linux-kernel - Re: [PATCH v9 2/5] rust: add bindings for bitops.h

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Message-ID: <CACQBu=XJ+21amU2tEkDYFO00RWt8PvOXsU173ZO_y3u9xmWUHw@mail.gmail.com>
Date: Mon, 2 Jun 2025 15:11:48 +0200
From: Burak Emir <bqe@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Yury Norov <yury.norov@...il.com>, Kees Cook <kees@...nel.org>, 
	Rasmus Villemoes <linux@...musvillemoes.dk>, Viresh Kumar <viresh.kumar@...aro.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	"Gustavo A . R . Silva" <gustavoars@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v9 2/5] rust: add bindings for bitops.h

On Tue, May 27, 2025 at 6:39 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Mon, May 26, 2025 at 03:01:31PM +0000, Burak Emir wrote:
> > --- /dev/null
> > +++ b/rust/helpers/bitops.c
> > @@ -0,0 +1,23 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <linux/bitops.h>
> > +
> > +void rust_helper___set_bit(unsigned int nr, unsigned long *addr)
>
> Why "unsigned int" instead of "unsigned long"? The C API uses
> "unsigned long" and in the Rust API, you uses "usize" for `nbits` and
> `index`s, therefore using "unsigned int" only introduces unnecessary "as
> u32" casting IMO, am I missing something here?

This is certainly a bit confusing to me.

As Jann points out here [1] the expand to asm macros arch___set_bit,
where `nr` is indeed unsigned long.

There is also the constraint that a bitmap's length must be shorter
than INT32_MAX bits, but that does not matter.

Being consistent with the C API, including parameter names, is what we
should do, so fixing this.
I found the underlying C API is just hard to find for these macros.
Thanks for catching.

cheers
- Burak

[1] https://lore.kernel.org/rust-for-linux/CAG48ez1NM7B8Vk7GOwhsitCipmfHi9eK6JNb3ve8aR4m8Cj0gA@mail.gmail.com/