Re: [PATCH v3 05/13] rust: init: wrap type checking struct initializers in a closure

[Gary Guo <gary@garyguo.net>]

On Sat, 29 Jul 2023 09:09:46 +0000
Benno Lossin <benno.lossin@proton.me> wrote:

> In the implementation of the init macros there is a `if false` statement
> that type checks the initializer to ensure every field is initialized.
> Since the next patch has a stack variable to store the struct, the
> function might allocate too much memory on debug builds. Putting the
> struct into a closure that is never executed ensures that even in debug
> builds no stack overflow error is caused. In release builds this was not
> a problem since the code was optimized away due to the `if false`.
> 
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
> v2 -> v3:
> - added Reviewed-by's from Martin and Alice.
> 
> v1 -> v2:
> - do not call the created closure.
> 
>  rust/kernel/init/macros.rs | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/rust/kernel/init/macros.rs b/rust/kernel/init/macros.rs
> index 454f31b8c614..2bad086cda0a 100644
> --- a/rust/kernel/init/macros.rs
> +++ b/rust/kernel/init/macros.rs
> @@ -1040,14 +1040,14 @@ macro_rules! __init_internal {
>                      // once, this struct initializer will still be type-checked and complain with a
>                      // very natural error message if a field is forgotten/mentioned more than once.
>                      #[allow(unreachable_code, clippy::diverging_sub_expression)]
> -                    if false {
> +                    let _ = || {
>                          $crate::__init_internal!(make_initializer:
>                              @slot(slot),
>                              @type_name($t),
>                              @munch_fields($($fields)*,),
>                              @acc(),
>                          );
> -                    }
> +                    };
>                  }
>                  Ok(__InitOk)
>              }
> @@ -1168,8 +1168,8 @@ macro_rules! __init_internal {
>          @acc($($acc:tt)*),
>      ) => {
>          // Endpoint, nothing more to munch, create the initializer.
> -        // Since we are in the `if false` branch, this will never get executed. We abuse `slot` to
> -        // get the correct type inference here:
> +        // Since we are in the closure that is never called, this will never get executed.
> +        // We abuse `slot` to get the correct type inference here:
>          unsafe {
>              ::core::ptr::write($slot, $t {
>                  $($acc)*