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c++: Fix up ubsan false positives on references [PR95693]

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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.
This commit is contained in:
Jakub Jelinek 2021-01-22 19:03:23 +01:00
parent 25fc4d01a8
commit a9ed18295b
3 changed files with 35 additions and 8 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

5
gcc/cp/init.c
View file

@ -288,10 +288,7 @@ 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)

12
gcc/cp/typeck2.c
View file

@ -1609,10 +1609,14 @@ 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 (fldtype, /*nelts=*/NULL_TREE,
/*static_storage_p=*/false);
if (!zero_init_p (fldtype) || skipped < 0)
{
if (TYPE_REF_P (fldtype))
next = build_zero_cst (fldtype);
else
next = build_zero_init (fldtype, /*nelts=*/NULL_TREE,
/*static_storage_p=*/false);
}
else
{
/* The default zero-initialization is fine for us; don't

26
gcc/testsuite/g++.dg/ubsan/pr95693.C Normal file
View file

@ -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;
}