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
