Re: [PATCH v2 1/7] rust: module: add `THIS_MODULE` const to `ModuleMetadata` trait

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alvin Sun" <alvin.sun@linux.dev> writes:

> Add a `THIS_MODULE` const to the `ModuleMetadata` trait so that
> modules can provide their `ThisModule` pointer usable in const
> contexts such as static file_operations.
>
> Move the `THIS_MODULE` static from the `module!` macro into the
> `ModuleMetadata` impl, and update `__init` to use
> `LocalModule::THIS_MODULE` instead.
>
> Signed-off-by: Alvin Sun <alvin.sun@linux.dev>
> ---
>  rust/kernel/lib.rs    |  3 +++
>  rust/macros/module.rs | 34 +++++++++++++++++-----------------
>  2 files changed, 20 insertions(+), 17 deletions(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index b72b2fbe046d6..f0cf0705d9697 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -184,6 +184,9 @@ fn init(module: &'static ThisModule) -> impl pin_init::PinInit<Self, error::Erro
>  pub trait ModuleMetadata {
>      /// The name of the module as specified in the `module!` macro.
>      const NAME: &'static crate::str::CStr;
> +
> +    /// The module's `THIS_MODULE` pointer.
> +    const THIS_MODULE: ThisModule;
>  }
>
>  /// Equivalent to `THIS_MODULE` in the C API.
> diff --git a/rust/macros/module.rs b/rust/macros/module.rs
> index 06c18e2075083..b6d7b3299fbf9 100644
> --- a/rust/macros/module.rs
> +++ b/rust/macros/module.rs
> @@ -497,28 +497,28 @@ pub(crate) fn module(info: ModuleInfo) -> Result<TokenStream> {
>          /// Used by the printing macros, e.g. [`info!`].
>          const __LOG_PREFIX: &[u8] = #name_cstr.to_bytes_with_nul();
>
> -        // SAFETY: `__this_module` is constructed by the kernel at load time and will not be
> -        // freed until the module is unloaded.
> -        #[cfg(MODULE)]
> -        static THIS_MODULE: ::kernel::ThisModule = unsafe {
> -            extern "C" {
> -                static __this_module: ::kernel::types::Opaque<::kernel::bindings::module>;
> -            };
> -
> -            ::kernel::ThisModule::from_ptr(__this_module.get())
> -        };
> -
> -        #[cfg(not(MODULE))]
> -        static THIS_MODULE: ::kernel::ThisModule = unsafe {
> -            ::kernel::ThisModule::from_ptr(::core::ptr::null_mut())
> -        };
> -
>          /// The `LocalModule` type is the type of the module created by `module!`,
>          /// `module_pci_driver!`, `module_platform_driver!`, etc.
>          type LocalModule = #type_;
>
>          impl ::kernel::ModuleMetadata for #type_ {
>              const NAME: &'static ::kernel::str::CStr = #name_cstr;
> +
> +            #[cfg(MODULE)]
> +            const THIS_MODULE: ::kernel::ThisModule = {
> +                extern "C" {
> +                    static __this_module: ::kernel::types::Opaque<::kernel::bindings::module>;
> +                }
> +
> +                // SAFETY: `__this_module` is constructed by the kernel at load time
> +                // and lives until the module is unloaded.
> +                unsafe { ::kernel::ThisModule::from_ptr(__this_module.get()) }
> +            };
> +
> +            #[cfg(not(MODULE))]
> +            const THIS_MODULE: ::kernel::ThisModule = unsafe {
> +                ::kernel::ThisModule::from_ptr(::core::ptr::null_mut())
> +            };
>          }
>
>          // Double nested modules, since then nobody can access the public items inside.
> @@ -616,7 +616,7 @@ pub extern "C" fn #ident_exit() {
>                  /// This function must only be called once.
>                  unsafe fn __init() -> ::kernel::ffi::c_int {
>                      let initer = <super::super::LocalModule as ::kernel::InPlaceModule>::init(
> -                        &super::super::THIS_MODULE
> +                        &<super::super::LocalModule as ::kernel::ModuleMetadata>::THIS_MODULE

Is it possible we could make this more ergonomic? Perhaps by adding a
helper:

  fn this_module<M: ::kernel::ModuleMetadata>() -> &'static ::kernel::ThisModule {
      &M::THIS_MODULE
  }

Then the invocation is a little better:

  let initer = <super::super::LocalModule as ::kernel::InPlaceModule>::init(
      this_module::<super::super::LocalModule>()
  );


Best regards,
Andreas Hindborg