linux-kernel - Re: [PATCH 01/10] x86/mm/numa: Remove uninitialized

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Date:   Thu, 4 Jun 2020 07:56:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Joe Perches <joe@...ches.com>,
        Andy Whitcroft <apw@...onical.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        drbd-dev@...ts.linbit.com, linux-block@...r.kernel.org,
        b43-dev@...ts.infradead.org,
        Network Development <netdev@...r.kernel.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        linux-ide@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-spi@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 01/10] x86/mm/numa: Remove uninitialized_var() usage

On Thu, Jun 04, 2020 at 01:41:07PM +0200, Miguel Ojeda wrote:
> On Thu, Jun 4, 2020 at 9:58 AM Thomas Gleixner <tglx@...utronix.de> 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