Re: [PATCH v2 0/7] rust: build_assert: document and fix use with function arguments

[Daniel Almeida]

> On 27 Nov 2025, at 23:11, Alexandre Courbot <acourbot@xxxxxxxxxx> 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, as pointed out by Gary Guo [3], 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.
> 
> This series fixes all possible points of failure in the kernel crate,
> and adds documentation to `build_assert` explaining how to properly
> inline functions for which this behavior may arise.
> 
> [1] https://lore.kernel.org/all/DEEUYUOAEZU3.1J1HM2YQ10EX1@xxxxxxxxxx/
> [2] https://lore.kernel.org/all/A1A280D4-836E-4D75-863E-30B1C276C80C@xxxxxxxxxxxxx/
> [3] https://lore.kernel.org/all/20251121143008.2f5acc33.gary@xxxxxxxxxxx/
> 
> Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
> ---
> Changes in v2:
> - Turn into a series and address other similar cases in the kernel crate.
> - Link to v1: https://patch.msgid.link/20251127-io-build-assert-v1-1-04237f2e5850@xxxxxxxxxx
> 
> ---
> Alexandre Courbot (7):
>      rust: build_assert: add instructions for use with function arguments
>      rust: io: always inline functions using build_assert with arguments
>      rust: cpufreq: always inline functions using build_assert with arguments
>      rust: bits: always inline functions using build_assert with arguments
>      rust: sync: refcount: always inline functions using build_assert with arguments
>      rust: irq: always inline functions using build_assert with arguments
>      rust: num: bounded: add missing comment for always inlined function
> 
> rust/kernel/bits.rs          | 6 ++++--
> rust/kernel/build_assert.rs  | 7 ++++++-
> rust/kernel/cpufreq.rs       | 2 ++
> rust/kernel/io.rs            | 9 ++++++---
> rust/kernel/io/resource.rs   | 2 ++
> rust/kernel/irq/flags.rs     | 2 ++
> rust/kernel/num/bounded.rs   | 1 +
> rust/kernel/sync/refcount.rs | 3 ++-
> 8 files changed, 25 insertions(+), 7 deletions(-)
> ---
> base-commit: 54e3eae855629702c566bd2e130d9f40e7f35bde
> change-id: 20251127-io-build-assert-3579a5bfb81c
> 
> Best regards,
> -- 
> Alexandre Courbot <acourbot@xxxxxxxxxx>
> 

Thanks for fixing this. I was _very_ confused when it happened to me.

Reviewed-by: Daniel Almeida <daniel.almeida@xxxxxxxxxxxxx>