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

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Message-ID: <87jzdopthc.fsf@kernel.org>
Date: Thu, 31 Oct 2024 09:27:43 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Tamir Duberstein" <tamird@...il.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>,  "Alex Gaynor"
 <alex.gaynor@...il.com>,  "Boqun Feng" <boqun.feng@...il.com>,  "Gary Guo"
 <gary@...yguo.net>,  =?utf-8?Q?Bj=C3=B6rn?=
 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`

"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.

>
>          // 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.

> +
>          // 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