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

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Message-ID: <20231201113204.2201389-1-aliceryhl@google.com>
Date:   Fri,  1 Dec 2023 11:32:04 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     benno.lossin@...ton.me
Cc:     a.hindborg@...sung.com, alex.gaynor@...il.com,
        aliceryhl@...gle.com, arve@...roid.com, bjorn3_gh@...tonmail.com,
        boqun.feng@...il.com, brauner@...nel.org, cmllamas@...gle.com,
        dan.j.williams@...el.com, dxu@...uu.xyz, gary@...yguo.net,
        gregkh@...uxfoundation.org, joel@...lfernandes.org,
        keescook@...omium.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, maco@...roid.com, ojeda@...nel.org,
        peterz@...radead.org, rust-for-linux@...r.kernel.org,
        surenb@...gle.com, tglx@...utronix.de, tkjos@...roid.com,
        viro@...iv.linux.org.uk, wedsonaf@...il.com, willy@...radead.org
Subject: Re: [PATCH 4/7] rust: file: add `FileDescriptorReservation`

Benno Lossin <benno.lossin@...ton.me> writes:
> 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.

I'll look into that for the next version.

>> +    /// 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.

That makes sense to me, but I don't think it needs to happen in this patchset.

Alice