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

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Date:   Wed, 29 Nov 2023 18:14:24 +0100
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     brauner@...nel.org
Cc:     a.hindborg@...sung.com, alex.gaynor@...il.com, arve@...roid.com,
        benno.lossin@...ton.me, bjorn3_gh@...tonmail.com,
        boqun.feng@...il.com, 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`

On Wed, Nov 29, 2023 at 5:55 PM Alice Ryhl <aliceryhl@...gle.com> wrote:

> >> +    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()) };
> >
> > Why file.0.get()? Where did that come from?
>
> This gets a raw pointer to the C type.
>
> The `.0` part is a field access. `ARef` struct is a tuple struct, so its
> fields are unnamed. However, the fields can still be accessed by index.

Oh, sorry, this is wrong. Let me try again:

This gets a raw pointer to the C type. The `.0` part accesses the
field of type `Opaque<bindings::file>` in the Rust wrapper. Recall
that File is defined like this:

pub struct File(Opaque<bindings::file>);

The above syntax defines a tuple struct, which means that the fields
are unnamed. The `.0` syntax accesses the first field of a tuple
struct [1].

The `.get()` method is from the `Opaque` struct, which returns a raw
pointer to the C type being wrapped.

Alice

[1]: https://doc.rust-lang.org/std/keyword.struct.html#:~:text=Tuple%20structs%20are%20similar%20to,with%20regular%20tuples%2C%20namely%20foo.