linux-kernel - Re: [PATCH 4/7] rust: file: add `FileDescriptorReservation`

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Date:   Thu, 30 Nov 2023 16:40:08 +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 4/7] rust: file: add `FileDescriptorReservation`

On 11/29/23 14:11, Alice Ryhl wrote:
> +impl FileDescriptorReservation {
> +    /// Creates a new file descriptor reservation.
> +    pub fn new(flags: u32) -> Result<Self> {
> +        // SAFETY: FFI call, there are no safety requirements on `flags`.
> +        let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
> +        if fd < 0 {
> +            return Err(Error::from_errno(fd));
> +        }

I think here we could also use the modified `to_result` function that
returns a `u32` if the value is non-negative.

> +        Ok(Self {
> +            fd: fd as _,
> +            _not_send_sync: PhantomData,
> +        })
> +    }
> +
> +    /// Returns the file descriptor number that was reserved.
> +    pub fn reserved_fd(&self) -> u32 {
> +        self.fd
> +    }
> +
> +    /// Commits the reservation.
> +    ///
> +    /// The previously reserved file descriptor is bound to `file`. This method consumes the
> +    /// [`FileDescriptorReservation`], so it will not be usable after this call.
> +    pub fn commit(self, file: ARef<File>) {
> +        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`, and `file.ptr` is
> +        // guaranteed to have an owned ref count by its type invariants.
> +        unsafe { bindings::fd_install(self.fd, file.0.get()) };
> +
> +        // `fd_install` consumes both the file descriptor and the file reference, so we cannot run
> +        // the destructors.
> +        core::mem::forget(self);
> +        core::mem::forget(file);

Would be useful to have an `ARef::into_raw` function that would do
the `forget` for us.

-- 
Cheers,
Benno