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Date:   Fri, 08 Dec 2023 09:48:30 +0000
To:     Alice Ryhl <aliceryhl@google.com>
From:   Benno Lossin <benno.lossin@proton.me>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
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        Peter Zijlstra <peterz@infradead.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Matthew Wilcox <willy@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Xu <dxu@dxuuu.xyz>, linux-kernel@vger.kernel.org,
        rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 1/7] rust: file: add Rust abstraction for `struct file`
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References: <20231206-alice-file-v2-0-af617c0d9d94@google.com> <20231206-alice-file-v2-1-af617c0d9d94@google.com>
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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 =3D ptr::NonNull::new(unsafe { bindings::fget(fd) }).ok_=
or(BadFdError)?;
> +
> +        // SAFETY: `fget` either returns null or a valid pointer to a fi=
le, 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-count=
ed.
> +unsafe impl AlwaysRefCounted for File {
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference means that the re=
fcount 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 i=
s nonzero.
> +        unsafe { bindings::fput(obj.cast().as_ptr()) }

The comment should also justify the cast.

--=20
Cheers,
Benno

> +    }
> +}