linux-kernel - Re: [PATCH v9 0/5] rust: adds Bitmap API, ID pool and bindings

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Message-ID: <aDXPhs79tQGTNU9b@yury>
Date: Tue, 27 May 2025 10:43:18 -0400
From: Yury Norov <yury.norov@...il.com>
To: Burak Emir <bqe@...gle.com>
Cc: 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>,
	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 0/5] rust: adds Bitmap API, ID pool and bindings

On Mon, May 26, 2025 at 03:01:29PM +0000, Burak Emir wrote:
> This series adds a Rust bitmap API for porting the approach from
> commit 15d9da3f818c ("binder: use bitmap for faster descriptor lookup")
> to Rust. The functionality in dbitmap.h makes use of bitmap and bitops.
> 
> The Rust bitmap API provides a safe abstraction to underlying bitmap
> and bitops operations. For now, only includes method necessary for
> dbitmap.h, more can be added later. We perform bounds checks for
> hardening, violations are programmer errors that result in panics.
> 
> We include set_bit_atomic and clear_bit_atomic operations. One has
> to avoid races with non-atomic operations, which is ensure by the
> Rust type system: either callers have shared references &bitmap in
> which case the mutations are atomic operations. Or there is a
> exclusive reference &mut bitmap, in which case there is no concurrent
> access.

Here I'm lost. In the other email you say:

> You also commented on the API. The weirdness of the API is all due to
> the separating "request to shrink/grow" from allocation.
> Since allocation can happen while other threads may mess with the id
> pool, one has to double check that the request to shrink/grow still
> makes sense.

And here you say:

> there is a
> exclusive reference &mut bitmap, in which case there is no concurrent
> access

So to me it sounds like if I want to resize, I just allocate a new bitmap,
take this exclusive reference, copy IDs, swap the pointers in
corresponding class, and that's it. What did I miss?