linux-kernel - Re: [PATCH 1/7] rust: file: add Rust abstraction for `struct file`

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Date:   Thu, 30 Nov 2023 14:53:35 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org,
        rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/7] rust: file: add Rust abstraction for `struct file`

On 11/29/23 13:51, Alice Ryhl wrote:
> +/// Flags associated with a [`File`].
> +pub mod flags {
> +    /// File is opened in append mode.
> +    pub const O_APPEND: u32 = bindings::O_APPEND;

Why do all of these constants begin with `O_`?

[...]

> +impl File {
> +    /// Constructs a new `struct file` wrapper from a file descriptor.
> +    ///
> +    /// The file descriptor belongs to the current process.
> +    pub fn from_fd(fd: u32) -> Result<ARef<Self>, BadFdError> {
> +        // SAFETY: FFI call, there are no requirements on `fd`.
> +        let ptr = ptr::NonNull::new(unsafe { bindings::fget(fd) }).ok_or(BadFdError)?;
> +
> +        // INVARIANT: `fget` increments the refcount before returning.
> +        Ok(unsafe { ARef::from_raw(ptr.cast()) })

Missing `SAFETY` comment.

> +    }
> +
> +    /// Creates a reference to a [`File`] from a valid pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that `ptr` points at a valid file and that its refcount does not
> +    /// reach zero during the lifetime 'a.
> +    pub unsafe fn from_ptr<'a>(ptr: *const bindings::file) -> &'a File {
> +        // INVARIANT: The safety requirements guarantee that the refcount does not hit zero during
> +        // 'a. The cast is okay because `File` is `repr(transparent)`.
> +        unsafe { &*ptr.cast() }

Missing `SAFETY` comment.

> +    }
> +
> +    /// Returns the flags associated with the file.
> +    ///
> +    /// The flags are a combination of the constants in [`flags`].
> +    pub fn flags(&self) -> u32 {
> +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
> +        //
> +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> +        //
> +        // TODO: Replace with `read_once` when available on the Rust side.
> +        unsafe { core::ptr::addr_of!((*self.0.get()).f_flags).read_volatile() }
> +    }
> +}
> +
> +// SAFETY: The type invariants guarantee that `File` is always ref-counted.
> +unsafe impl AlwaysRefCounted for File {
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> +        unsafe { bindings::get_file(self.0.get()) };
> +    }
> +
> +    unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
> +        // SAFETY: The safety requirements guarantee that the refcount is nonzero.
> +        unsafe { bindings::fput(obj.cast().as_ptr()) }
> +    }
> +}
> +
> +/// Represents the `EBADF` error code.
> +///
> +/// Used for methods that can only fail with `EBADF`.
> +pub struct BadFdError;
> +
> +impl From<BadFdError> for Error {
> +    fn from(_: BadFdError) -> Error {
> +        EBADF
> +    }
> +}
> +
> +impl core::fmt::Debug for BadFdError {
> +    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
> +        f.pad("EBADF")
> +    }
> +}

Do we want to generalize this to the other errors as well? We could modify
the `declare_error!` macro in `error.rs` to create these unit structs.

-- 
Cheers,
Benno