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From: Alice Ryhl <aliceryhl@google.com>
Date: Mon, 12 Feb 2024 10:56:43 +0100
Message-ID: <CAH5fLgjP98pS1wsP=YXP5Yr79Y62VF7XPKjbj+G75B3SOFt80g@mail.gmail.com>
Subject: Re: [PATCH 1/6] rust: add `container_of!` macro
To: Trevor Gross <tmgross@umich.edu>
Cc: mattgilbride@google.com, 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>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Benno Lossin <benno.lossin@proton.me>, Andreas Hindborg <a.hindborg@samsung.com>, 
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>, =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>, 
	Todd Kjos <tkjos@android.com>, Martijn Coenen <maco@android.com>, 
	Joel Fernandes <joel@joelfernandes.org>, Carlos Llamas <cmllamas@google.com>, 
	Suren Baghdasaryan <surenb@google.com>, Christian Brauner <brauner@kernel.org>, Rob Landley <rob@landley.net>, 
	Davidlohr Bueso <dave@stgolabs.net>, Michel Lespinasse <michel@lespinasse.org>, 
	rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org
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On Sat, Feb 10, 2024 at 9:05=E2=80=AFAM Trevor Gross <tmgross@umich.edu> wr=
ote:
>
> On Mon, Feb 5, 2024 at 9:50=E2=80=AFAM <mattgilbride@google.com> wrote:
> > +macro_rules! container_of {
> > +    ($ptr:expr, $type:ty, $($f:tt)*) =3D> {{
> > +        let ptr =3D $ptr as *const _ as *const u8;
> > +        let offset: usize =3D ::core::mem::offset_of!($type, $($f)*);
>
> `offset_of` will be stable in 1.77 BUT only for a single field [1]. I
> don't know if there are other blockers in the kernel already, but if
> this could be changed to call `offset_of!` recursively then  it will
> work with the stable version.
>
> We might want an `offset_of_many!(a, b, c)` macro somewhere if there
> are other places that need this nesting.
>
> [1]: https://github.com/rust-lang/rust/pull/118799

Rust Binder *does* need that in one place. Creating a nested
offset_of! is kind of tricky, because you have to fail to compile when
you're following a pointer with the Deref trait. I haven't figured out
how to do that yet.

> > +        ptr.sub(offset) as *const $type
>
> Instead of casting to and from `u8`, you should be able to use `byte_sub`

Casting to and from u8 also has the side-effect of making it not work
when the target type is !Sized. Not allowing this might be desirable.

Alice