From 8e915901deb3518d4bef73ea52eab2ece7a2bbf6 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Fri, 29 May 2020 11:59:33 -0400
Subject: [PATCH] c++: vptr ubsan and derived class [PR95311].

We weren't able to find OBJ_TYPE_REF_OBJECT walking through
OBJ_TYPE_REF_EXPR because we had folded away the ADDR_EXPR.

gcc/cp/ChangeLog:

	PR c++/95311
	PR c++/95221
	* class.c (build_vfn_ref): Don't fold the INDIRECT_REF.

gcc/testsuite/ChangeLog:

	PR c++/95311
	* g++.dg/ubsan/vptr-16.C: New test.
---
 gcc/cp/class.c                       |  8 ++++++--
 gcc/testsuite/g++.dg/ubsan/vptr-16.C | 14 ++++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/ubsan/vptr-16.C

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index bab15524a60..ca492cdbd40 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -729,9 +729,13 @@ build_vtbl_ref (tree instance, tree idx)
 tree
 build_vfn_ref (tree instance_ptr, tree idx)
 {
-  tree aref;
+  tree obtype = TREE_TYPE (TREE_TYPE (instance_ptr));
+
+  /* Leave the INDIRECT_REF unfolded so cp_ubsan_maybe_instrument_member_call
+     can find instance_ptr.  */
+  tree ind = build1 (INDIRECT_REF, obtype, instance_ptr);
 
-  aref = build_vtbl_ref (cp_build_fold_indirect_ref (instance_ptr), idx);
+  tree aref = build_vtbl_ref (ind, idx);
 
   /* When using function descriptors, the address of the
      vtable entry is treated as a function pointer.  */
diff --git a/gcc/testsuite/g++.dg/ubsan/vptr-16.C b/gcc/testsuite/g++.dg/ubsan/vptr-16.C
new file mode 100644
index 00000000000..a3db66e9140
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ubsan/vptr-16.C
@@ -0,0 +1,14 @@
+// PR c++/95311
+// { dg-additional-options -fsanitize=undefined }
+
+class a {
+  virtual long b() const;
+};
+class c : a {
+public:
+  long b() const;
+};
+class d : c {
+  long e();
+};
+long d::e() { b(); return 0; }
-- 
2.30.2