linux-kernel - Re: [PATCH v12 3/5] rust: add bitmap API.

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Message-ID: <CACQBu=UHdyxiqngiqE-5KPVyn4VFmCDapKiy2W27ic46uNO6sw@mail.gmail.com>
Date: Thu, 19 Jun 2025 11:49:06 +0200
From: Burak Emir <bqe@...gle.com>
To: Alice Ryhl <aliceryhl@...gle.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>, 
	Boqun Feng <boqun.feng@...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>, 
	Trevor Gross <tmgross@...ch.edu>, "Gustavo A . R . Silva" <gustavoars@...nel.org>, 
	Carlos LLama <cmllamas@...gle.com>, Pekka Ristola <pekkarr@...tonmail.com>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v12 3/5] rust: add bitmap API.

On Mon, Jun 16, 2025 at 12:49 PM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> On Wed, Jun 11, 2025 at 9:48 PM Burak Emir <bqe@...gle.com> wrote:
> > +impl core::ops::DerefMut for Bitmap {
> > +    fn deref_mut(&mut self) -> &mut CBitmap {
> > +        let ptr = if self.nbits <= bindings::BITS_PER_LONG as _ {
> > +            // SAFETY: Bitmap is represented inline.
> > +            unsafe { core::ptr::addr_of_mut!(self.repr.bitmap) }
> > +        } else {
> > +            // SAFETY: Bitmap is represented as array of `unsigned long`.
> > +            unsafe { self.repr.ptr.as_mut() }
>
> Nit: You want NonNull::as_mut_ptr() here.

Can you explain? That seems to be an unstable method that exists so
one can get pointer to slice buffer.

The repr.ptr case is NonNull<usize>, not a slice - though "morally",
it is actually an owned C array.
Are you suggesting we could/should represent it as a Rust one?
However, we'd like it to use the C API to free etc.

Thanks,
Burak