LIST: [PATCH v3 1/2] rust: time: Implement Add<Delta>/Sub<Delta> for Instant
In order to copy the behavior rust currently follows for basic arithmetic
operations and panic if the result of an addition or subtraction results in
a value that would violate the invariants of Instant, but only if the
kernel has overflow checking for rust enabled.
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250820203704.731588-2-lyude@redhat.com
diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 7320d87..642c42a 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -25,6 +25,7 @@
//! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
use core::marker::PhantomData;
+use core::ops;
pub mod delay;
pub mod hrtimer;
@@ -224,7 +225,7 @@ pub(crate) unsafe fn from_ktime(ktime: bindings::ktime_t) -> Self {
}
}
-impl<C: ClockSource> core::ops::Sub for Instant<C> {
+impl<C: ClockSource> ops::Sub for Instant<C> {
type Output = Delta;
// By the type invariant, it never overflows.
@@ -236,6 +237,46 @@ fn sub(self, other: Instant<C>) -> Delta {
}
}
+impl<T: ClockSource> ops::Add<Delta> for Instant<T> {
+ type Output = Self;
+
+ #[inline]
+ fn add(self, rhs: Delta) -> Self::Output {
+ // INVARIANT: With arithmetic over/underflow checks enabled, this will panic if we overflow
+ // (e.g. go above `KTIME_MAX`)
+ let res = self.inner + rhs.nanos;
+
+ // INVARIANT: With overflow checks enabled, we verify here that the value is >= 0
+ #[cfg(CONFIG_RUST_OVERFLOW_CHECKS)]
+ assert!(res >= 0);
+
+ Self {
+ inner: res,
+ _c: PhantomData,
+ }
+ }
+}
+
+impl<T: ClockSource> ops::Sub<Delta> for Instant<T> {
+ type Output = Self;
+
+ #[inline]
+ fn sub(self, rhs: Delta) -> Self::Output {
+ // INVARIANT: With arithmetic over/underflow checks enabled, this will panic if we overflow
+ // (e.g. go above `KTIME_MAX`)
+ let res = self.inner - rhs.nanos;
+
+ // INVARIANT: With overflow checks enabled, we verify here that the value is >= 0
+ #[cfg(CONFIG_RUST_OVERFLOW_CHECKS)]
+ assert!(res >= 0);
+
+ Self {
+ inner: res,
+ _c: PhantomData,
+ }
+ }
+}
+
/// A span of time.
///
/// This struct represents a span of time, with its value stored as nanoseconds.
