`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@nvidia.com/ [2] https://lore.kernel.org/all/A1A280D4-836E-4D75-863E-30B1C276C80C@collabora.c... [3] https://lore.kernel.org/all/20251121143008.2f5acc33.gary@garyguo.net/
Signed-off-by: Alexandre Courbot acourbot@nvidia.com --- Changes in v3: - Add "Fixes:" tags. - CC stable on fixup patches. - Link to v2: https://patch.msgid.link/20251128-io-build-assert-v2-0-a9ea9ce7d45d@nvidia.c...
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@nvidia.c...
--- 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: ba65a4e7120a616d9c592750d9147f6dcafedffa change-id: 20251127-io-build-assert-3579a5bfb81c
Best regards,