linux-kernel - Re: [PATCH] rust: pin-init: add references to previously initialized fields

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Message-ID: <20251203220516.GA592465@robin.jannau.net>
Date: Wed, 3 Dec 2025 23:05:16 +0100
From: Janne Grunau <j@...nau.net>
To: Benno Lossin <lossin@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>, Fiona Behrens <me@...enk.dev>,
	Alban Kurti <kurti@...icto.ai>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, asahi@...ts.linux.de
Subject: Re: [PATCH] rust: pin-init: add references to previously initialized
 fields

On Fri, Sep 05, 2025 at 04:00:46PM +0200, Benno Lossin wrote:
> After initializing a field in an initializer macro, create a variable
> holding a reference that points at that field. The type is either
> `Pin<&mut T>` or `&mut T` depending on the field's structural pinning
> kind.

just as a heads up: creating references broke part of the agx firmware
init structs which uses a `#[repr(C, packed)]` struct as field in
another struct. This fails with

| error[E0793]: reference to packed field is unaligned
|    --> ../drivers/gpu/drm/asahi/initdata.rs:722:28
|     |
| 722 |                       sub <- try_init!(raw::GlobalsSub::ver {
|     |  ____________________________^
| 723 | |                         unk_54: cfg.global_unk_54,
| 724 | |                         unk_56: 40,
| 725 | |                         unk_58: 0xffff,
| ...   |
| 731 | |                         ..Zeroable::init_zeroed()
| 732 | |                     }),
|     | |______________________^
|     |
|     = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
|     = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
|     = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
|     = note: this error originates in the macro `$crate::__init_internal` which comes from the expansion of the macro `try_init` (in Nightly builds, run with -Z macro-backtrace for more info)

This was easy enough to work around, I don't see how this embedding of a
`#[repr(C, packed)]` struct was necessary or at least helpful. The code
is not expected to be included in the upstream driver so it no worth
spending effort on this.

I don't think it's likely that anyone else will run into this but I
thought I mention it at least.

The asahi driver also ran into the discussed variable shadowing issue (a
variable was used to initialize a field of the same name and was later
used to initialize another field). This was trivially fixed by renaming
the variable.

Janne