[PATCH v1 3/4] rust: hrtimer: Restrict expires() to safe contexts

From: FUJITA Tomonori <[email protected]>

HrTimer::expires() previously read node.expires via a volatile load, which
can race with C-side updates. Rework the API so it is only callable with
exclusive access or from the callback context.

Introduce expires_unchecked() with an explicit safety contract, switch
HrTimer::expires() to Pin<&mut Self>, add
HrTimerCallbackContext::expires(), and route the read through
hrtimer_get_expires() via a Rust helper.

Fixes: 4b0147494275 ("rust: hrtimer: Add HrTimer::expires()")
Closes: https://lore.kernel.org/rust-for-linux/[email protected]/
Acked-by: Andreas Hindborg <[email protected]>
Signed-off-by: FUJITA Tomonori <[email protected]>
---
 rust/helpers/time.c         |  6 +++++
 rust/kernel/time/hrtimer.rs | 46 ++++++++++++++++++++++++++-----------
 2 files changed, 39 insertions(+), 13 deletions(-)

diff --git a/rust/helpers/time.c b/rust/helpers/time.c
index 32f495970493..ef8999621399 100644
--- a/rust/helpers/time.c
+++ b/rust/helpers/time.c
@@ -2,6 +2,7 @@
 
 #include <linux/delay.h>
 #include <linux/ktime.h>
+#include <linux/hrtimer.h>
 #include <linux/timekeeping.h>
 
 __rust_helper void rust_helper_fsleep(unsigned long usecs)
@@ -38,3 +39,8 @@ __rust_helper void rust_helper_udelay(unsigned long usec)
 {
 	udelay(usec);
 }
+
+__rust_helper ktime_t rust_helper_hrtimer_get_expires(const struct hrtimer *timer)
+{
+	return hrtimer_get_expires(timer);
+}
diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index d94275f2e93f..59e9559e7099 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -557,27 +557,36 @@ pub fn forward_now(self: Pin<&mut Self>, interval: Delta) -> u64
         self.forward(HrTimerInstant::<T>::now(), interval)
     }
 
+    /// Return the time expiry for this [`HrTimer`].
+    ///
+    /// # Safety
+    ///
+    /// The caller must either have exclusive access to `self`, or be within the context of the
+    /// timer callback.
+    #[inline]
+    unsafe fn expires_unchecked(&self) -> HrTimerInstant<T>
+    where
+        T: HasHrTimer<T>,
+    {
+        // SAFETY:
+        // - The C API requirements for this function are fulfilled by our safety contract.
+        // - Timers cannot have negative `ktime_t` values as their expiration time.
+        unsafe { Instant::from_ktime(bindings::hrtimer_get_expires(Self::raw_get(self))) }
+    }
+
     /// Return the time expiry for this [`HrTimer`].
     ///
     /// This value should only be used as a snapshot, as the actual expiry time could change after
     /// this function is called.
-    pub fn expires(&self) -> HrTimerInstant<T>
+    pub fn expires(self: Pin<&mut Self>) -> HrTimerInstant<T>
     where
         T: HasHrTimer<T>,
     {
-        // SAFETY: `self` is an immutable reference and thus always points to a valid `HrTimer`.
-        let c_timer_ptr = unsafe { HrTimer::raw_get(self) };
+        // SAFETY: `expires_unchecked` does not move `Self`.
+        let this = unsafe { self.get_unchecked_mut() };
 
-        // SAFETY:
-        // - Timers cannot have negative ktime_t values as their expiration time.
-        // - There's no actual locking here, a racy read is fine and expected
-        unsafe {
-            Instant::from_ktime(
-                // This `read_volatile` is intended to correspond to a READ_ONCE call.
-                // FIXME(read_once): Replace with `read_once` when available on the Rust side.
-                core::ptr::read_volatile(&raw const ((*c_timer_ptr).node.expires)),
-            )
-        }
+        // SAFETY: By existence of `Pin<&mut Self>`, we have exclusive access to `Self`.
+        unsafe { this.expires_unchecked() }
     }
 }
 
@@ -1060,6 +1069,17 @@ pub fn forward(&mut self, now: HrTimerInstant<T>, interval: Delta) -> u64 {
     pub fn forward_now(&mut self, duration: Delta) -> u64 {
         self.forward(HrTimerInstant::<T>::now(), duration)
     }
+
+    /// Return the time expiry for the timer.
+    ///
+    /// This function is identical to [`HrTimer::expires()`] except that it may only be used from
+    /// within the context of a [`HrTimer`] callback.
+    pub fn expires(&self) -> HrTimerInstant<T> {
+        // SAFETY:
+        // - We are guaranteed to be within the context of a timer callback by our type invariants.
+        // - By our type invariants, `self.0` always points to a valid `HrTimer<T>`.
+        unsafe { self.0.as_ref().expires_unchecked() }
+    }
 }
 
 /// Use to implement the [`HasHrTimer<T>`] trait.
-- 
2.43.0