Re: [PATCH v3 1/2] rust: time: Implement Add<Delta>/Sub<Delta> for Instant

[Daniel Almeida]

Hi Lyude,


> On 20 Aug 2025, at 17:26, Lyude Paul <lyude@xxxxxxxxxx> wrote:
> 
> 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@xxxxxxxxxx>
> 
> ---
> 
> 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.
> V3:
> * Don't forget to update the commit message this time!
> 
> 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;

Shouldn’t we clamp here instead of..
> +
> +        // INVARIANT: With overflow checks enabled, we verify here that the value is >= 0
> +        #[cfg(CONFIG_RUST_OVERFLOW_CHECKS)]
> +        assert!(res >= 0);

..relying on this?

> +
> +        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);

Same here?

> +
> +        Self {
> +            inner: res,
> +            _c: PhantomData,
> +        }
> +    }
> +}
> +
> /// A span of time.
> ///
> /// This struct represents a span of time, with its value stored as nanoseconds.
> -- 
> 2.50.0
> 
>