stackleak: Register the 'stackleak_cleanup' pass before the '*free_cfg' pass

Currently the 'stackleak_cleanup' pass deleting a CALL insn is executed
after the 'reload' pass. That allows gcc to do some weird optimization in
function prologues and epilogues, which are generated later [1].

Let's avoid that by registering the 'stackleak_cleanup' pass before
the '*free_cfg' pass. It's the moment when the stack frame size is
already final, function prologues and epilogues are generated, and the
machine-dependent code transformations are not done.

[1] https://www.openwall.com/lists/kernel-hardening/2018/11/23/2Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Alexander Popov <alex.popov@linux.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

scripts/gcc-plugins/stackleak_plugin.c

@@ -363,10 +363,12 @@ __visible int plugin_init(struct plugin_name_args *plugin_info,
 						PASS_POS_INSERT_BEFORE);
 
 	/*
-	 * The stackleak_cleanup pass should be executed after the
-	 * "reload" pass, when the stack frame size is final.
+	 * The stackleak_cleanup pass should be executed before the "*free_cfg"
+	 * pass. It's the moment when the stack frame size is already final,
+	 * function prologues and epilogues are generated, and the
+	 * machine-dependent code transformations are not done.
 	 */
-	PASS_INFO(stackleak_cleanup, "reload", 1, PASS_POS_INSERT_AFTER);
+	PASS_INFO(stackleak_cleanup, "*free_cfg", 1, PASS_POS_INSERT_BEFORE);
 
 	if (!plugin_default_version_check(version, &gcc_version)) {
 		error(G_("incompatible gcc/plugin versions"));