linux-kernel - Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87bjutdefk.fsf@kernel.org>
Date: Sat, 22 Feb 2025 19:25:51 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Tamir Duberstein" <tamird@...il.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>,  "Anna-Maria Behnsen"
 <anna-maria@...utronix.de>,  "Frederic Weisbecker" <frederic@...nel.org>,
  "Thomas Gleixner" <tglx@...utronix.de>,  "Danilo Krummrich"
 <dakr@...nel.org>,  "Alex Gaynor" <alex.gaynor@...il.com>,  "Boqun Feng"
 <boqun.feng@...il.com>,  "Gary Guo" <gary@...yguo.net>,  Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,  "Benno Lossin" <benno.lossin@...ton.me>,
  "Alice Ryhl" <aliceryhl@...gle.com>,  "Trevor Gross" <tmgross@...ch.edu>,
  "Lyude Paul" <lyude@...hat.com>,  "Guangbo Cui" <2407018371@...com>,
  "Dirk Behme" <dirk.behme@...il.com>,  "Daniel Almeida"
 <daniel.almeida@...labora.com>,  <rust-for-linux@...r.kernel.org>,
  <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

"Tamir Duberstein" <tamird@...il.com> writes:

> On Fri, Feb 21, 2025 at 8:29 AM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>
>> Andreas Hindborg <a.hindborg@...nel.org> writes:
>>
>> > "Tamir Duberstein" <tamird@...il.com> writes:
>> >
>> >> On Thu, Feb 20, 2025 at 4:19 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>> >>>
>> >>> "Tamir Duberstein" <tamird@...il.com> writes:
>> >>>
>> >>> > On Tue, Feb 18, 2025 at 8:29 AM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>
>> [...]
>>
>> >>> >> +    /// Get a pointer to the contained `bindings::hrtimer`.
>> >>> >> +    ///
>> >>> >> +    /// # Safety
>> >>> >> +    ///
>> >>> >> +    /// `ptr` must point to a live allocation of at least the size of `Self`.
>> >>> >> +    unsafe fn raw_get(ptr: *const Self) -> *mut bindings::hrtimer {
>> >>> >> +        // SAFETY: The field projection to `timer` does not go out of bounds,
>> >>> >> +        // because the caller of this function promises that `ptr` points to an
>> >>> >> +        // allocation of at least the size of `Self`.
>> >>> >> +        unsafe { Opaque::raw_get(core::ptr::addr_of!((*ptr).timer)) }
>> >>> >> +    }
>> >>> >
>> >>> > Can you help me understand why the various functions here operate on
>> >>> > *const Self? I understand the need to obtain a C pointer to interact
>> >>> > with bindings, but I don't understand why we're dealing in raw
>> >>> > pointers to the abstraction rather than references.
>> >>>
>> >>> We cannot reference the `bindings::hrtimer` without wrapping it in
>> >>> `Opaque`. This would be the primary reason. At other times, we cannot
>> >>> produce references because we might not be able to prove that we satisfy
>> >>> the safety requirements for turning a pointer into a reference. If we
>> >>> are just doing offset arithmetic anyway, we don't need a reference.
>> >>
>> >> Why do we have a pointer, rather than a reference, to Self in the
>> >> first place? I think this is the key thing I don't understand.
>> >
>> > Perhaps it makes more sense if you look at the context. One of the entry
>> > points to `HrTimer::raw_get` is via `<ArcHrTimerHandle as
>> > HrTimerHandle>::cancel`. This user facing method takes `&mut self`. The
>> > handle contains an arc to a type that contains a `Timer` and implements
>> > `HasHrTImer`. To get to the timer, we need to do pointer manipulation.
>> > We only know how to get the `Timer` field via the `OFFSET`. The natural
>> > return value from the offset operation is a raw pointer. Rather than
>> > convert back to a reference, we stay in pointer land when we call
>> > `HrTimer::raw_cancel`, because we need a pointer to the
>> > `bindings::hrtimer` anyway, not a reference.
>>
>> I changed `HasHrTimer::start` to take a reference, and I think that
>> makes sense 👍 Taking an `impl AsRef` does not work out when `Self` is
>> `Pin<&T>`. I'll go over the whole thing and see of other places could
>> benefit.
>
> Can you elaborate please? Pin<&T>::get_ref returns &T, which gets you
> to the AsRef you need, no?

Right, that works. But if I pass in &self it's not really needed. You
can see for yourself in next version.

Hopefully I did not break any provenance rules.


Best regards,
Andreas Hindborg