linux-kernel - Re: [PATCH 1/6] rust: add `container

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Date: Mon, 12 Feb 2024 10:56:43 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Trevor Gross <tmgross@...ch.edu>
Cc: mattgilbride@...gle.com, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Arve Hjønnevåg <arve@...roid.com>, 
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Christian Brauner <brauner@...nel.org>, Rob Landley <rob@...dley.net>, 
	Davidlohr Bueso <dave@...olabs.net>, Michel Lespinasse <michel@...pinasse.org>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] rust: add `container_of!` macro

On Sat, Feb 10, 2024 at 9:05 AM Trevor Gross <tmgross@...ch.edu> wrote:
>
> On Mon, Feb 5, 2024 at 9:50 AM <mattgilbride@...gle.com> wrote:
> > +macro_rules! container_of {
> > +    ($ptr:expr, $type:ty, $($f:tt)*) => {{
> > +        let ptr = $ptr as *const _ as *const u8;
> > +        let offset: usize = ::core::mem::offset_of!($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