linux-kernel - Re: [PATCH] rust: workaround `bindgen` issue with forward references to `enum` types

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Message-ID: <CANiq72nDtkpC6zKLT3dNH-u8uUXRqTGpS+GPotOrSV=pNR-39A@mail.gmail.com>
Date: Wed, 21 May 2025 15:45:18 +0200
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Andreas Hindborg <a.hindborg@...nel.org>, Miguel Ojeda <ojeda@...nel.org>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH] rust: workaround `bindgen` issue with forward references
 to `enum` types

On Tue, Mar 25, 2025 at 7:43 PM Miguel Ojeda <ojeda@...nel.org> wrote:
>
> `bindgen` currently generates the wrong type for an `enum` when there
> is a forward reference to it. For instance:
>
>     enum E;
>     enum E { A };
>
> generates:
>
>     pub const E_A: E = 0;
>     pub type E = i32;
>
> instead of the expected:
>
>     pub const E_A: E = 0;
>     pub type E = ffi::c_uint;
>
> The issue was reported to upstream `bindgen` [1].
>
> Now, both GCC and Clang support silently these forward references to
> `enum` types, unless `-Wpedantic` is passed, and it turns out that some
> headers in the kernel depend on them.
>
> Thus, depending on how the headers are included, which in turn may depend
> on the kernel configuration or the architecture, we may get a different
> type on the Rust side for a given C `enum`.
>
> That can be quite confusing, to say the least, especially since
> developers may only notice issues when building for other architectures
> like in [2]. In particular, they may end up forcing a cast and adding
> an `#[allow(clippy::unnecessary_cast)]` like it was done in commit
> 94e05a66ea3e ("rust: hrtimer: allow timer restart from timer handler"),
> which isn't great.
>
> Instead, let's have a section at the top of our `bindings_helper.h` that
> `#include`s the headers with the affected types -- hopefully there are
> not many cases and there is a single ordering that covers all cases.
>
> This allows us to remove the cast and the `#[allow]`, thus keeping the
> correct code in the source files. When the issue gets resolved in upstream
> `bindgen` (and we update our minimum `bindgen` version), we can easily
> remove this section at the top.
>
> Link: https://github.com/rust-lang/rust-bindgen/issues/3179 [1]
> Link: https://lore.kernel.org/rust-for-linux/87tt7md1s6.fsf@kernel.org/ [2]
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>

Andreas: are you OK with something along the lines of this change?

If you don't have anything against it, then I think it would be nice
to put it in, mainly so that the issue is documented, and so that we
don't forget about this possible workaround. Plus it removes an
`allow` and a cast, which is also nice.

The only downside, as far as I know, is that the workaround cannot be
always applied (see my sibling reply), but at least for this case, it
can be.

Thanks!

Cheers,
Miguel