[clang][Interp] Don't allow reading from non-const blocks when returning

Author: tbaederr

clang/lib/AST/Interp/EvalEmitter.cpp

@@ -151,6 +151,11 @@ template <> bool EvalEmitter::emitRet<PT_Ptr>(const SourceInfo &Info) {
 
   // Implicitly convert lvalue to rvalue, if requested.
   if (ConvertResultToRValue) {
+    // Never allow reading from a non-const pointer, unless the memory
+    // has been created in this evaluation.
+    if (!Ptr.isConst() && Ptr.block()->getEvalID() != Ctx.getEvalID())
+      return false;
+
     if (std::optional<APValue> V = Ptr.toRValue(Ctx)) {
       EvalResult.setValue(*V);
     } else {

clang/test/AST/Interp/const-temporaries.cpp

@@ -90,3 +90,12 @@ struct R { mutable long x; };
 struct Z2 { const R &x, y; };
 Z2 z2 = { R{1}, z2.x.x = 10 };
 
+// CHECK: __cxa_atexit({{.*}} @_ZN1BD1Ev, {{.*}} @b
+
+// CHECK: define
+// CHECK-NOT: @_ZGRN21ModifyStaticTemporary1cE_
+// CHECK: store {{.*}} @_ZGRN21ModifyStaticTemporary1cE_, {{.*}} @_ZN21ModifyStaticTemporary1cE
+// CHECK: add
+// CHECK: store
+// CHECK: load {{.*}} @_ZN21ModifyStaticTemporary1bE
+// CHECK: store {{.*}} @_ZN21ModifyStaticTemporary1cE