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

Author: gbMattN

compiler-rt/lib/tysan/tysan.cpp

@@ -228,21 +228,26 @@ __tysan_check(void *addr, int size, tysan_type_descriptor *td, int flags) {
     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;
+          }
         }
       }
     }