linux-kernel - Re: [PATCH v7 7/7] rust: enable `clippy::ref_as

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Message-ID: <D8QI804Q3DAS.2BV4WSL81H52Z@proton.me>
Date: Wed, 26 Mar 2025 21:09:47 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Tamir Duberstein <tamird@...il.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, 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>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Brendan Higgins <brendan.higgins@...ux.dev>, David Gow <davidgow@...gle.com>, Rae Moar <rmoar@...gle.com>, Bjorn Helgaas <bhelgaas@...gle.com>, Luis Chamberlain <mcgrof@...nel.org>, Russ Weight <russ.weight@...ux.dev>, Rob Herring <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>, Abdiel Janulgue <abdiel.janulgue@...il.com>, Daniel Almeida <daniel.almeida@...labora.com>, Robin Murphy <robin.murphy@....com>, Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, FUJITA Tomonori <fujita.tomonori@...il.com>, linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com, linux-pci@...r.kernel.org, linux-block@...r.kernel.org, devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org, netdev@...r.kernel.org
Subject: Re: [PATCH v7 7/7] rust: enable `clippy::ref_as_ptr` lint

On Wed Mar 26, 2025 at 8:06 PM CET, Tamir Duberstein wrote:
> On Wed, Mar 26, 2025 at 1:36 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>> On Wed Mar 26, 2025 at 5:57 PM CET, Tamir Duberstein wrote:
>> > In the current code you're looking at, yes. But in the code I have
>> > locally I'm transmuting `[u8]` to `BStr`. See my earlier reply where I
>> > said "Hmm, looking at this again we can just transmute ref-to-ref and
>> > avoid pointers entirely. We're already doing that in
>> > `CStr::from_bytes_with_nul_unchecked`".
>>
>> `CStr::from_bytes_with_nul_unchecked` does the transmute with
>> references. That is a usage that the docs of `transmute` explicitly
>> recommend to change to an `as` cast [1].
>
> RIght. That guidance was written in 2016
> (https://github.com/rust-lang/rust/pull/34609) and doesn't present any
> rationale for `as` casts being preferred to transmute. I posted a
> comment in the most relevant issue I could find:
> https://github.com/rust-lang/rust/issues/34249#issuecomment-2755316610.

Not sure if that's the correct issue, maybe we should post one on the
UCG (unsafe code guidelines). But before that we probably should ask on
zulip...

>> No idea about provenance still.
>
> Well that's not surprising, nobody was thinking about provenance in
> 2016. But I really don't think we should blindly follow the advice in
> this case. It doesn't make an iota of sense to me - does it make sense
> to you?

For ptr-to-int transmutes, I know that they will probably remove
provenance, hence I am a bit cautious about using them for ptr-to-ptr or
ref-to-ref.

>> [1]: https://doc.rust-lang.org/std/mem/fn.transmute.html#alternatives
>>
>> >> I tried to find some existing issues about the topic and found that
>> >> there exists a clippy lint `transmute_ptr_to_ptr`. There is an issue
>> >> asking for a better justification [1] and it seems like nobody provided
>> >> one there. Maybe we should ask the opsem team what happens to provenance
>> >> when transmuting?
>> >
>> > Yeah, we should do this - but again: not relevant in this discussion.
>>
>> I think it's pretty relevant.
>
> It's not relevant because we're no longer talking about transmuting
> pointer to pointer. The two options are:
> 1. transmute reference to reference.
> 2. coerce reference to pointer, `as` cast pointer to pointer (triggers
> `ptr_as_ptr`), reborrow pointer to reference.
>
> If anyone can help me understand why (2) is better than (1), I'd
> certainly appreciate it.

I am very confident that (2) is correct. With (1) I'm not sure (see
above), so that's why I mentioned it.

---
Cheers,
Benno