Subject: Re: [PATCH 01/10] x86/mm/numa: Remove uninitialized_var() usage

From: Kees Cook <[email-masked]>
Date: Thu, 4 Jun 2020 07:56:40 -0700

On Thu, Jun 04, 2020 at 01:41:07PM +0200, Miguel Ojeda wrote:
> On Thu, Jun 4, 2020 at 9:58 AM Thomas Gleixner <[email-masked]> wrote:
> >
> > but if we ever lose the 1 then the above will silently compile the code
> > within the IS_ENABLED() section out.
> 
> Yeah, I believe `IS_ENABLED()` is only meant for Kconfig symbols, not
> macro defs in general. A better option would be `__is_defined()` which
> works for defined-to-nothing too.

Er? That's not what it looked like to me:

#define IS_BUILTIN(option) __is_defined(option)
#define IS_ENABLED(option) __or(IS_BUILTIN(option), IS_MODULE(option))

But just to be sure, I just tested in with a real build:

[    3.242160] IS_ENABLED(TEST_UNDEF) false
[    3.242691] __is_defined(TEST_UNDEF) false
[    3.243240] IS_ENABLED(TEST_VALUE_EMPTY) false
[    3.243794] __is_defined(TEST_VALUE_EMPTY) false
[    3.244353] IS_ENABLED(TEST_VALUE_1) true
[    3.244848] __is_defined(TEST_VALUE_1) true

and nope, it only works with a defined value present.

diff --git a/init/main.c b/init/main.c
index 03371976d387..378a9e54b6dc 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1406,6 +1406,34 @@ static int __ref kernel_init(void *unused)
 	 */
 	pti_finalize();
 
+#undef TEST_UNDEF
+	if (IS_ENABLED(TEST_UNDEF))
+		pr_info("IS_ENABLED(TEST_UNDEF) true\n");
+	else
+		pr_info("IS_ENABLED(TEST_UNDEF) false\n");
+	if (__is_defined(TEST_UNDEF))
+		pr_info("__is_defined(TEST_UNDEF) true\n");
+	else
+		pr_info("__is_defined(TEST_UNDEF) false\n");
+#define TEST_VALUE_EMPTY
+	if (IS_ENABLED(TEST_VALUE_EMPTY))
+		pr_info("IS_ENABLED(TEST_VALUE_EMPTY) true\n");
+	else
+		pr_info("IS_ENABLED(TEST_VALUE_EMPTY) false\n");
+	if (__is_defined(TEST_VALUE_EMPTY))
+		pr_info("__is_defined(TEST_VALUE_EMPTY) true\n");
+	else
+		pr_info("__is_defined(TEST_VALUE_EMPTY) false\n");
+#define TEST_VALUE_1 1
+	if (IS_ENABLED(TEST_VALUE_1))
+		pr_info("IS_ENABLED(TEST_VALUE_1) true\n");
+	else
+		pr_info("IS_ENABLED(TEST_VALUE_1) false\n");
+	if (__is_defined(TEST_VALUE_1))
+		pr_info("__is_defined(TEST_VALUE_1) true\n");
+	else
+		pr_info("__is_defined(TEST_VALUE_1) false\n");
+
 	system_state = SYSTEM_RUNNING;
 	numa_default_policy();

which means a few other __is_defined() users are not correct too...

-- 
Kees Cook