linux-kernel - Re: [PATCH 1/5] rust: arc: use `NonNull::new

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Message-ID: <CAJ-ks9=ph2ejGEyi_nS9vfA662xQS-yFFxT3t2yDkGaV9AqYGw@mail.gmail.com>
Date: Thu, 31 Oct 2024 07:50:08 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Miguel Ojeda <ojeda@...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>, Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] rust: arc: use `NonNull::new_unchecked`

On Thu, Oct 31, 2024 at 4:50 AM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> "Tamir Duberstein" <tamird@...il.com> writes:
>
> > There is no need to check (and panic on violations of) the safety
> > requirements on `ForeignOwnable` functions. Avoiding the check is
> > consistent with the implementation of `ForeignOwnable` for `Box`.
> >
> > Signed-off-by: Tamir Duberstein <tamird@...il.com>
> > ---
> >  rust/kernel/sync/arc.rs | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> > index db9da352d588f65348aa7a5204abbb165b70197f..4857230bd8d410bcca97b2081c3ce2f617ee7921 100644
> > --- a/rust/kernel/sync/arc.rs
> > +++ b/rust/kernel/sync/arc.rs
> > @@ -337,9 +337,9 @@ fn into_foreign(self) -> *const core::ffi::c_void {
> >      }
> >
> >      unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
> > -        // By the safety requirement of this function, we know that `ptr` came from
> > -        // a previous call to `Arc::into_foreign`.
> > -        let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
> > +        // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
> > +        // call to `Self::into_foreign`.
> > +        let inner = unsafe { NonNull::new_unchecked(ptr as _) };
>
> Please use an explicit cast.

This changes to .cast() in a subsequent patch.

>
> >
> >          // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
> >          // for the lifetime of the returned value.
> > @@ -347,10 +347,14 @@ unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
> >      }
> >
> >      unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
> > +        // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
> > +        // call to `Self::into_foreign`.
> > +        let inner = unsafe { NonNull::new_unchecked(ptr as _) };
>
> Please use an explicit cast.

This changes in a subsequent patch, here it is matching the prevailing
convention. I will restore the type ascription in v2.


>
> > +
> >          // SAFETY: By the safety requirement of this function, we know that `ptr` came from
> >          // a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
> >          // holds a reference count increment that is transferrable to us.
> > -        unsafe { Self::from_inner(NonNull::new(ptr as _).unwrap()) }
> > +        unsafe { Self::from_inner(inner) }
> >      }
> >  }
>
> Otherwise lgtm.
>
> Best regards,
> Andreas
>