rust: hrtimer: add UnsafeHrTimerPointer

Add a trait to allow unsafely queuing stack allocated timers.

Acked-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250224-hrtimer-v3-v6-12-rc2-v9-5-5bd3bf0ce6cc@kernel.org

Author: Andreas Hindborg

rust/kernel/time/hrtimer.rs

@@ -181,6 +181,37 @@ pub trait HrTimerPointer: Sync + Sized {
     fn start(self, expires: Ktime) -> Self::TimerHandle;
 }
 
+/// Unsafe version of [`HrTimerPointer`] for situations where leaking the
+/// [`HrTimerHandle`] returned by `start` would be unsound. This is the case for
+/// stack allocated timers.
+///
+/// Typical implementers are pinned references such as [`Pin<&T>`].
+///
+/// # Safety
+///
+/// Implementers of this trait must ensure that instances of types implementing
+/// [`UnsafeHrTimerPointer`] outlives any associated [`HrTimerPointer::TimerHandle`]
+/// instances.
+pub unsafe trait UnsafeHrTimerPointer: Sync + Sized {
+    /// A handle representing a running timer.
+    ///
+    /// # Safety
+    ///
+    /// If the timer is running, or if the timer callback is executing when the
+    /// handle is dropped, the drop method of [`Self::TimerHandle`] must not return
+    /// until the timer is stopped and the callback has completed.
+    type TimerHandle: HrTimerHandle;
+
+    /// Start the timer after `expires` time units. If the timer was already
+    /// running, it is restarted at the new expiry time.
+    ///
+    /// # Safety
+    ///
+    /// Caller promises keep the timer structure alive until the timer is dead.
+    /// Caller can ensure this by not leaking the returned [`Self::TimerHandle`].
+    unsafe fn start(self, expires: Ktime) -> Self::TimerHandle;
+}
+
 /// Implemented by [`HrTimerPointer`] implementers to give the C timer callback a
 /// function to call.
 // This is split from `HrTimerPointer` to make it easier to specify trait bounds.