[gcc r9-9408] c++: Fix up ubsan false positives on references [PR95693]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:4ccdb3fdbc14102c91b6148bcbe09d0763726ae0

commit r9-9408-g4ccdb3fdbc14102c91b6148bcbe09d0763726ae0
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jan 22 19:03:23 2021 +0100

    c++: Fix up ubsan false positives on references [PR95693]
    
    Alex' 2 years old change to build_zero_init_1 to return NULL pointer with
    reference type for references breaks the sanitizers, the assignment of NULL
    to a reference typed member is then instrumented before it is overwritten
    with a non-NULL address later on.
    That change has been done to fix error recovery ICE during
    process_init_constructor_record, where we:
              if (TYPE_REF_P (fldtype))
                {
                  if (complain & tf_error)
                    error ("member %qD is uninitialized reference", field);
                  else
                    return PICFLAG_ERRONEOUS;
                }
    a few lines earlier, but then continue and ICE when build_zero_init returns
    NULL.
    
    The following patch reverts the build_zero_init_1 change and instead creates
    the NULL with reference type constants during the error recovery.
    
    The pr84593.C testcase Alex' change was fixing still works as before.
    
    2021-01-22  Jakub Jelinek  <jakub@redhat.com>
    
            PR sanitizer/95693
            * init.c (build_zero_init_1): Revert the 2018-03-06 change to
            return build_zero_cst for reference types.
            * typeck2.c (process_init_constructor_record): Instead call
            build_zero_cst here during error recovery instead of build_zero_init.
    
            * g++.dg/ubsan/pr95693.C: New test.
    
    (cherry picked from commit e5750f847158e7f9bdab770fd9c5fff58c5074d3)

Diff:
---
 gcc/cp/init.c                        |  5 +----
 gcc/cp/typeck2.c                     | 12 ++++++++----
 gcc/testsuite/g++.dg/ubsan/pr95693.C | 26 ++++++++++++++++++++++++++
 3 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/gcc/cp/init.c b/gcc/cp/init.c
index c5a01c83093..e3f190c6114 100644
--- a/gcc/cp/init.c
+++ b/gcc/cp/init.c
@@ -286,10 +286,7 @@ build_zero_init_1 (tree type, tree nelts, bool static_storage_p,
   else if (VECTOR_TYPE_P (type))
     init = build_zero_cst (type);
   else
-    {
-      gcc_assert (TYPE_REF_P (type));
-      init = build_zero_cst (type);
-    }
+    gcc_assert (TYPE_REF_P (type));
 
   /* In all cases, the initializer is a constant.  */
   if (init)
diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index b406368a5d6..aae5ff24c98 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -1637,10 +1637,14 @@ process_init_constructor_record (tree type, tree init, int nested, int flags,
 	    warning (OPT_Wmissing_field_initializers,
 		     "missing initializer for member %qD", field);
 
-	  if (!zero_init_p (fldtype)
-	      || skipped < 0)
-	    next = build_zero_init (TREE_TYPE (field), /*nelts=*/NULL_TREE,
-				    /*static_storage_p=*/false);
+	  if (!zero_init_p (fldtype) || skipped < 0)
+	    {
+	      if (TYPE_REF_P (fldtype))
+		next = build_zero_cst (TREE_TYPE (field));
+	      else
+		next = build_zero_init (TREE_TYPE (field), /*nelts=*/NULL_TREE,
+					/*static_storage_p=*/false);
+	    }
 	  else
 	    {
 	      /* The default zero-initialization is fine for us; don't
diff --git a/gcc/testsuite/g++.dg/ubsan/pr95693.C b/gcc/testsuite/g++.dg/ubsan/pr95693.C
new file mode 100644
index 00000000000..13f688e8ec0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ubsan/pr95693.C
@@ -0,0 +1,26 @@
+// PR sanitizer/95693
+// { dg-do run }
+// { dg-options "-O2 -fsanitize=undefined -fno-sanitize-recover=undefined" }
+
+int g = 9;
+
+struct A {
+  A () : a(g) {}
+private:
+  int &a;
+};
+
+struct B {
+  A payload;
+};
+
+struct C : public B {
+  C () : B () {}
+  A p;
+};
+
+int
+main ()
+{
+  C t;
+}