Removed false positive/segfault when accessing member of global for the first time

Author: gbMattN

compiler-rt/lib/tysan/tysan.cpp

@@ -227,22 +227,26 @@ __tysan_check(void *addr, int size, tysan_type_descriptor *td, int flags) {
     int i = -((sptr)OldTD);
     OldTDPtr -= i;
     OldTD = *OldTDPtr;
-
-    // When shadow memory is set for global objects, the entire object is tagged
-    // with the struct type This means that when you access a member variable,
-    // tysan reads that as you accessing a struct midway through, with 'i' being
-    // the offset Therefore, if you are accessing a struct, we need to find the
-    // member type. We can go through the members of the struct type and see if
-    // there is a member at the offset you are accessing the struct by. If there
-    // is indeed a member starting at offset 'i' in the struct, we should check
-    // aliasing legality with that type. If there isn't, we run alias checking
-    // on the struct which will give us the correct error.
+  
     tysan_type_descriptor *AccessedType = OldTD;
-    if (OldTD->Tag == TYSAN_STRUCT_TD) {
-      for (int j = 0; j < OldTD->Struct.MemberCount; ++j) {
-        if (OldTD->Struct.Members[j].Offset == i) {
-          AccessedType = OldTD->Struct.Members[j].Type;
-          break;
+    
+    // Only check if we are accessing members if the type exists
+    if(OldTD != nullptr){
+      // When shadow memory is set for global objects, the entire object is tagged
+      // with the struct type This means that when you access a member variable,
+      // tysan reads that as you accessing a struct midway through, with 'i' being
+      // the offset Therefore, if you are accessing a struct, we need to find the
+      // member type. We can go through the members of the struct type and see if
+      // there is a member at the offset you are accessing the struct by. If there
+      // is indeed a member starting at offset 'i' in the struct, we should check
+      // aliasing legality with that type. If there isn't, we run alias checking
+      // on the struct which will give us the correct error.
+      if (OldTD->Tag == TYSAN_STRUCT_TD) {
+        for (int j = 0; j < OldTD->Struct.MemberCount; ++j) {
+          if (OldTD->Struct.Members[j].Offset == i) {
+            AccessedType = OldTD->Struct.Members[j].Type;
+            break;
+          }
         }
       }
     }