Re: [PATCH v3 3/4] rust: miscdevice: Provide additional abstractions for iov_iter and kiocb structures

[Andreas Hindborg <a.hindborg@kernel.org>]

"Alice Ryhl" <aliceryhl@google.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@kernel.org>
> Co-developed-by: Lee Jones <lee@kernel.org>
> Signed-off-by: Lee Jones <lee@kernel.org>
> Signed-off-by: Alice Ryhl <aliceryhl@google.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