gcc-plugins: Add structleak for more stack initialization

This plugin detects any structures that contain __user attributes and
makes sure it is being fully initialized so that a specific class of
information exposure is eliminated. (This plugin was originally designed
to block the exposure of siginfo in CVE-2013-2141.)

Ported from grsecurity/PaX. This version adds a verbose option to the
plugin and the Kconfig.

Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/compiler.h

@@ -27,7 +27,11 @@ extern void __chk_user_ptr(const volatile void __user *);
 extern void __chk_io_ptr(const volatile void __iomem *);
 # define ACCESS_PRIVATE(p, member) (*((typeof((p)->member) __force *) &(p)->member))
 #else /* __CHECKER__ */
-# define __user
+# ifdef STRUCTLEAK_PLUGIN
+#  define __user __attribute__((user))
+# else
+#  define __user
+# endif
 # define __kernel
 # define __safe
 # define __force