Re: [PATCH] c++: ICE with -fsanitize=vptr and constexpr dynamic_cast [PR98103]

[Marek Polacek <polacek@redhat.com>]

On Wed, Dec 02, 2020 at 09:01:48PM -0500, Jason Merrill wrote:
> On 12/2/20 6:18 PM, Marek Polacek wrote:
> > -fsanitize=vptr initializes all vtable pointers to null so that it can
> > catch invalid calls; see cp_ubsan_maybe_initialize_vtbl_ptrs.  That
> > means that evaluating a vtable reference can produce a null pointer
> > in this mode, so cxx_eval_dynamic_cast_fn should check that.
> 
> Yes, but we shouldn't accept it silently; sanitize is supposed to flag
> undefined behavior, not allow it.  If we see a null vptr, we should complain
> and set *non_constant_p.

True, I shouldn't have left it for the run-time diagnostic.  How's this, then?

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
-fsanitize=vptr initializes all vtable pointers to null so that it can
catch invalid calls; see cp_ubsan_maybe_initialize_vtbl_ptrs.  That
means that evaluating a vtable reference can produce a null pointer
in this mode, so cxx_eval_dynamic_cast_fn should check that and give
and error.

gcc/cp/ChangeLog:

	PR c++/98103
	* constexpr.c (cxx_eval_dynamic_cast_fn): If the evaluating of vtable
	yields a null pointer, give an error and return.  Use objtype.

gcc/testsuite/ChangeLog:

	PR c++/98103
	* g++.dg/ubsan/vptr-18.C: New test.
---
 gcc/cp/constexpr.c                   | 11 ++++++++++-
 gcc/testsuite/g++.dg/ubsan/vptr-18.C | 25 +++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/ubsan/vptr-18.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index e0d358027c9..c413313fbe1 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1998,11 +1998,20 @@ cxx_eval_dynamic_cast_fn (const constexpr_ctx *ctx, tree call,
      to the object under construction or destruction, this object is
      considered to be a most derived object that has the type of the
      constructor or destructor's class.  */
-  tree vtable = build_vfield_ref (obj, TREE_TYPE (obj));
+  tree vtable = build_vfield_ref (obj, objtype);
   vtable = cxx_eval_constant_expression (ctx, vtable, /*lval*/false,
 					 non_constant_p, overflow_p);
   if (*non_constant_p)
     return call;
+  /* With -fsanitize=vptr, we initialize all vtable pointers to null,
+     so it's possible that we got a null pointer now.  */
+  if (integer_zerop (vtable))
+    {
+      if (!ctx->quiet)
+	error_at (loc, "virtual table pointer is used uninitialized");
+      *non_constant_p = true;
+      return integer_zero_node;
+    }
   /* VTABLE will be &_ZTV1A + 16 or similar, get _ZTV1A.  */
   vtable = extract_obj_from_addr_offset (vtable);
   const tree mdtype = DECL_CONTEXT (vtable);
diff --git a/gcc/testsuite/g++.dg/ubsan/vptr-18.C b/gcc/testsuite/g++.dg/ubsan/vptr-18.C
new file mode 100644
index 00000000000..cd2ca0a9fb6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ubsan/vptr-18.C
@@ -0,0 +1,25 @@
+// PR c++/98103
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fsanitize=vptr -fno-sanitize-recover=vptr" }
+// Modified constexpr-dynamic17.C.
+
+struct V {
+  virtual void f();
+};
+
+struct A : V { };
+
+struct B : V {
+  constexpr B(V*, A*);
+};
+
+struct D : B, A {
+  constexpr D() : B((A*)this, this) { }
+};
+
+constexpr B::B(V* v, A* a)
+{
+  dynamic_cast<B*>(a); // { dg-error "uninitialized" }
+}
+
+constexpr D d;

base-commit: df933e307b1950ce12472660dcac1765b8eb431d
-- 
2.28.0