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

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Message-ID: <20251206170214.GE1097212@robin.jannau.net>
Date: Sat, 6 Dec 2025 18:02:14 +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.dev
Subject: Re: [PATCH] rust: pin-init: add references to previously initialized
 fields

On Sat, Dec 06, 2025 at 09:23:08AM +0100, Benno Lossin wrote:
> On Wed Dec 3, 2025 at 11:05 PM CET, Janne Grunau wrote:
> > 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.
> 
> Thanks for the report, I expected the latter kind of error, but was not
> aware of the packed struct issue. If anyone needs a proper workaround
> from pin-init, let me know.

I spoke too soon. The packed struct issue is also present in the capture
driver for Macbook microphones (aop_audio). Working around this issue
there is less obvious and more effort. I think it might be enough to use
unaligned u32 / u64 types already present in asahi [1].

I'm not sure how prevalent packed structs are outside of Apple's
firmware interfaces. I was surprised running into the same issue in a
second driver but I shouldn't have been. There are plans for another
driver where this isssue will be present.

A workaround on pin-init side would be appreciated. Due to the nature of
these packed structs I do not see a need to have access to previously
initialized fields. An optional way to supress the references would be
good enough for the cases I'm aware off.

Thanks

Janne

1: https://github.com/AsahiLinux/linux/blob/asahi-6.17.9-1/drivers/gpu/drm/asahi/fw/types.rs#L48