int ff_h263_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
                         AVPacket *avpkt)
{
    const uint8_t *buf = avpkt->data;
    int buf_size       = avpkt->size;
    MpegEncContext *s  = avctx->priv_data;
    int ret;
    int slice_ret = 0;
    AVFrame *pict = data;

    /* no supplementary picture */
    if (buf_size == 0) {
        /* special case for last picture */
        if (s->low_delay == 0 && s->next_picture_ptr) {
            if ((ret = av_frame_ref(pict, s->next_picture_ptr->f)) < 0)
                return ret;
            s->next_picture_ptr = NULL;

            *got_frame = 1;
        }

        return 0;
    }

    if (s->avctx->flags & AV_CODEC_FLAG_TRUNCATED) {
        int next;

        if (CONFIG_MPEG4_DECODER && s->codec_id == AV_CODEC_ID_MPEG4) {
            next = ff_mpeg4_find_frame_end(&s->parse_context, buf, buf_size);
        } else if (CONFIG_H263_DECODER && s->codec_id == AV_CODEC_ID_H263) {
            next = ff_h263_find_frame_end(&s->parse_context, buf, buf_size);
        } else if (CONFIG_H263P_DECODER && s->codec_id == AV_CODEC_ID_H263P) {
            next = ff_h263_find_frame_end(&s->parse_context, buf, buf_size);
        } else {
            av_log(s->avctx, AV_LOG_ERROR,
                   "this codec does not support truncated bitstreams\n");
            return AVERROR(ENOSYS);
        }

        if (ff_combine_frame(&s->parse_context, next, (const uint8_t **)&buf,
                             &buf_size) < 0)
            return buf_size;
    }

retry:
    if (s->divx_packed && s->bitstream_buffer_size) {
        int i;
        for(i=0; i < buf_size-3; i++) {
            if (buf[i]==0 && buf[i+1]==0 && buf[i+2]==1) {
                if (buf[i+3]==0xB0) {
                    av_log(s->avctx, AV_LOG_WARNING, "Discarding excessive bitstream in packed xvid\n");
                    s->bitstream_buffer_size = 0;
                }
                break;
            }
        }
    }

    if (s->bitstream_buffer_size && (s->divx_packed || buf_size <= MAX_NVOP_SIZE)) // divx 5.01+/xvid frame reorder
        ret = init_get_bits8(&s->gb, s->bitstream_buffer,
                             s->bitstream_buffer_size);
    else
        ret = init_get_bits8(&s->gb, buf, buf_size);

    s->bitstream_buffer_size = 0;
    if (ret < 0)
        return ret;

    if (!s->context_initialized)
        ff_mpv_idct_init(s);

    /* let's go :-) */
    if (CONFIG_WMV2_DECODER && s->msmpeg4_version == 5) {
        ret = ff_wmv2_decode_picture_header(s);
    } else if (CONFIG_MSMPEG4_DECODER && s->msmpeg4_version) {
        ret = ff_msmpeg4_decode_picture_header(s);
    } else if (CONFIG_MPEG4_DECODER && avctx->codec_id == AV_CODEC_ID_MPEG4) {
        if (s->avctx->extradata_size && s->picture_number == 0) {
            GetBitContext gb;

            if (init_get_bits8(&gb, s->avctx->extradata, s->avctx->extradata_size) >= 0 )
                ff_mpeg4_decode_picture_header(avctx->priv_data, &gb);
        }
        ret = ff_mpeg4_decode_picture_header(avctx->priv_data, &s->gb);
    } else if (CONFIG_H263I_DECODER && s->codec_id == AV_CODEC_ID_H263I) {
        ret = ff_intel_h263_decode_picture_header(s);
    } else if (CONFIG_FLV_DECODER && s->h263_flv) {
        ret = ff_flv_decode_picture_header(s);
    } else {
        ret = ff_h263_decode_picture_header(s);
    }

    if (ret < 0 || ret == FRAME_SKIPPED) {
        if (   s->width  != avctx->coded_width
            || s->height != avctx->coded_height) {
                av_log(s->avctx, AV_LOG_WARNING, "Reverting picture dimensions change due to header decoding failure\n");
                s->width = avctx->coded_width;
                s->height= avctx->coded_height;
        }
    }
    if (ret == FRAME_SKIPPED)
        return get_consumed_bytes(s, buf_size);

    /* skip if the header was thrashed */
    if (ret < 0) {
        av_log(s->avctx, AV_LOG_ERROR, "header damaged\n");
        return ret;
    }

    if (!s->context_initialized) {
        avctx->pix_fmt = h263_get_format(avctx);
        if ((ret = ff_mpv_common_init(s)) < 0)
            return ret;
    }

    if (!s->current_picture_ptr || s->current_picture_ptr->f->data[0]) {
        int i = ff_find_unused_picture(s->avctx, s->picture, 0);
        if (i < 0)
            return i;
        s->current_picture_ptr = &s->picture[i];
    }

    avctx->has_b_frames = !s->low_delay;

    if (CONFIG_MPEG4_DECODER && avctx->codec_id == AV_CODEC_ID_MPEG4) {
        if (ff_mpeg4_workaround_bugs(avctx) == 1)
            goto retry;
        if (s->studio_profile != (s->idsp.idct == NULL))
            ff_mpv_idct_init(s);
    }

    /* After H.263 & MPEG-4 header decode we have the height, width,
     * and other parameters. So then we could init the picture.
     * FIXME: By the way H.263 decoder is evolving it should have
     * an H263EncContext */
    if (s->width  != avctx->coded_width  ||
        s->height != avctx->coded_height ||
        s->context_reinit) {
        /* H.263 could change picture size any time */
        s->context_reinit = 0;

        ret = ff_set_dimensions(avctx, s->width, s->height);
        if (ret < 0)
            return ret;

        ff_set_sar(avctx, avctx->sample_aspect_ratio);

        if ((ret = ff_mpv_common_frame_size_change(s)))
            return ret;

        if (avctx->pix_fmt != h263_get_format(avctx)) {
            av_log(avctx, AV_LOG_ERROR, "format change not supported\n");
            avctx->pix_fmt = AV_PIX_FMT_NONE;
            return AVERROR_UNKNOWN;
        }
    }

    if (s->codec_id == AV_CODEC_ID_H263  ||
        s->codec_id == AV_CODEC_ID_H263P ||
        s->codec_id == AV_CODEC_ID_H263I)
        s->gob_index = H263_GOB_HEIGHT(s->height);

    s->current_picture.f->pict_type = s->pict_type;
    s->current_picture.f->key_frame = s->pict_type == AV_PICTURE_TYPE_I;

    /* skip B-frames if we don't have reference frames */
    if (!s->last_picture_ptr &&
        (s->pict_type == AV_PICTURE_TYPE_B || s->droppable))
        return get_consumed_bytes(s, buf_size);
    if ((avctx->skip_frame >= AVDISCARD_NONREF &&
         s->pict_type == AV_PICTURE_TYPE_B)    ||
        (avctx->skip_frame >= AVDISCARD_NONKEY &&
         s->pict_type != AV_PICTURE_TYPE_I)    ||
        avctx->skip_frame >= AVDISCARD_ALL)
        return get_consumed_bytes(s, buf_size);

    if (s->next_p_frame_damaged) {
        if (s->pict_type == AV_PICTURE_TYPE_B)
            return get_consumed_bytes(s, buf_size);
        else
            s->next_p_frame_damaged = 0;
    }

    if ((!s->no_rounding) || s->pict_type == AV_PICTURE_TYPE_B) {
        s->me.qpel_put = s->qdsp.put_qpel_pixels_tab;
        s->me.qpel_avg = s->qdsp.avg_qpel_pixels_tab;
    } else {
        s->me.qpel_put = s->qdsp.put_no_rnd_qpel_pixels_tab;
        s->me.qpel_avg = s->qdsp.avg_qpel_pixels_tab;
    }

    if ((ret = ff_mpv_frame_start(s, avctx)) < 0)
        return ret;

    if (!s->divx_packed)
        ff_thread_finish_setup(avctx);

    if (avctx->hwaccel) {
        ret = avctx->hwaccel->start_frame(avctx, s->gb.buffer,
                                          s->gb.buffer_end - s->gb.buffer);
        if (ret < 0 )
            return ret;
    }

    ff_mpeg_er_frame_start(s);

    /* the second part of the wmv2 header contains the MB skip bits which
     * are stored in current_picture->mb_type which is not available before
     * ff_mpv_frame_start() */
    if (CONFIG_WMV2_DECODER && s->msmpeg4_version == 5) {
        ret = ff_wmv2_decode_secondary_picture_header(s);
        if (ret < 0)
            return ret;
        if (ret == 1)
            goto frame_end;
    }

    /* decode each macroblock */
    s->mb_x = 0;
    s->mb_y = 0;

    slice_ret = decode_slice(s);
    while (s->mb_y < s->mb_height) {
        if (s->msmpeg4_version) {
            if (s->slice_height == 0 || s->mb_x != 0 || slice_ret < 0 ||
                (s->mb_y % s->slice_height) != 0 || get_bits_left(&s->gb) < 0)
                break;
        } else {
            int prev_x = s->mb_x, prev_y = s->mb_y;
            if (ff_h263_resync(s) < 0)
                break;
            if (prev_y * s->mb_width + prev_x < s->mb_y * s->mb_width + s->mb_x)
                s->er.error_occurred = 1;
        }

        if (s->msmpeg4_version < 4 && s->h263_pred)
            ff_mpeg4_clean_buffers(s);

        if (decode_slice(s) < 0)
            slice_ret = AVERROR_INVALIDDATA;
    }

    if (s->msmpeg4_version && s->msmpeg4_version < 4 &&
        s->pict_type == AV_PICTURE_TYPE_I)
        if (!CONFIG_MSMPEG4_DECODER ||
            ff_msmpeg4_decode_ext_header(s, buf_size) < 0)
            s->er.error_status_table[s->mb_num - 1] = ER_MB_ERROR;
 
     av_assert1(s->bitstream_buffer_size == 0);
 frame_end:
    if (!s->studio_profile)
        ff_er_frame_end(&s->er);
 
     if (avctx->hwaccel) {
         ret = avctx->hwaccel->end_frame(avctx);
        if (ret < 0)
            return ret;
    }

    ff_mpv_frame_end(s);

    if (CONFIG_MPEG4_DECODER && avctx->codec_id == AV_CODEC_ID_MPEG4)
        ff_mpeg4_frame_end(avctx, buf, buf_size);

    if (!s->divx_packed && avctx->hwaccel)
        ff_thread_finish_setup(avctx);

    av_assert1(s->current_picture.f->pict_type == s->current_picture_ptr->f->pict_type);
    av_assert1(s->current_picture.f->pict_type == s->pict_type);
    if (s->pict_type == AV_PICTURE_TYPE_B || s->low_delay) {
        if ((ret = av_frame_ref(pict, s->current_picture_ptr->f)) < 0)
            return ret;
        ff_print_debug_info(s, s->current_picture_ptr, pict);
        ff_mpv_export_qp_table(s, pict, s->current_picture_ptr, FF_QSCALE_TYPE_MPEG1);
    } else if (s->last_picture_ptr) {
        if ((ret = av_frame_ref(pict, s->last_picture_ptr->f)) < 0)
            return ret;
        ff_print_debug_info(s, s->last_picture_ptr, pict);
        ff_mpv_export_qp_table(s, pict, s->last_picture_ptr, FF_QSCALE_TYPE_MPEG1);
    }

    if (s->last_picture_ptr || s->low_delay) {
        if (   pict->format == AV_PIX_FMT_YUV420P
            && (s->codec_tag == AV_RL32("GEOV") || s->codec_tag == AV_RL32("GEOX"))) {
            int x, y, p;
            av_frame_make_writable(pict);
            for (p=0; p<3; p++) {
                int w = AV_CEIL_RSHIFT(pict-> width, !!p);
                int h = AV_CEIL_RSHIFT(pict->height, !!p);
                int linesize = pict->linesize[p];
                for (y=0; y<(h>>1); y++)
                    for (x=0; x<w; x++)
                        FFSWAP(int,
                               pict->data[p][x + y*linesize],
                               pict->data[p][x + (h-1-y)*linesize]);
            }
        }
        *got_frame = 1;
    }

    if (slice_ret < 0 && (avctx->err_recognition & AV_EF_EXPLODE))
        return slice_ret;
    else
        return get_consumed_bytes(s, buf_size);
}

The product contains an assert() or similar statement that can be triggered by an attacker, which leads to an application exit or other behavior that is more severe than necessary.