linux-kernel - Re: [PATCH] rust: time: Seal the ClockSource trait

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Message-ID: <aFIEAiDKnxsZQ8s4@tardis.local>
Date: Tue, 17 Jun 2025 17:10:42 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: a.hindborg@...nel.org, alex.gaynor@...il.com, ojeda@...nel.org,
	aliceryhl@...gle.com, anna-maria@...utronix.de,
	bjorn3_gh@...tonmail.com, dakr@...nel.org, frederic@...nel.org,
	gary@...yguo.net, jstultz@...gle.com, linux-kernel@...r.kernel.org,
	lossin@...nel.org, lyude@...hat.com, rust-for-linux@...r.kernel.org,
	sboyd@...nel.org, tglx@...utronix.de, tmgross@...ch.edu
Subject: Re: [PATCH] rust: time: Seal the ClockSource trait

On Wed, Jun 18, 2025 at 08:20:53AM +0900, FUJITA Tomonori wrote:
> Prevent downstream crates or drivers from implementing `ClockSource`
> for arbitrary types, which could otherwise leads to unsupported
> behavior.
> 

Hmm.. I don't think other impl of `ClockSource` is a problem, IIUC, as
long as the ktime_get() can return a value in [0, i64::MAX). Also this
means ClockSource should be an `unsafe` trait, because the correct
implementaion relies on ktime_get() returns the correct value. This is
needed even if you sealed ClockSource trait.

Could you drop this and fix that the ClockSource trait instead? Thanks!

Regards,
Boqun

> Introduce a `private::Sealed` trait and implement it for all types
> that implement `ClockSource`.
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
> ---
>  rust/kernel/time.rs | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index eaa6d9ab5737..b1961652c884 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -51,6 +51,15 @@ pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
>      unsafe { bindings::__msecs_to_jiffies(msecs) }
>  }
>  
> +mod private {
> +    pub trait Sealed {}
> +
> +    impl Sealed for super::Monotonic {}
> +    impl Sealed for super::RealTime {}
> +    impl Sealed for super::BootTime {}
> +    impl Sealed for super::Tai {}
> +}
> +
>  /// Trait for clock sources.
>  ///
>  /// Selection of the clock source depends on the use case. In some cases the usage of a
> @@ -58,7 +67,7 @@ pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
>  /// cases the user of the clock has to decide which clock is best suited for the
>  /// purpose. In most scenarios clock [`Monotonic`] is the best choice as it
>  /// provides a accurate monotonic notion of time (leap second smearing ignored).
> -pub trait ClockSource {
> +pub trait ClockSource: private::Sealed {
>      /// The kernel clock ID associated with this clock source.
>      ///
>      /// This constant corresponds to the C side `clockid_t` value.
> 
> base-commit: 994393295c89711531583f6de8f296a30b0d944a
> -- 
> 2.43.0
>