linux-kernel - Re: [PATCH v4 3/4] rust: miscdevice: Provide additional abstractions for iov

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Message-Id: <DC19RGJ4EPSK.1QRTOMYUKY34@kernel.org>
Date: Wed, 13 Aug 2025 13:39:12 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>, "Alexander Viro"
 <viro@...iv.linux.org.uk>, "Arnd Bergmann" <arnd@...db.de>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Trevor Gross" <tmgross@...ch.edu>, "Matthew Maurer" <mmaurer@...gle.com>,
 "Lee Jones" <lee@...nel.org>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, "Benno Lossin" <lossin@...nel.org>,
 "Christian Brauner" <brauner@...nel.org>
Subject: Re: [PATCH v4 3/4] rust: miscdevice: Provide additional
 abstractions for iov_iter and kiocb structures

On Wed Aug 13, 2025 at 10:27 AM CEST, Alice Ryhl wrote:
> diff --git a/rust/kernel/fs.rs b/rust/kernel/fs.rs
> index 0121b38c59e63d01a89f22c8ef6983ef5c3234de..6ba6bdf143cb991c6e78215178eb585260215da0 100644
> --- a/rust/kernel/fs.rs
> +++ b/rust/kernel/fs.rs
> @@ -6,3 +6,6 @@
>  
>  pub mod file;
>  pub use self::file::{File, LocalFile};
> +
> +mod kiocb;
> +pub use self::kiocb::Kiocb;
> diff --git a/rust/kernel/fs/kiocb.rs b/rust/kernel/fs/kiocb.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..84c936cd69b0e9b490d54c87d8c7279b27d4476a
> --- /dev/null
> +++ b/rust/kernel/fs/kiocb.rs
> @@ -0,0 +1,68 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2024 Google LLC.
> +
> +//! Kernel IO callbacks.
> +//!
> +//! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h)
> +
> +use core::marker::PhantomData;
> +use core::ptr::NonNull;
> +use kernel::types::ForeignOwnable;
> +
> +/// Wrapper for the kernel's `struct kiocb`.
> +///
> +/// Currently this abstractions is incomplete and is essentially just a tuple containing a
> +/// reference to a file and a file position.
> +///
> +/// The type `T` represents the filesystem or driver specific data associated with the file.
> +///
> +/// # Invariants
> +///
> +/// `inner` points at a valid `struct kiocb` whose file has the type `T` as its private data.
> +pub struct Kiocb<'a, T> {
> +    inner: NonNull<bindings::kiocb>,
> +    _phantom: PhantomData<&'a T>,
> +}
> +
> +impl<'a, T: ForeignOwnable> Kiocb<'a, T> {
> +    /// Create a `Kiocb` from a raw pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The pointer must reference a valid `struct kiocb` for the duration of `'a`. The private
> +    /// data of the file must be `T`.
> +    pub unsafe fn from_raw(kiocb: *mut bindings::kiocb) -> Self {
> +        Self {
> +            // SAFETY: If a pointer is valid it is not null.
> +            inner: unsafe { NonNull::new_unchecked(kiocb) },
> +            _phantom: PhantomData,
> +        }
> +    }
> +
> +    /// Access the underlying `struct kiocb` directly.
> +    pub fn as_raw(&self) -> *mut bindings::kiocb {
> +        self.inner.as_ptr()
> +    }
> +
> +    /// Get the filesystem or driver specific data associated with the file.
> +    pub fn file(&self) -> <T as ForeignOwnable>::Borrowed<'a> {
> +        // SAFETY: We have shared access to this kiocb and hence the underlying file, so we can
> +        // read the file's private data.
> +        let private = unsafe { (*(*self.as_raw()).ki_filp).private_data };
> +        // SAFETY: The kiocb has shared access to the private data.
> +        unsafe { <T as ForeignOwnable>::borrow(private) }
> +    }
> +
> +    /// Gets the current value of `ki_pos`.
> +    pub fn ki_pos(&self) -> i64 {
> +        // SAFETY: We have shared access to the kiocb, so we can read its `ki_pos` field.
> +        unsafe { (*self.as_raw()).ki_pos }
> +    }
> +
> +    /// Gets a mutable reference to the `ki_pos` field.
> +    pub fn ki_pos_mut(&mut self) -> &mut i64 {
> +        // SAFETY: We have exclusive access to the kiocb, so we can write to `ki_pos`.
> +        unsafe { &mut (*self.as_raw()).ki_pos }
> +    }
> +}

I think this should be a separate commit.