linux-kernel - Re: [PATCH v3 0/5] Check Rust signatures at compile time

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Message-ID: <CANiq72kY+h9jsFbVBq2_Z4YDUyPCSWuFX0Q5hUyfxsraNV-C4Q@mail.gmail.com>
Date: Sun, 9 Mar 2025 21:47:35 +0100
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	Rasmus Villemoes <linux@...musvillemoes.dk>, Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Tamir Duberstein <tamird@...il.com>, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, Simona Vetter <simona.vetter@...ll.ch>
Subject: Re: [PATCH v3 0/5] Check Rust signatures at compile time

On Mon, Mar 3, 2025 at 9:45 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> 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.
>
> However, we only use bindgen in the kernel. 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.
>
> Introducing cbindgen as a mandatory dependency to build the kernel would
> be a rather complex and large change, so we do not consider that at this
> time. Instead, 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.
>
> The main commit of this series is "rust: add #[export] macro".
>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>

Applied to `rust-next` -- thanks everyone!

    [ Removed period as requested by Andy. - Miguel ]

    [ Fixed `rustfmt`. Moved on top the unsafe requirement comment to follow
      the usual style, and slightly reworded it for clarity. Formatted
      bindings helper comment. - Miguel ]

Cheers,
Miguel