linux-kernel - Re: [PATCH 2/2] rust: use `#[used(compiler)]` to fix build and `modpost` with Rust >= 1.89.0

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Message-ID: <CAH5fLgiZsdW+98-_kMmGcdujQzusDcLMdGJzPk-6VG7pkC2bcA@mail.gmail.com>
Date: Mon, 14 Jul 2025 10:55:03 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Masahiro Yamada <masahiroy@...nel.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev, 
	David Wood <david@...idtw.co>, Wesley Wiser <wwiser@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] rust: use `#[used(compiler)]` to fix build and
 `modpost` with Rust >= 1.89.0

On Sat, Jul 12, 2025 at 6:02 PM Miguel Ojeda <ojeda@...nel.org> wrote:
>
> Starting with Rust 1.89.0 (expected 2025-08-07), the Rust compiler fails
> to build the `rusttest` target due to undefined references such as:
>
>     kernel...-cgu.0:(.text....+0x116): undefined reference to
>     `rust_helper_kunit_get_current_test'
>
> Moreover, tooling like `modpost` gets confused:
>
>     WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/nova/nova.o
>     ERROR: modpost: missing MODULE_LICENSE() in drivers/gpu/nova-core/nova_core.o
>
> The reason behind both issues is that the Rust compiler will now [1]
> treat `#[used]` as `#[used(linker)]` instead of `#[used(compiler)]`
> for our targets. This means that the retain section flag (`R`,
> `SHF_GNU_RETAIN`) will be used and that they will be marked as `unique`
> too, with different IDs. In turn, that means we end up with undefined
> references that did not get discarded in `rusttest` and that multiple
> `.modinfo` sections are generated, which confuse tooling like `modpost`
> because they only expect one.
>
> Thus start using `#[used(compiler)]` to keep the previous behavior
> and to be explicit about what we want. Sadly, it is an unstable feature
> (`used_with_arg`) [2] -- we will talk to upstream Rust about it. The good
> news is that it has been available for a long time (Rust >= 1.60) [3].
>
> The changes should also be fine for previous Rust versions, since they
> behave the same way as before [4].
>
> Alternatively, we could use `#[no_mangle]` or `#[export_name = ...]`
> since those still behave like `#[used(compiler)]`, but of course it is
> not really what we want to express, and it requires other changes to
> avoid symbol conflicts.
>
> Cc: Björn Roy Baron <bjorn3_gh@...tonmail.com>
> Cc: David Wood <david@...idtw.co>
> Cc: Wesley Wiser <wwiser@...il.com>
> Cc: stable@...r.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
> Link: https://github.com/rust-lang/rust/pull/140872 [1]
> Link: https://github.com/rust-lang/rust/issues/93798 [2]
> Link: https://github.com/rust-lang/rust/pull/91504 [3]
> Link: https://godbolt.org/z/sxzWTMfzW [4]
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>