linux-kernel - [PATCH v2 1/2] rust: time: Implement Add<Delta>/Sub<Delta> for Instant

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Message-ID: <20250807190649.3078875-1-lyude@redhat.com>
Date: Thu,  7 Aug 2025 15:06:35 -0400
From: Lyude Paul <lyude@...hat.com>
To: rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Andreas Hindborg <a.hindborg@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	FUJITA Tomonori <fujita.tomonori@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	John Stultz <jstultz@...gle.com>,
	Stephen Boyd <sboyd@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH v2 1/2] rust: time: Implement Add<Delta>/Sub<Delta> for Instant

In order to maintain the invariants of Instant, we use saturating
addition/subtraction that is clamped to the valid value range for a
non-negative Ktime.

Signed-off-by: Lyude Paul <lyude@...hat.com>

---
V2:
* Change behavior in ops::{Add,Sub}<Delta> so that we panic on overflows
  under the same conditions that arithmetic operations in rust would panic
  by default.
---
 rust/kernel/time.rs | 43 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 64c8dcf548d63..4bd7a8a009f3e 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;
@@ -202,7 +203,7 @@ pub(crate) fn as_nanos(&self) -> i64 {
     }
 }
 
-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.
@@ -214,6 +215,46 @@ 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.

base-commit: 479058002c32b77acac43e883b92174e22c4be2d
-- 
2.50.0