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.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
gcc/cp/ChangeLog:
PR c++/98103
* constexpr.c (cxx_eval_dynamic_cast_fn): If the evaluating of vtable
yields a null pointer, return.
gcc/testsuite/ChangeLog:
PR c++/98103
* g++.dg/ubsan/vptr-18.C: New test.
---
gcc/cp/constexpr.c | 4 +++-
gcc/testsuite/g++.dg/ubsan/vptr-18.C | 27 +++++++++++++++++++++++++++
2 files changed, 30 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 9a1a1db1267..8c6a9cf2b40 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1996,7 +1996,9 @@ cxx_eval_dynamic_cast_fn (const constexpr_ctx *ctx, tree
call,
tree vtable = build_vfield_ref (obj, TREE_TYPE (obj));
vtable = cxx_eval_constant_expression (ctx, vtable, /*lval*/false,
non_constant_p, overflow_p);
- if (*non_constant_p)
+ /* With -fsanitize=vptr, we initialize all vtable pointers to null,
+ so it's possible that we got a null pointer now. */
+ if (*non_constant_p || integer_zerop (vtable))
return call;
/* VTABLE will be &_ZTV1A + 16 or similar, get _ZTV1A. */
vtable = extract_obj_from_addr_offset (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..9f421c269bc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ubsan/vptr-18.C
@@ -0,0 +1,27 @@
+// PR c++/98103
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fsanitize=vptr" }
+// Modified constexpr-dynamic17.C. -fsanitize=vptr initializes all vtable
+// pointers to null, so we don't get the "accessing uninitialized member"
+// error.
+
+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);
+}
+
+constexpr D d;
base-commit: dc2b372ed1b1e9af6db45051cff95478c7616807
