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

[Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx>]

On Thu, Jun 4, 2020 at 4:56 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> 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.

You are right, it follows the Kconfig logic, returning false for
defined-but-to-0 too.

We should probably add an `IS_DEFINED()` macro kernel-wide for this
(and add it to the `coding-guidelines.rst` since `IS_ENABLED()` is
mentioned there, with a warning not to mix it with `__is_defined()`
which looks it was only intended as an implementation detail for
`include/linux/kconfig.h`).

CC'ing Masahiro by the way.

Cheers,
Miguel