> +
> +        /* make sure the data size is consistent with N byte pairs. */
> +        if (data_size % 2 != 0) {
> +            ret = AVERROR_INVALIDDATA;
> +            break;
> +        }
> +
> +        if (!cc_field) {
> +            /* neither cdat or cdt2 ... skip it */
> +            avio_skip(pb, data_size);
> +            src_size -= data_size;
> +            continue;
> +        }
> +
> +        for (int32_t i = 0; i < data_size; i += 2) {
> +            pkt->data[out_size] = (0x1F << 3) | (1 << 2) | (cc_field - 1);
> +            pkt->data[out_size + 1] = avio_r8(pb);
> +            pkt->data[out_size + 2] = avio_r8(pb);
> +            out_size += 3;
> +            src_size -= 2;
> +        }
>      }
>
> -    return 0;
> +    if (src_size > 0)
> +        /* skip any remaining unread portion of the input payload */
> +        avio_skip(pb, src_size);
> +
> +    av_shrink_packet(pkt, out_size);
> +    return ret;
>  }
>
>  static int mov_finalize_packet(AVFormatContext *s, AVStream *st,
> AVIndexEntry *sample,
> --
> 2.43.0
>
>
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