Re: [PATCH v4 2/2] rust: hrtimer: Make HrTimer repr(transparent)

[FUJITA Tomonori <tomo@flapping.org>]

On Mon, 10 Aug 2026 13:07:20 +0200
Andreas Hindborg <a.hindborg@kernel.org> wrote:

> "FUJITA Tomonori" <tomo@flapping.org> writes:
> 
>> From: FUJITA Tomonori <fujita.tomonori@gmail.com>
>>
>> HrTimerCallbackContext acquires a &HrTimer<T> from a
>> NonNull<HrTimer<T>> while a &mut HrTimer<T> can exist at the same
>> time. This is sound only because HrTimer's sole field is
>> Opaque<bindings::hrtimer>, which puts every byte behind an UnsafeCell.
>> Adding a field to HrTimer that is not Opaque would make acquiring that
>> shared reference unsound.
>>
>> Make HrTimer repr(transparent), which prevents multiple fields, so that
>> such a refactor fails to compile instead of silently introducing
>> unsoundness. This does not guarantee the remaining field stays behind
>> Opaque, but it rules out the likely way of getting there.
>>
>> repr(transparent) cannot be combined with repr(C), so drop the latter.
>>
>> Suggested-by: Miguel Ojeda <ojeda@kernel.org>
>> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
>> ---
>>  rust/kernel/time/hrtimer.rs         | 6 +++++-
>>  rust/kernel/time/hrtimer/arc.rs     | 2 +-
>>  rust/kernel/time/hrtimer/pin.rs     | 2 +-
>>  rust/kernel/time/hrtimer/pin_mut.rs | 2 +-
>>  rust/kernel/time/hrtimer/tbox.rs    | 2 +-
>>  5 files changed, 9 insertions(+), 5 deletions(-)
>>
>> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
>> index 1db84cd4cbe8..04f47af3541b 100644
>> --- a/rust/kernel/time/hrtimer.rs
>> +++ b/rust/kernel/time/hrtimer.rs
>> @@ -418,8 +418,12 @@
>>  /// # Invariants
>>  ///
>>  /// * `self.timer` is initialized by `bindings::hrtimer_setup`.
>> +// `repr(transparent)` is not merely about layout. `HrTimerCallbackContext` acquires a
>> +// `&HrTimer<T>` while a `&mut HrTimer<T>` may exist, which is sound only because every byte of
>> +// this type sits inside `Opaque`. Being transparent rejects a second field at compile time,
>> +// but it does not enforce that the remaining field stays `Opaque`.
> 
> Missing bullet. With that fixed:
> 
> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>

This is a plain `//` comment, not documentation. Did you mean that you
want it documented as one of the `# Invariants` bullets instead?