Re: Extend -fstack-protector-strong to cover calls with return slot

[Florian Weimer <fweimer at redhat dot com>]

On 01/07/2014 02:37 PM, Jakub Jelinek wrote:
> On Tue, Jan 07, 2014 at 02:27:04PM +0100, Florian Weimer wrote:
> > gimplify_modify_expr_rhs, in the CALL_EXPR case:
> >
> > 	      if (use_target)
> > 		{
> > 		  CALL_EXPR_RETURN_SLOT_OPT (*from_p) = 1;
> > 		  mark_addressable (*to_p);
> > 		}
>
> Yeah, that sets it in some cases too, not in other testcases.
>
> Just look at how the flag is used when actually expanding it:
>
>          if (target && MEM_P (target) && CALL_EXPR_RETURN_SLOT_OPT (exp))
>            structure_value_addr = XEXP (target, 0);
>          else
>            {
>              /* For variable-sized objects, we must be called with a target
>                 specified.  If we were to allocate space on the stack here,
>                 we would have no way of knowing when to free it.  */
>              rtx d = assign_temp (rettype, 1, 1);
>              structure_value_addr = XEXP (d, 0);
>              target = 0;
>            }

Okay, I'm beginning to understand.  I tried to actually reach the second 
branch, and ended up with PR59711. :)

foo12 in the new C testcase covers it in part without a variable-sized 
object.

> so, if it is set, the address of the var on the LHS is passed to the
> function as hidden argument, if it is not set, we pass address of
> a stack temporary instead.  Both the automatic var and the stack temporary
> can overflow, if the callee does something wrong.

What about the attached version?  It still does not exactly match your 
original suggestion because gimple_call_lhs (stmt) can be NULL_TREE if 
the result is ignored and this case needs instrumentation, as you 
explained, so I use the function return type in the aggregate_value_p check.

Testing is still under way, but looks good so far.  I'm bootstrapping 
with BOOT_CFLAGS="-O2 -g -fstack-protector-strong" with Ada enabled, for 
additional coverage.

--
Florian Weimer / Red Hat Product Security Team

gcc/

2014-01-08  Florian Weimer  <fweimer@redhat.com>

	* cfgexpand.c (stack_protect_decl_p): New function, extracted from
	expand_used_vars.
	(stack_protect_return_slot_p): New function.
	(expand_used_vars): Call stack_protect_decl_p and
	stack_protect_return_slot_p for -fstack-protector-strong.

gcc/testsuite/

2014-01-08  Florian Weimer  <fweimer@redhat.com>

	* gcc.dg/fstack-protector-strong.c: Add coverage for return slots.
	* g++.dg/fstack-protector-strong.C: Likewise.
	* gcc.target/i386/ssp-strong-reg.c: New file.

Index: gcc/cfgexpand.c
===================================================================
--- gcc/cfgexpand.c	(revision 206311)
+++ gcc/cfgexpand.c	(working copy)
@@ -1599,6 +1599,52 @@
   return 0;
 }
 
+/* Check if the current function has local referenced variables that
+   have their addresses taken, contain an array, or are arrays.  */
+
+static bool
+stack_protect_decl_p ()
+{
+  unsigned i;
+  tree var;
+
+  FOR_EACH_LOCAL_DECL (cfun, i, var)
+    if (!is_global_var (var))
+      {
+	tree var_type = TREE_TYPE (var);
+	if (TREE_CODE (var) == VAR_DECL
+	    && (TREE_CODE (var_type) == ARRAY_TYPE
+		|| TREE_ADDRESSABLE (var)
+		|| (RECORD_OR_UNION_TYPE_P (var_type)
+		    && record_or_union_type_has_array_p (var_type))))
+	  return true;
+      }
+  return false;
+}
+
+/* Check if the current function has calls that use a return slot.  */
+
+static bool
+stack_protect_return_slot_p ()
+{
+  basic_block bb;
+  
+  FOR_ALL_BB_FN (bb, cfun)
+    for (gimple_stmt_iterator gsi = gsi_start_bb (bb);
+	 !gsi_end_p (gsi); gsi_next (&gsi))
+      {
+	gimple stmt = gsi_stmt (gsi);
+	/* This assumes that calls to internal-only functions never
+	   use a return slot.  */
+	if (is_gimple_call (stmt)
+	    && !gimple_call_internal_p (stmt)
+	    && aggregate_value_p (TREE_TYPE (gimple_call_fntype (stmt)),
+				  gimple_call_fndecl (stmt)))
+	  return true;
+      }
+  return false;
+}
+
 /* Expand all variables used in the function.  */
 
 static rtx
@@ -1669,22 +1715,8 @@
   pointer_map_destroy (ssa_name_decls);
 
   if (flag_stack_protect == SPCT_FLAG_STRONG)
-    FOR_EACH_LOCAL_DECL (cfun, i, var)
-      if (!is_global_var (var))
-	{
-	  tree var_type = TREE_TYPE (var);
-	  /* Examine local referenced variables that have their addresses taken,
-	     contain an array, or are arrays.  */
-	  if (TREE_CODE (var) == VAR_DECL
-	      && (TREE_CODE (var_type) == ARRAY_TYPE
-		  || TREE_ADDRESSABLE (var)
-		  || (RECORD_OR_UNION_TYPE_P (var_type)
-		      && record_or_union_type_has_array_p (var_type))))
-	    {
-	      gen_stack_protect_signal = true;
-	      break;
-	    }
-	}
+      gen_stack_protect_signal
+	= stack_protect_decl_p () || stack_protect_return_slot_p ();
 
   /* At this point all variables on the local_decls with TREE_USED
      set are not associated with any block scope.  Lay them out.  */
Index: gcc/testsuite/g++.dg/fstack-protector-strong.C
===================================================================
--- gcc/testsuite/g++.dg/fstack-protector-strong.C	(revision 206311)
+++ gcc/testsuite/g++.dg/fstack-protector-strong.C	(working copy)
@@ -32,4 +32,52 @@
   return global_func (a);
 }
 
-/* { dg-final { scan-assembler-times "stack_chk_fail" 2 } } */
+/* Frame addressed exposed through return slot. */
+
+struct B
+{
+  /* Discourage passing this struct in registers. */
+  int a1, a2, a3, a4, a5, a6, a7, a8, a9, a10;
+  int method ();
+  B return_slot();
+};
+
+B global_func ();
+void noop ();
+
+int foo3 ()
+{
+  return global_func ().a1;
+}
+
+int foo4 ()
+{
+  try {
+    noop ();
+    return 0;
+  } catch (...) {
+    return global_func ().a1;
+  }
+}
+
+int foo5 ()
+{
+  try {
+    return global_func ().a1;
+  } catch (...) {
+    return 0;
+  }
+}
+
+int foo6 ()
+{
+  B b;
+  return b.method ();
+}
+
+int foo7 (B *p)
+{
+  return p->return_slot ().a1;
+}
+
+/* { dg-final { scan-assembler-times "stack_chk_fail" 7 } } */
Index: gcc/testsuite/gcc.dg/fstack-protector-strong.c
===================================================================
--- gcc/testsuite/gcc.dg/fstack-protector-strong.c	(revision 206311)
+++ gcc/testsuite/gcc.dg/fstack-protector-strong.c	(working copy)
@@ -131,4 +131,22 @@
   return bb.three;
 }
 
-/* { dg-final { scan-assembler-times "stack_chk_fail" 10 } } */
+struct B
+{
+  /* Discourage passing this struct in registers. */
+  int a1, a2, a3, a4, a5, a6, a7, a8, a9, a10;
+};
+
+struct B global3 (void);
+
+int foo11 ()
+{
+  return global3 ().a1;
+}
+
+void foo12 ()
+{
+  global3 ();
+}
+
+/* { dg-final { scan-assembler-times "stack_chk_fail" 12 } } */
Index: gcc/testsuite/gcc.target/i386/ssp-strong-reg.c
===================================================================
--- gcc/testsuite/gcc.target/i386/ssp-strong-reg.c	(revision 0)
+++ gcc/testsuite/gcc.target/i386/ssp-strong-reg.c	(working copy)
@@ -0,0 +1,19 @@
+/* Test that structs returned in registers do not lead to
+   instrumentation with -fstack-protector-strong.  */
+
+/* { dg-do compile { target { ! { ia32 } } } } */
+/* { dg-options "-O2 -fstack-protector-strong" } */
+
+struct S {
+  int a;
+  int b;
+};
+
+struct S f (void);
+
+int g (void)
+{
+  return f ().a;
+}
+
+/* { dg-final { scan-assembler-times "stack_chk_fail" 0 } } */