c++: Further fix for -fsanitize=vptr [PR94325]

For -fsanitize=vptr, we insert a NULL store into the vptr instead of just
adding a CLOBBER of this.  build_clobber_this makes the CLOBBER conditional
on in_charge (implicit) parameter whenever CLASSTYPE_VBASECLASSES, but when
adding this conditionalization to the -fsanitize=vptr code in PR87095,
I wanted it to catch some more cases when the class has CLASSTYPE_VBASECLASSES,
but the vptr is still not shared with something else, otherwise the
sanitization would be less effective.
The following testcase shows that the chosen test that CLASSTYPE_PRIMARY_BINFO
is non-NULL and has BINFO_VIRTUAL_P set wasn't sufficient,
the D class has still sizeof(D) == sizeof(void*) and thus contains just
a single vptr, but while in B::~B() this results in the vptr not being
cleared, in C::~C() this condition isn't true, as CLASSTYPE_PRIMARY_BINFO
in that case is B and is not BINFO_VIRTUAL_P, so it clears the vptr, but the
D::~D() dtor after invoking C::~C() invokes A::~A() with an already cleared
vptr, which is then reported.
The following patch is just a shot in the dark, keep looking through
CLASSTYPE_PRIMARY_BINFO until we find BINFO_VIRTUAL_P, but it works on the
existing testcase as well as this new one.

2020-04-08  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94325
	* decl.c (begin_destructor_body): For CLASSTYPE_VBASECLASSES class
	dtors, if CLASSTYPE_PRIMARY_BINFO is non-NULL, but not BINFO_VIRTUAL_P,
	look at CLASSTYPE_PRIMARY_BINFO of its BINFO_TYPE if it is not
	BINFO_VIRTUAL_P, and so on.

	* g++.dg/ubsan/vptr-15.C: New test.

gcc/cp/ChangeLog

+2020-04-08  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/94325
+	* decl.c (begin_destructor_body): For CLASSTYPE_VBASECLASSES class
+	dtors, if CLASSTYPE_PRIMARY_BINFO is non-NULL, but not BINFO_VIRTUAL_P,
+	look at CLASSTYPE_PRIMARY_BINFO of its BINFO_TYPE if it is not
+	BINFO_VIRTUAL_P, and so on.
+
 2020-04-08  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94478 - ICE with defaulted comparison operator

gcc/cp/decl.c

@@ -16663,14 +16663,20 @@ begin_destructor_body (void)
 	    /* If the vptr is shared with some virtual nearly empty base,
 	       don't clear it if not in charge, the dtor of the virtual
 	       nearly empty base will do that later.  */
-	    if (CLASSTYPE_VBASECLASSES (current_class_type)
-		&& CLASSTYPE_PRIMARY_BINFO (current_class_type)
-		&& BINFO_VIRTUAL_P
-			  (CLASSTYPE_PRIMARY_BINFO (current_class_type)))
+	    if (CLASSTYPE_VBASECLASSES (current_class_type))
 	      {
-		stmt = convert_to_void (stmt, ICV_STATEMENT,
-					tf_warning_or_error);
-		stmt = build_if_in_charge (stmt);
+		tree c = current_class_type;
+		while (CLASSTYPE_PRIMARY_BINFO (c))
+		  {
+		    if (BINFO_VIRTUAL_P (CLASSTYPE_PRIMARY_BINFO (c)))
+		      {
+			stmt = convert_to_void (stmt, ICV_STATEMENT,
+						tf_warning_or_error);
+			stmt = build_if_in_charge (stmt);
+			break;
+		      }
+		    c = BINFO_TYPE (CLASSTYPE_PRIMARY_BINFO (c));
+		  }
 	      }
 	    finish_decl_cleanup (NULL_TREE, stmt);
 	  }

gcc/testsuite/ChangeLog

+2020-04-08  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/94325
+	* g++.dg/ubsan/vptr-15.C: New test.
+
 2020-04-08  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94478 - ICE with defaulted comparison operator

gcc/testsuite/g++.dg/ubsan/vptr-15.C

+// PR c++/94325
+// { dg-do run { target c++11 } }
+// { dg-options "-fsanitize=vptr -fno-sanitize-recover=vptr" }
+
+struct A { virtual ~A () = default; };
+struct B : public virtual A {};
+struct C : public B {};
+struct D : public C {};
+
+int
+main ()
+{
+  D a;
+}