mirror of https://git.ffmpeg.org/ffmpeg.git
mpeg12dec: move setting first_field to mpeg_field_start()
For field picture, the first_field is set based on its previous value. Before this commit, first_field is set when reading the picture coding extension. However, in corrupted files there may be multiple picture coding extension headers, so the final value of first_field that is actually used during decoding can be wrong. That can lead to various undefined behaviour, like predicting from a non-existing field. Fix this problem, by setting first_field in mpeg_field_start(), which should be called exactly once per field. CC: libav-stable@libav.org Bug-ID: 999
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@ -1536,10 +1536,8 @@ static void mpeg_decode_picture_coding_extension(Mpeg1Context *s1)
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av_log(s->avctx, AV_LOG_WARNING, "invalid frame_pred_frame_dct\n");
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if (s->picture_structure == PICT_FRAME) {
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s->first_field = 0;
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s->v_edge_pos = 16 * s->mb_height;
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} else {
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s->first_field ^= 1;
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s->v_edge_pos = 8 * s->mb_height;
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memset(s->mbskip_table, 0, s->mb_stride * s->mb_height);
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}
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@ -1570,6 +1568,11 @@ static int mpeg_field_start(MpegEncContext *s, const uint8_t *buf, int buf_size)
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Mpeg1Context *s1 = (Mpeg1Context *) s;
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int ret;
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if (s->picture_structure == PICT_FRAME)
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s->first_field = 0;
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else
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s->first_field ^= 1;
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/* start frame decoding */
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if (s->first_field || s->picture_structure == PICT_FRAME) {
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AVFrameSideData *pan_scan;
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