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

[Florian Weimer <fweimer@redhat.com>]

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On 01/03/2014 10:43 PM, Florian Weimer wrote:

>> Lastly, I wonder if gimple_call_return_slot_opt_p is really what you are
>> after, why does NRV matter here?
>
> The C code we generate does not construct the returned value in place
> (presumably because the partial write would be visible with threads,
> longjmp etc.), unlike the C++ code.
>
> That's why I'm interested in instrumenting NRV-able calls only.  But
> gimple_call_return_slot_opt_p doesn't actually give me that.  The GIMPLE
> from the C test case looks like this (before and after applying your
> proposal):

I thought about this some more and I think it makes sense to add the 
instrumentation each time the return slot is used, both for C and C++. 
We don't if the called function is implemented in C or C++, so 
language-specific instrumentation is not entirely accurate.

I'm attaching a second version of the patch, splitting out the decl and 
bb analysis and using is_gimple_call.  Bootstrapped and 
regression-tested on x86_64-redhat-linux-gnu.

-- 
Florian Weimer / Red Hat Product Security Team

[-- Attachment #2: ssp-strong-return-slot.patch --]
[-- Type: text/x-patch, Size: 4169 bytes --]

gcc/

2014-01-07  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/

2014-01-07  Florian Weimer  <fweimer@redhat.com>

	* gcc.dg/fstack-protector-strong.c: Add coverage for return slots.
	* g++.dg/fstack-protector-strong.C: Likewise.

Index: gcc/cfgexpand.c
===================================================================
--- gcc/cfgexpand.c	(revision 206311)
+++ gcc/cfgexpand.c	(working copy)
@@ -1599,6 +1599,47 @@
   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);
+	if (is_gimple_call (stmt) && gimple_call_return_slot_opt_p (stmt))
+	  return true;
+      }
+  return false;
+}
+
 /* Expand all variables used in the function.  */
 
 static rtx
@@ -1669,22 +1710,8 @@
   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 +32,39 @@
   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;
+};
+
+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;
+  }
+}
+
+/* { dg-final { scan-assembler-times "stack_chk_fail" 5 } } */
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 +131,17 @@
   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;
+}
+
+/* { dg-final { scan-assembler-times "stack_chk_fail" 11 } } */