linux-kernel - Re: [PATCH v2 1/7] rust: file: add Rust abstraction for `struct file`

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Message-ID: <9q-gcPBFqAZ1mAEZ333ax8Y16e8foTXUWsMijcJyvMhBVu91g4cBo3xRVXVFJeMUW3_67bCukA-bfAzpCwXdbHqwEdciNa8UJBJaCL2q2nw=@proton.me>
Date:   Fri, 08 Dec 2023 09:48:30 +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 v2 1/7] rust: file: add Rust abstraction for `struct file`

On 12/6/23 12:59, Alice Ryhl wrote:
> +impl File {
> +    /// Constructs a new `struct file` wrapper from a file descriptor.
> +    ///
> +    /// The file descriptor belongs to the current process.
> +    pub fn fget(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)?;
> +
> +        // SAFETY: `fget` either returns null or a valid pointer to a file, and we checked for null
> +        // above.

Since now both the Rust and C functions are called `fget`, I think you
should refer to `bindings::fget`.

> +        //
> +        // INVARIANT: `fget` increments the refcount before returning.
> +        Ok(unsafe { ARef::from_raw(ptr.cast()) })
> +    }

[...]

> +// 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.as_ptr()) };
> +    }
> +
> +    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()) }

The comment should also justify the cast.

-- 
Cheers,
Benno

> +    }
> +}