Re: [PATCH v2 3/5] rust: add #[export] macro

Andreas Hindborg
2025-02-28T18:58:40Z

"Alice Ryhl" <aliceryhl@google.com> writes:

> Rust has two different tools for generating function declarations to
> call across the FFI boundary:
>
> * bindgen. Generates Rust declarations from a C header.
> * cbindgen. Generates C headers from Rust declarations.
>
> In the kernel, we only use bindgen. This is because cbindgen assumes a
> cargo-based buildsystem, so it is not compatible with the kernel's build
> system. This means that when C code calls a Rust function by name, its
> signature must be duplicated in both Rust code and a C header, and the
> signature needs to be kept in sync manually.
>
> To eliminate this manual checking, introduce a new macro that verifies
> at compile time that the two function declarations use the same
> signature. The idea is to run the C declaration through bindgen, and
> then have rustc verify that the function pointers have the same type.
>
> The signature must still be written twice, but at least you can no
> longer get it wrong. If the signatures don't match, you will get errors
> that look like this:
>
> error[E0308]: `if` and `else` have incompatible types
> --> <linux>/rust/kernel/print.rs:22:22
> |
> 21 | #[export]
> | --------- expected because of this
> 22 | unsafe extern "C" fn rust_fmt_argument(
> | ^^^^^^^^^^^^^^^^^ expected `u8`, found `i8`
> |
> = note: expected fn item `unsafe extern "C" fn(*mut u8, *mut u8, *mut c_void) -> *mut u8 {bindings::rust_fmt_argument}`
> found fn item `unsafe extern "C" fn(*mut i8, *mut i8, *const c_void) -> *mut i8 {print::rust_fmt_argument}`
>
> It is unfortunate that the error message starts out by saying "`if` and
> `else` have incompatible types", but I believe the rest of the error
> message is reasonably clear and not too confusing.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>

Best regards,
Andreas Hindborg