Re: [PATCH] warn-access: Prevent -Wuse-after-free on ARM [PR104213]

[Martin Sebor via Gcc-patches]

On 1/26/22 08:24, Jason Merrill via Gcc-patches wrote:

On 1/25/22 18:33, Marek Polacek wrote:

Here, -Wuse-after-free warns about using 'this' which, on ARM, cdtors
return, as mandated by the EABI.  To be entirely correct, it only
requires it for C1 and C2 ctors and D2 and D1 dtors, but I don't feel
like changing that now and possibly running into issues later on.

This patch uses suppress_warning on 'this' for certain cdtor_returns_this
cases in the C++ FE, and then warn_invalid_pointer makes use of this
information and doesn't warn.

In my first attempt I tried suppress_warning the MODIFY_EXPR or RETURN_EXPR

we build in build_delete_destructor_body, but the complication is that
the suppress_warning bits don't always survive gimplification; see e.g.
gimplify_modify_expr where we do

  6130       if (COMPARISON_CLASS_P (*from_p))
  6131         copy_warning (assign, *from_p);

but here we're not dealing with a comparison.  Removing that check
regresses uninit-pr74762.C.  Adding copy_warning (assign, *expr_p)
regresses c-c++-common/uninit-17.c.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK if Martin doesn't have any suggestions.

Nothing further from me.

Thanks
Martin

    PR target/104213

gcc/cp/ChangeLog:

    * decl.cc (finish_constructor_body): Suppress -Wuse-after-free.
    (finish_destructor_body): Likewise.
    * optimize.cc (build_delete_destructor_body): Likewise.

gcc/ChangeLog:

    * gimple-ssa-warn-access.cc (pass_waccess::warn_invalid_pointer): Don't

    warn when the SSA_NAME_VAR of REF has supressed -Wuse-after-free.

gcc/testsuite/ChangeLog:

    * g++.dg/warn/Wuse-after-free2.C: New test.
---
  gcc/cp/decl.cc                               |  2 ++
  gcc/cp/optimize.cc                           |  1 +
  gcc/gimple-ssa-warn-access.cc                | 14 +++++++++++---
  gcc/testsuite/g++.dg/warn/Wuse-after-free2.C | 10 ++++++++++
  4 files changed, 24 insertions(+), 3 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/warn/Wuse-after-free2.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 22d3dd1e2ad..6534a7fd320 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -17315,6 +17315,7 @@ finish_constructor_body (void)
        add_stmt (build_stmt (input_location, LABEL_EXPR, cdtor_label));
        val = DECL_ARGUMENTS (current_function_decl);
+      suppress_warning (val, OPT_Wuse_after_free);
        val = build2 (MODIFY_EXPR, TREE_TYPE (val),
              DECL_RESULT (current_function_decl), val);
        /* Return the address of the object.  */
@@ -17408,6 +17409,7 @@ finish_destructor_body (void)
        tree val;
        val = DECL_ARGUMENTS (current_function_decl);
+      suppress_warning (val, OPT_Wuse_after_free);
        val = build2 (MODIFY_EXPR, TREE_TYPE (val),
              DECL_RESULT (current_function_decl), val);
        /* Return the address of the object.  */
diff --git a/gcc/cp/optimize.cc b/gcc/cp/optimize.cc
index 4ad3f1dc9aa..13ab8b7361e 100644
--- a/gcc/cp/optimize.cc
+++ b/gcc/cp/optimize.cc

@@ -166,6 +166,7 @@ build_delete_destructor_body (tree delete_dtor, tree complete_dtor)

    if (targetm.cxx.cdtor_returns_this ())
      {
        tree val = DECL_ARGUMENTS (delete_dtor);
+      suppress_warning (val, OPT_Wuse_after_free);
        val = build2 (MODIFY_EXPR, TREE_TYPE (val),
                      DECL_RESULT (delete_dtor), val);
        add_stmt (build_stmt (0, RETURN_EXPR, val));

diff --git a/gcc/gimple-ssa-warn-access.cc b/gcc/gimple-ssa-warn-access.cc

index 8bc33eeb6fa..484bcd34c25 100644
--- a/gcc/gimple-ssa-warn-access.cc
+++ b/gcc/gimple-ssa-warn-access.cc

@@ -3880,9 +3880,17 @@ pass_waccess::warn_invalid_pointer (tree ref, gimple *use_stmt,

                      bool maybe, bool equality /* = false */)
  {
    /* Avoid printing the unhelpful "<unknown>" in the diagnostics.  */
-  if (ref && TREE_CODE (ref) == SSA_NAME
-      && (!SSA_NAME_VAR (ref) || DECL_ARTIFICIAL (SSA_NAME_VAR (ref))))
-    ref = NULL_TREE;
+  if (ref && TREE_CODE (ref) == SSA_NAME)
+    {
+      tree var = SSA_NAME_VAR (ref);
+      if (!var)
+    ref = NULL_TREE;

+      /* Don't warn for cases like when a cdtor returns 'this' on ARM.  */

+      else if (warning_suppressed_p (var, OPT_Wuse_after_free))
+    return;
+      else if (DECL_ARTIFICIAL (var))
+    ref = NULL_TREE;
+    }
    location_t use_loc = gimple_location (use_stmt);
    if (use_loc == UNKNOWN_LOCATION)

diff --git a/gcc/testsuite/g++.dg/warn/Wuse-after-free2.C b/gcc/testsuite/g++.dg/warn/Wuse-after-free2.C

new file mode 100644
index 00000000000..6d5f2bf01b5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wuse-after-free2.C
@@ -0,0 +1,10 @@
+// PR target/104213
+// { dg-do compile }
+// { dg-options "-Wuse-after-free" }
+
+class C
+{
+    virtual ~C();
+};
+
+C::~C() {} // { dg-bogus "used after" }

base-commit: aeac414923aa1e87986c7fc6f9b921d89a9b86cf