avformat/mxfdec: Remove this_partition

Suggested-by: Tomas Härdin <git@haerdin.se>
Fixes: 51896/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-5130394286817280

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mxfdec.c

@@ -102,7 +102,6 @@ typedef struct MXFPartition {
     uint64_t previous_partition;
     int index_sid;
     int body_sid;
-    int64_t this_partition;
     int64_t essence_offset;         ///< absolute offset of essence
     int64_t essence_length;
     int32_t kag_size;
@@ -727,10 +726,13 @@ static int mxf_read_partition_pack(void *arg, AVIOContext *pb, int tag, int size
     UID op;
     uint64_t footer_partition;
     uint32_t nb_essence_containers;
+    uint64_t this_partition;
 
     if (mxf->partitions_count >= INT_MAX / 2)
         return AVERROR_INVALIDDATA;
 
+    av_assert0(klv_offset >= mxf->run_in);
+
     tmp_part = av_realloc_array(mxf->partitions, mxf->partitions_count + 1, sizeof(*mxf->partitions));
     if (!tmp_part)
         return AVERROR(ENOMEM);
@@ -773,7 +775,13 @@ static int mxf_read_partition_pack(void *arg, AVIOContext *pb, int tag, int size
     partition->complete = uid[14] > 2;
     avio_skip(pb, 4);
     partition->kag_size = avio_rb32(pb);
-    partition->this_partition = avio_rb64(pb);
+    this_partition = avio_rb64(pb);
+    if (this_partition != klv_offset - mxf->run_in) {
+        av_log(mxf->fc, AV_LOG_ERROR,
+               "this_partition %"PRId64" mismatches %"PRId64"\n",
+               this_partition, klv_offset - mxf->run_in);
+        return AVERROR_INVALIDDATA;
+    }
     partition->previous_partition = avio_rb64(pb);
     footer_partition = avio_rb64(pb);
     partition->header_byte_count = avio_rb64(pb);
@@ -793,8 +801,8 @@ static int mxf_read_partition_pack(void *arg, AVIOContext *pb, int tag, int size
         av_dict_set(&s->metadata, "operational_pattern_ul", str, 0);
     }
 
-    if (partition->this_partition &&
-        partition->previous_partition == partition->this_partition) {
+    if (this_partition &&
+        partition->previous_partition == this_partition) {
         av_log(mxf->fc, AV_LOG_ERROR,
                "PreviousPartition equal to ThisPartition %"PRIx64"\n",
                partition->previous_partition);
@@ -802,11 +810,11 @@ static int mxf_read_partition_pack(void *arg, AVIOContext *pb, int tag, int size
         if (!mxf->parsing_backward && mxf->last_forward_partition > 1) {
             MXFPartition *prev =
                 mxf->partitions + mxf->last_forward_partition - 2;
-            partition->previous_partition = prev->this_partition;
+            partition->previous_partition = prev->pack_ofs - mxf->run_in;
         }
         /* if no previous body partition are found point to the header
          * partition */
-        if (partition->previous_partition == partition->this_partition)
+        if (partition->previous_partition == this_partition)
             partition->previous_partition = 0;
         av_log(mxf->fc, AV_LOG_ERROR,
                "Overriding PreviousPartition with %"PRIx64"\n",
@@ -828,7 +836,7 @@ static int mxf_read_partition_pack(void *arg, AVIOContext *pb, int tag, int size
             "PartitionPack: ThisPartition = 0x%"PRIX64
             ", PreviousPartition = 0x%"PRIX64", "
             "FooterPartition = 0x%"PRIX64", IndexSID = %i, BodySID = %i\n",
-            partition->this_partition,
+            this_partition,
             partition->previous_partition, footer_partition,
             partition->index_sid, partition->body_sid);
 
@@ -902,7 +910,7 @@ static uint64_t partition_score(MXFPartition *p)
         score = 3;
     else
         score = 1;
-    return (score << 60) | ((uint64_t)p->this_partition >> 4);
+    return (score << 60) | ((uint64_t)p->pack_ofs >> 4);
 }
 
 static int mxf_add_metadata_set(MXFContext *mxf, MXFMetadataSet **metadata_set)
@@ -3539,14 +3547,14 @@ static void mxf_compute_essence_containers(AVFormatContext *s)
 
             /* essence container spans to the next partition */
             if (x < mxf->partitions_count - 1)
-                p->essence_length = mxf->partitions[x+1].this_partition - p->essence_offset;
+                p->essence_length = mxf->partitions[x+1].pack_ofs - mxf->run_in - p->essence_offset;
 
             if (p->essence_length < 0) {
                 /* next ThisPartition < essence_offset */
                 p->essence_length = 0;
                 av_log(mxf->fc, AV_LOG_ERROR,
                        "partition %i: bad ThisPartition = %"PRIX64"\n",
-                       x+1, mxf->partitions[x+1].this_partition);
+                       x+1, mxf->partitions[x+1].pack_ofs - mxf->run_in);
             }
         }
     }