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

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Date:   Thu, 30 Nov 2023 09:23:22 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     brauner@...nel.org
Cc:     a.hindborg@...sung.com, alex.gaynor@...il.com,
        aliceryhl@...gle.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`

Christian Brauner <brauner@...nel.org> writes:
> On Wed, Nov 29, 2023 at 06:14:24PM +0100, Alice Ryhl wrote:
> > 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.
> 
> It'd be nice if this could be written in a more obvious/elegant way. And
> if not a comment would help. I know there'll be more text then code but
> until this is second nature to read I personally won't mind... Because
> searching for this specific syntax isn't really possible.

Adding a comment to every instance of this is probably not realisitic.
This kind of code will be very common in abstraction code. However,
there are two other options that I think are reasonable:

1. I can change the definition of `File` so that the field has a name:

struct File {
    inner: Opaque<bindings::file>,
}

Then, it would say `file.inner.get()`.

2. Alternatively, I can add a method to file:

impl File {
    #[inline]
    pub fn as_ptr(&self) -> *mut bindings::file {
        self.0.get()
    }
}

And then write `file.as_ptr()` whenever I want a pointer.

Alice