linux-kernel - Re: [PATCH v2 1/7] rust: build_assert: add instructions for use with function arguments

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Message-ID: <db6a6b28-c0be-4bd8-a0b4-70431df7737a@nvidia.com>
Date: Sun, 30 Nov 2025 13:44:50 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Alexandre Courbot <acourbot@...dia.com>,
 Danilo Krummrich <dakr@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Daniel Almeida <daniel.almeida@...labora.com>,
 Miguel Ojeda <ojeda@...nel.org>, 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 <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>, "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>, Will Deacon <will@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Mark Rutland <mark.rutland@....com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 1/7] rust: build_assert: add instructions for use with
 function arguments

On 11/27/25 6:11 PM, Alexandre Courbot wrote:
> `build_assert` relies on the compiler to optimize out its error path,
> lest build fails with the dreaded error:
> 
>      ERROR: modpost: "rust_build_error" [path/to/module.ko] undefined!
> 
> It has been observed that very trivial code performing I/O accesses
> (sometimes even using an immediate value) would seemingly randomly fail
> with this error whenever `CLIPPY=1` was set. The same behavior was also
> observed until different, very similar conditions [1][2].
> 
> The cause appears to be that the failing function is eventually using
> `build_assert` with its argument, but is only annotated with
> `#[inline]`. This gives the compiler freedom to not inline the function,
> which it notably did when Clippy was active, triggering the error.
> 
> The fix is to annotate functions passing their argument to
> `build_assert` with `#[inline(always)]`, telling the compiler to be as
> aggressive as possible with their inlining. This is also the correct
> behavior as inlining is mandatory for correct behavior in these cases.

Very interesting. So by adding a partially faulty build_assert!() call,
these functions were actually wrong when they created! Maybe a Fixes:
tag is warranted.

thanks,
-- 
John Hubbard

> 
> Add a paragraph instructing to annotate such functions with
> `#[inline(always)]` in `build_assert`'s documentation, and split its
> example to illustrate.
> 
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
>   rust/kernel/build_assert.rs | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/build_assert.rs b/rust/kernel/build_assert.rs
> index 6331b15d7c4d..f8124dbc663f 100644
> --- a/rust/kernel/build_assert.rs
> +++ b/rust/kernel/build_assert.rs
> @@ -61,8 +61,13 @@ macro_rules! build_error {
>   ///     build_assert!(N > 1); // Build-time check
>   ///     assert!(N > 1); // Run-time check
>   /// }
> +/// ```
>   ///
> -/// #[inline]
> +/// When a condition depends on a function argument, the function must be annotated with
> +/// `#[inline(always)]`. Without this attribute, the compiler may choose to not inline the
> +/// function, preventing it from optimizing out the error path.
> +/// ```
> +/// #[inline(always)]
>   /// fn bar(n: usize) {
>   ///     // `static_assert!(n > 1);` is not allowed
>   ///     build_assert!(n > 1); // Build-time check
>