On 10/22/24 2:17 PM, Jakub Jelinek wrote:
Hi!
The following testcase shows that the previous get_member_function_from_ptrfunc
changes weren't sufficient and we still have cases where
-fsanitize=undefined with pointers to member functions can cause wrong code
being generated and related false positive warnings.
The problem is that save_expr doesn't always create SAVE_EXPR, it can skip
some invariant arithmetics and in the end it could be really large
expressions which would be evaluated several times (and what is worse, with
-fsanitize=undefined those expressions then can have SAVE_EXPRs added to
their subparts for -fsanitize=bounds or -fsanitize=null or
-fsanitize=alignment instrumentation). Tried to just build1 a SAVE_EXPR
+ add TREE_SIDE_EFFECTS instead of save_expr, but that doesn't work either,
because cp_fold happily optimizes those SAVE_EXPRs away when it sees
SAVE_EXPR operand is tree_invariant_p.
Hmm, when is that be a problem? I wouldn't expect instance_ptr to be
tree_invariant_p.
So, the following patch instead of using save_expr or building SAVE_EXPR
manually builds a TARGET_EXPR. Both types are pointers, so it doesn't need
to be destroyed in any way, but TARGET_EXPR is what doesn't get optimized
away immediately.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
2024-10-22 Jakub Jelinek <ja...@redhat.com>
PR c++/117259
* typeck.cc (get_member_function_from_ptrfunc): Use force_target_expr
rather than save_expr for instance_ptr and function. Don't call it
for TREE_CONSTANT.
* g++.dg/ubsan/pr117259.C: New test.
--- gcc/cp/typeck.cc.jj 2024-10-16 14:42:58.835725318 +0200
+++ gcc/cp/typeck.cc 2024-10-22 16:12:58.462731292 +0200
@@ -4193,24 +4193,27 @@ get_member_function_from_ptrfunc (tree *
if (!nonvirtual && is_dummy_object (instance_ptr))
nonvirtual = true;
- /* Use save_expr even when instance_ptr doesn't have side-effects,
- unless it is a simple decl (save_expr won't do anything on
- constants), so that we don't ubsan instrument the expression
- multiple times. See PR116449. */
+ /* Use force_target_expr even when instance_ptr doesn't have
+ side-effects, unless it is a simple decl or constant, so
+ that we don't ubsan instrument the expression multiple times.
+ Don't use save_expr, as save_expr can avoid building a SAVE_EXPR
+ and building a SAVE_EXPR manually can be optimized away during
+ cp_fold. See PR116449 and PR117259. */
if (TREE_SIDE_EFFECTS (instance_ptr)
- || (!nonvirtual && !DECL_P (instance_ptr)))
- {
- instance_save_expr = save_expr (instance_ptr);
- if (instance_save_expr == instance_ptr)
- instance_save_expr = NULL_TREE;
- else
- instance_ptr = instance_save_expr;
- }
+ || (!nonvirtual
+ && !DECL_P (instance_ptr)
+ && !TREE_CONSTANT (instance_ptr)))
+ instance_ptr = instance_save_expr
+ = force_target_expr (TREE_TYPE (instance_ptr), instance_ptr,
+ complain);
/* See above comment. */
if (TREE_SIDE_EFFECTS (function)
- || (!nonvirtual && !DECL_P (function)))
- function = save_expr (function);
+ || (!nonvirtual
+ && !DECL_P (function)
+ && !TREE_CONSTANT (function)))
+ function
+ = force_target_expr (TREE_TYPE (function), function, complain);
/* Start by extracting all the information from the PMF itself. */
e3 = pfn_from_ptrmemfunc (function);
--- gcc/testsuite/g++.dg/ubsan/pr117259.C.jj 2024-10-22 17:00:52.156114344
+0200
+++ gcc/testsuite/g++.dg/ubsan/pr117259.C 2024-10-22 17:05:20.470324367
+0200
@@ -0,0 +1,13 @@
+// PR c++/117259
+// { dg-do compile }
+// { dg-options "-Wuninitialized -fsanitize=undefined" }
+
+struct A { void foo () {} };
+struct B { void (A::*b) (); B (void (A::*x) ()) : b(x) {}; };
+const B c[1] = { &A::foo };
+
+void
+foo (A *x, int y)
+{
+ (x->*c[y].b) ();
+}
Jakub
