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

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Message-ID: <87cyb084df.fsf@kernel.org>
Date: Thu, 19 Jun 2025 11:31:08 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "FUJITA Tomonori" <fujita.tomonori@...il.com>
Cc: <boqun.feng@...il.com>,  <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

"FUJITA Tomonori" <fujita.tomonori@...il.com> writes:

> On Wed, 18 Jun 2025 21:13:07 +0200
> Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
>> "Boqun Feng" <boqun.feng@...il.com> writes:
>>
>>> On Tue, Jun 17, 2025 at 05:10:42PM -0700, Boqun Feng wrote:
>>>> 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!
>>>>
>>>
>>> For example:
>>>
>>>     /// Trait for clock sources.
>>>     ///
>>>     /// ...
>>>     /// # Safety
>>>     ///
>>>     /// Implementers must ensure `ktime_get()` return a value in [0,
>>>     //  KTIME_MAX (i.e. i64::MAX)).
>>>     pub unsafe trait ClockSource {
>>>         ...
>>>     }
>>
>> Nice catch, it definitely needs to be unsafe. We should also require
>> correlation between ID and the value fetched by `ktime_get`.
>
> What's ID?


  pub trait ClockSource {
      /// The kernel clock ID associated with this clock source.
      ///
      /// This constant corresponds to the C side `clockid_t` value.
      const ID: bindings::clockid_t;

The constant used to identify the clock source when calling into C APIs.


Best regards,
Andreas Hindborg