[TySan] Fix struct access with different bases

Author: Matthew Nagy

compiler-rt/lib/tysan/tysan.cpp

@@ -128,6 +128,10 @@ static bool isAliasingLegalUp(tysan_type_descriptor *TDA,
           break;
       }
 
+      //You can't have negative offset, you must be partially inside the last type
+      if (TDA->Struct.Members[Idx].Offset > OffsetA)
+        Idx -=1;
+        
       OffsetA -= TDA->Struct.Members[Idx].Offset;
       TDA = TDA->Struct.Members[Idx].Type;
     } else {

compiler-rt/test/tysan/struct-offset-different-base.cpp

@@ -0,0 +1,31 @@
+// RUN: %clangxx_tysan -O0 %s -o %t && %run %t >%t.out 2>&1
+// RUN: FileCheck %s < %t.out
+
+#include <stdio.h>
+
+struct inner {
+	char buffer;
+	int i;
+};
+
+void init_inner(inner *iPtr) {
+	iPtr->i = 0;
+}
+
+struct outer {
+	inner foo;
+    char buffer;
+};
+
+int main(void) {
+    outer *l = new outer();
+
+    init_inner(&l->foo);
+    
+    int access_offsets_with_different_base = l->foo.i;
+    printf("%d\n", access_offsets_with_different_base);
+    
+    return 0;
+}
+
+// CHECK-NOT: ERROR: TypeSanitizer: type-aliasing-violation