avcodec: Avoid splitting side data repeatedly

Fixes Timeout
Fixes: 508/clusterfuzz-testcase-6245747678773248

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/avpacket.c

@@ -462,7 +462,32 @@ int av_packet_split_side_data(AVPacket *pkt){
     }
     return 0;
 }
+#endif
+
+#if FF_API_MERGE_SD
+int ff_packet_split_and_drop_side_data(AVPacket *pkt){
+    if (!pkt->side_data_elems && pkt->size >12 && AV_RB64(pkt->data + pkt->size - 8) == FF_MERGE_MARKER){
+        int i;
+        unsigned int size;
+        uint8_t *p;
 
+        p = pkt->data + pkt->size - 8 - 5;
+        for (i=1; ; i++){
+            size = AV_RB32(p);
+            if (size>INT_MAX - 5 || p - pkt->data < size)
+                return 0;
+            if (p[4]&128)
+                break;
+            if (p - pkt->data < size + 5)
+                return 0;
+            p-= size+5;
+        }
+        pkt->size = p - pkt->data - size;
+        av_assert0(pkt->size >= 0);
+        return 1;
+    }
+    return 0;
+}
 #endif
 
 uint8_t *av_packet_pack_dictionary(AVDictionary *dict, int *size)

libavcodec/decode.c

@@ -392,7 +392,9 @@ static int decode_simple_internal(AVCodecContext *avctx, AVFrame *frame)
     tmp = *pkt;
 #if FF_API_MERGE_SD
 FF_DISABLE_DEPRECATION_WARNINGS
-    did_split = av_packet_split_side_data(&tmp);
+    did_split = avci->compat_decode_partial_size ?
+                ff_packet_split_and_drop_side_data(&tmp) :
+                av_packet_split_side_data(&tmp);
 
     if (did_split) {
         ret = extract_packet_props(avctx->internal, &tmp);
@@ -961,6 +963,7 @@ int avcodec_decode_subtitle2(AVCodecContext *avctx, AVSubtitle *sub,
                              AVPacket *avpkt)
 {
     int i, ret = 0;
+    AVCodecInternal *avci = avctx->internal;
 
     if (!avpkt->data && avpkt->size) {
         av_log(avctx, AV_LOG_ERROR, "invalid packet: NULL data, size != 0\n");
@@ -981,7 +984,9 @@ int avcodec_decode_subtitle2(AVCodecContext *avctx, AVSubtitle *sub,
         AVPacket tmp = *avpkt;
 #if FF_API_MERGE_SD
 FF_DISABLE_DEPRECATION_WARNINGS
-        int did_split = av_packet_split_side_data(&tmp);
+        int did_split = avci->compat_decode_partial_size ?
+                        ff_packet_split_and_drop_side_data(&tmp) :
+                        av_packet_split_side_data(&tmp);
         //apply_param_change(avctx, &tmp);
 
         if (did_split) {

libavcodec/internal.h

@@ -356,6 +356,10 @@ int ff_set_sar(AVCodecContext *avctx, AVRational sar);
 int ff_side_data_update_matrix_encoding(AVFrame *frame,
                                         enum AVMatrixEncoding matrix_encoding);
 
+#if FF_API_MERGE_SD
+int ff_packet_split_and_drop_side_data(AVPacket *pkt);
+#endif
+
 /**
  * Select the (possibly hardware accelerated) pixel format.
  * This is a wrapper around AVCodecContext.get_format() and should be used