[RFC PATCH] -Wmemset-transposed-args (PR middle-end/61294)

Jakub Jelinek jakub@redhat.com
Tue Jul 8 12:50:00 GMT 2014 

Hi!

This is an attempt to move the warning about transposed memset arguments
from the glibc headers to gcc FEs.  The problem with the warning in glibc
is that it uses __builtin_constant_p and e.g. jump threading very often
makes the warning trigger even on code where it is very unlikely a user
swapped arguments.  See e.g.
https://gcc.gnu.org/PR51744
https://gcc.gnu.org/PR56977
https://gcc.gnu.org/PR61294
https://bugzilla.redhat.com/452219
https://bugs.kde.org/show_bug.cgi?id=311098
https://bugzilla.mozilla.org/show_bug.cgi?id=581227
and many others.  Thus, I'd like to warn in the FEs instead, and
once we have a GCC release with that warning in, disable the glibc
bits/string3.h:
  if (__builtin_constant_p (__len) && __len == 0
      && (!__builtin_constant_p (__ch) || __ch != 0))
    {
      __warn_memset_zero_len ();
      return __dest;
    }
warning for GCC versions with that new warning in.

Any thoughts on this?

If you are ok with it, either we can add it only for 4.10/5.0 and
later only, or perhaps 4.9.2 too, or even 4.9.1.  For -D_FORTIFY_SOURCE=2
built code with glibc it shouldn't make a difference (other than having
fewer false positives), but for other non-fortified -Wall compilation
it would make a difference (introducing new warnings), so perhaps
doing it only for 4.10/5.0+ is best.

2014-07-08  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/61294
gcc/c-family/
	* c.opt (Wmemset-transposed-args): New warning.
gcc/c/
	* c-typeck.c (c_build_function_call_vec): Handle
	-Wmemset-transposed-args.
gcc/cp/
	* semantics.c (finish_call_expr): Handle -Wmemset-transposed-args.
gcc/
	* doc/invoke.texi (-Wmemset-transposed-args): Document.
gcc/testsuite/
	* c-c++-common/Wmemset-transposed-args1.c: New test.
	* g++.dg/warn/Wmemset-transposed-args-1.C: New test.

--- gcc/c-family/c.opt.jj	2014-07-07 10:39:43.000000000 +0200
+++ gcc/c-family/c.opt	2014-07-08 13:12:07.755536537 +0200
@@ -518,6 +518,10 @@ Wmain
 LangEnabledBy(C ObjC C++ ObjC++,Wpedantic, 2, 0)
 ;
 
+Wmemset-transposed-args
+C ObjC C++ ObjC++ Var(warn_memset_transposed_args) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
+Warn about suspicious call to memset where the third argument is constant zero and second is not zero
+
 Wmissing-braces
 C ObjC C++ ObjC++ Var(warn_missing_braces) Warning LangEnabledBy(C ObjC,Wall)
 Warn about possibly missing braces around initializers
--- gcc/c/c-typeck.c.jj	2014-07-07 10:39:43.000000000 +0200
+++ gcc/c/c-typeck.c	2014-07-08 13:22:36.846564329 +0200
@@ -2987,6 +2987,16 @@ c_build_function_call_vec (location_t lo
   /* Convert anything with function type to a pointer-to-function.  */
   if (TREE_CODE (function) == FUNCTION_DECL)
     {
+      if (warn_memset_transposed_args
+	  && DECL_BUILT_IN_CLASS (function) == BUILT_IN_NORMAL
+	  && DECL_FUNCTION_CODE (function) == BUILT_IN_MEMSET
+	  && vec_safe_length (params) == 3
+	  && integer_zerop ((*params)[2])
+	  && !integer_zerop ((*params)[1]))
+	warning_at (loc, OPT_Wmemset_transposed_args,
+		    "%<memset%> used with constant zero length parameter; "
+		    "this could be due to transposed parameters");
+
       /* Implement type-directed function overloading for builtins.
 	 resolve_overloaded_builtin and targetm.resolve_overloaded_builtin
 	 handle all the type checking.  The result is a complete expression
--- gcc/cp/semantics.c.jj	2014-07-02 09:04:13.000000000 +0200
+++ gcc/cp/semantics.c	2014-07-08 14:02:45.936782580 +0200
@@ -2361,6 +2361,18 @@ finish_call_expr (tree fn, vec<tree, va_
 		 sizeof_arg, same_type_ignoring_top_level_qualifiers_p);
 	    }
 
+	  if (warn_memset_transposed_args
+	      && !processing_template_decl
+	      && TREE_CODE (fn) == FUNCTION_DECL
+	      && DECL_BUILT_IN_CLASS (fn) == BUILT_IN_NORMAL
+	      && DECL_FUNCTION_CODE (fn) == BUILT_IN_MEMSET
+	      && vec_safe_length (*args) == 3
+	      && integer_zerop ((**args)[2])
+	      && !integer_zerop ((**args)[1]))
+	    warning (OPT_Wmemset_transposed_args,
+		     "%<memset%> used with constant zero length parameter; "
+		     "this could be due to transposed parameters");
+
 	  /* A call to a namespace-scope function.  */
 	  result = build_new_function_call (fn, args, koenig_p, complain);
 	}
--- gcc/doc/invoke.texi.jj	2014-07-08 11:36:14.000000000 +0200
+++ gcc/doc/invoke.texi	2014-07-08 14:20:09.932799699 +0200
@@ -257,8 +257,8 @@ Objective-C and Objective-C++ Dialects}.
 -Wno-int-to-pointer-cast -Wno-invalid-offsetof @gol
 -Winvalid-pch -Wlarger-than=@var{len}  -Wunsafe-loop-optimizations @gol
 -Wlogical-op -Wlogical-not-parentheses -Wlong-long @gol
--Wmain -Wmaybe-uninitialized -Wmissing-braces  -Wmissing-field-initializers @gol
--Wmissing-include-dirs @gol
+-Wmain -Wmaybe-uninitialized -Wmemset-transposed-args  -Wmissing-braces @gol
+-Wmissing-field-initializers -Wmissing-include-dirs @gol
 -Wno-multichar  -Wnonnull  -Wno-overflow -Wopenmp-simd @gol
 -Woverlength-strings  -Wpacked  -Wpacked-bitfield-compat  -Wpadded @gol
 -Wparentheses  -Wpedantic-ms-format -Wno-pedantic-ms-format @gol
@@ -4683,6 +4683,15 @@ Warn when the @code{sizeof} operator is
 declared as an array in a function definition.  This warning is enabled by
 default for C and C++ programs.
 
+@item -Wmemset-transposed-args
+@opindex Wmemset-transposed-args
+@opindex Wno-memset-transposed-args
+Warn for suspicious calls to the memset built-in function, if the
+second argument is not zero and third argument is zero.  This warns e.g.@
+about @code{memset (buf, sizeof buf, 0);} where most probably
+@code{memset (buf, 0, sizeof buf);} was meant instead.  This warning is
+enabled by @option{-Wall}.
+
 @item -Waddress
 @opindex Waddress
 @opindex Wno-address
--- gcc/testsuite/c-c++-common/Wmemset-transposed-args1.c.jj	2014-07-08 13:46:19.381765644 +0200
+++ gcc/testsuite/c-c++-common/Wmemset-transposed-args1.c	2014-07-08 13:46:14.868798956 +0200
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-Wall" } */
+
+typedef __SIZE_TYPE__ size_t;
+extern
+#ifdef __cplusplus
+"C"
+#endif
+void *memset (void *, int, size_t);
+char buf[1024];
+
+void
+foo ()
+{
+  memset (buf, sizeof buf, 0);	/* { dg-warning ".memset. used with constant zero length parameter; this could be due to transposed parameters" } */
+  memset (buf, sizeof buf, '\0'); /* { dg-warning ".memset. used with constant zero length parameter; this could be due to transposed parameters" } */
+  memset (buf, 1, 1 - 1);	/* { dg-warning ".memset. used with constant zero length parameter; this could be due to transposed parameters" } */
+  memset (buf, 0, 0);
+  memset (buf, 1 - 1, 0);
+}
--- gcc/testsuite/g++.dg/warn/Wmemset-transposed-args-1.C.jj	2014-07-08 13:50:22.685624346 +0200
+++ gcc/testsuite/g++.dg/warn/Wmemset-transposed-args-1.C	2014-07-08 13:51:59.837968475 +0200
@@ -0,0 +1,25 @@
+// { dg-do compile }
+// { dg-options "-Wall" }
+
+typedef __SIZE_TYPE__ size_t;
+extern "C" void *memset (void *, int, size_t);
+char buf[1024];
+
+template <int N>
+void
+foo ()
+{
+  memset (buf, sizeof buf, 0);	// { dg-warning ".memset. used with constant zero length parameter; this could be due to transposed parameters" }
+  memset (buf, sizeof buf, '\0'); // { dg-warning ".memset. used with constant zero length parameter; this could be due to transposed parameters" }
+  memset (buf, sizeof buf, N);	// { dg-warning ".memset. used with constant zero length parameter; this could be due to transposed parameters" }
+  memset (buf, 1, 1 - 1);	// { dg-warning ".memset. used with constant zero length parameter; this could be due to transposed parameters" }
+  memset (buf, 1, N - N);	// { dg-warning ".memset. used with constant zero length parameter; this could be due to transposed parameters" }
+  memset (buf, 0, 0);
+  memset (buf, 1 - 1, 0);
+}
+
+void
+bar ()
+{
+  foo<0> ();
+}

	Jakub

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