int ff_mpeg4_decode_video_packet_header(Mpeg4DecContext *ctx)
{
    MpegEncContext *s = &ctx->m;

    int mb_num_bits      = av_log2(s->mb_num - 1) + 1;
    int header_extension = 0, mb_num, len;

    /* is there enough space left for a video packet + header */
    if (get_bits_count(&s->gb) > s->gb.size_in_bits - 20)
        return AVERROR_INVALIDDATA;

    for (len = 0; len < 32; len++)
        if (get_bits1(&s->gb))
            break;

    if (len != ff_mpeg4_get_video_packet_prefix_length(s)) {
        av_log(s->avctx, AV_LOG_ERROR, "marker does not match f_code\n");
        return AVERROR_INVALIDDATA;
    }

    if (ctx->shape != RECT_SHAPE) {
        header_extension = get_bits1(&s->gb);
    }

    mb_num = get_bits(&s->gb, mb_num_bits);
    if (mb_num >= s->mb_num || !mb_num) {
        av_log(s->avctx, AV_LOG_ERROR,
               "illegal mb_num in video packet (%d %d) \n", mb_num, s->mb_num);
        return AVERROR_INVALIDDATA;
    }

    s->mb_x = mb_num % s->mb_width;
    s->mb_y = mb_num / s->mb_width;

    if (ctx->shape != BIN_ONLY_SHAPE) {
        int qscale = get_bits(&s->gb, s->quant_precision);
        if (qscale)
            s->chroma_qscale = s->qscale = qscale;
    }

    if (ctx->shape == RECT_SHAPE)
        header_extension = get_bits1(&s->gb);

    if (header_extension) {
        int time_incr = 0;

        while (get_bits1(&s->gb) != 0)
            time_incr++;

        check_marker(s->avctx, &s->gb, "before time_increment in video packed header");
        skip_bits(&s->gb, ctx->time_increment_bits);      /* time_increment */
        check_marker(s->avctx, &s->gb, "before vop_coding_type in video packed header");

        skip_bits(&s->gb, 2); /* vop coding type */

        if (ctx->shape != BIN_ONLY_SHAPE) {
            skip_bits(&s->gb, 3); /* intra dc vlc threshold */
            if (s->pict_type == AV_PICTURE_TYPE_S &&
                ctx->vol_sprite_usage == GMC_SPRITE) {
                if (mpeg4_decode_sprite_trajectory(ctx, &s->gb) < 0)
                    return AVERROR_INVALIDDATA;
                av_log(s->avctx, AV_LOG_ERROR, "untested\n");
            }


            if (s->pict_type != AV_PICTURE_TYPE_I) {
                int f_code = get_bits(&s->gb, 3);       /* fcode_for */
                if (f_code == 0)
                    av_log(s->avctx, AV_LOG_ERROR,
                           "Error, video packet header damaged (f_code=0)\n");
            }
            if (s->pict_type == AV_PICTURE_TYPE_B) {
                int b_code = get_bits(&s->gb, 3);
                if (b_code == 0)
                    av_log(s->avctx, AV_LOG_ERROR,
                           "Error, video packet header damaged (b_code=0)\n");
            }
        }
    }
    if (ctx->new_pred)
        decode_new_pred(ctx, &s->gb);

    return 0;
}

The product reads data past the end, or before the beginning, of the intended buffer.