linux-kernel - Re: [PATCH 06/10] rust: xarray: simplify `Guard::load`

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Message-ID: <CAJ-ks9=VO1nypKFtOaGWxOUqzzJ7-Wb5qLd9pzQuFCSfk8PkZA@mail.gmail.com>
Date: Mon, 29 Dec 2025 11:07:56 -0500
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 <lossin@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Daniel Gomez <da.gomez@...nel.org>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/10] rust: xarray: simplify `Guard::load`

On Wed, Dec 3, 2025 at 5:27 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> Simplify the implementation by removing the closure-based API from
> `Guard::load` in favor of returning `Option<NonNull<c_void>>` directly.

This is not sound. The returned pointer can now outlive the guard and
mutation through that pointer is trivial.

>
> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> ---
>  rust/kernel/xarray.rs | 23 +++++++++--------------
>  1 file changed, 9 insertions(+), 14 deletions(-)
>
> diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
> index 0f69a523b72bf..ca97134ba2bd0 100644
> --- a/rust/kernel/xarray.rs
> +++ b/rust/kernel/xarray.rs
> @@ -211,16 +211,12 @@ fn from(value: StoreError<T>) -> Self {
>  }
>
>  impl<'a, T: ForeignOwnable> Guard<'a, T> {
> -    fn load<F, U>(&self, index: usize, f: F) -> Option<U>
> -    where
> -        F: FnOnce(NonNull<c_void>) -> U,
> -    {
> +    fn load(&self, index: usize) -> Option<NonNull<c_void>> {
>          let mut state = XArrayState::new(self, index);
>          // SAFETY: `state.state` is always valid by the type invariant of
>          // `XArrayState`.
>          let ptr = unsafe { bindings::xas_load(&raw mut state.state) };
> -        let ptr = NonNull::new(ptr.cast())?;
> -        Some(f(ptr))
> +        NonNull::new(ptr)
>      }
>
>      /// Checks if the XArray contains an element at the specified index.
> @@ -246,18 +242,17 @@ pub fn contains_index(&self, index: usize) -> bool {
>
>      /// Provides a reference to the element at the given index.
>      pub fn get(&self, index: usize) -> Option<T::Borrowed<'_>> {
> -        self.load(index, |ptr| {
> -            // SAFETY: `ptr` came from `T::into_foreign`.
> -            unsafe { T::borrow(ptr.as_ptr()) }
> -        })
> +        let ptr = self.load(index)?;
> +        // SAFETY: `ptr` came from `T::into_foreign`.
> +        Some(unsafe { T::borrow(ptr.as_ptr()) })
>      }
>
>      /// Provides a mutable reference to the element at the given index.
>      pub fn get_mut(&mut self, index: usize) -> Option<T::BorrowedMut<'_>> {
> -        self.load(index, |ptr| {
> -            // SAFETY: `ptr` came from `T::into_foreign`.
> -            unsafe { T::borrow_mut(ptr.as_ptr()) }
> -        })
> +        let ptr = self.load(index)?;
> +
> +        // SAFETY: `ptr` came from `T::into_foreign`.
> +        Some(unsafe { T::borrow_mut(ptr.as_ptr()) })
>      }
>
>      /// Removes and returns the element at the given index.
>
> --
> 2.51.2
>
>