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Message-ID: <87v7n27yw6.fsf@kernel.org>
Date: Tue, 05 Aug 2025 14:10:01 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>, Greg Kroah-Hartman
<gregkh@...uxfoundation.org>, Alexander Viro <viro@...iv.linux.org.uk>,
Arnd Bergmann <arnd@...db.de>, Miguel Ojeda <ojeda@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
Björn
Roy Baron <bjorn3_gh@...tonmail.com>, Trevor Gross <tmgross@...ch.edu>,
Danilo Krummrich <dakr@...nel.org>, Matthew Maurer <mmaurer@...gle.com>,
Lee Jones <lee@...nel.org>, linux-kernel@...r.kernel.org,
rust-for-linux@...r.kernel.org, Alice
Ryhl <aliceryhl@...gle.com>, Benno Lossin <lossin@...nel.org>, Christian
Brauner <brauner@...nel.org>
Subject: Re: [PATCH v3 3/4] rust: miscdevice: Provide additional
abstractions for iov_iter and kiocb structures
"Alice Ryhl" <aliceryhl@...gle.com> writes:
> These will be used for the read_iter() and write_iter() callbacks, which
> are now the preferred back-ends for when a user operates on a char device
> with read() and write() respectively.
>
> Cc: Christian Brauner <brauner@...nel.org>
> Co-developed-by: Lee Jones <lee@...nel.org>
> Signed-off-by: Lee Jones <lee@...nel.org>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
> rust/kernel/fs.rs | 3 +++
> rust/kernel/fs/kiocb.rs | 67 +++++++++++++++++++++++++++++++++++++++++++++++
> rust/kernel/miscdevice.rs | 63 +++++++++++++++++++++++++++++++++++++++++++-
> 3 files changed, 132 insertions(+), 1 deletion(-)
>
> 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..837f4be7cb8fbca6e3f9aeff74d1c904df3ff7ff
> --- /dev/null
> +++ b/rust/kernel/fs/kiocb.rs
> @@ -0,0 +1,67 @@
> +// 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 private data of the underlying file.
In my opinion, this paragraph could use some clarification:
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 Rust data stored in the private data of the file.
I would suggest:
Get the filesystem or driver specific data associated with the file.
> + pub fn file(&self) -> <T as ForeignOwnable>::Borrowed<'a> {
> + // SAFETY: The `kiocb` lets us access the private data.
This safety comment is strange. I like what you did in patch 1:
++ // SAFETY: We have shared access to this IO vector, so we can read its `count` field.
> + 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: The `kiocb` can access `ki_pos`.
Same.
> + 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: The `kiocb` can access `ki_pos`.
Same.
Best regards,
Andreas Hindborg
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