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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAJ-ks9=dgQE_UF--Kv0HVTF_d1JzqK1gMoeE8GO8EGVtM-yt_Q@mail.gmail.com>
Date: Wed, 26 Mar 2025 16:47:20 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
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 3:06 PM Tamir Duberstein <tamird@...il.com> 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.
>
> > 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?
>
> >
> > [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.

Turns out there's a tortured past even in the standard library. In
2017 someone replaces trasmutes with pointer casts:

https://github.com/rust-lang/rust/commit/2633b85ab2c89822d2c227fc9e81c6ec1c0ed9b6

In 2020 someone changes it back to transmute:

https://github.com/rust-lang/rust/pull/75157/files

See also https://github.com/rust-lang/rust/pull/34609#issuecomment-230559871
which makes my point better than I have, particularly this snippet:
"In addition, casting through raw pointers removes the check that both
types have the same size that transmute does provide.".