linux-hardening - Re: [PATCH v12 4/5] rust: add find_bit_benchmark

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Message-ID: <aEqWvIffNhh6Kzp8@google.com>
Date: Thu, 12 Jun 2025 08:58:36 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Burak Emir <bqe@...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 4/5] rust: add find_bit_benchmark_rust module.

On Wed, Jun 11, 2025 at 07:48:37PM +0000, Burak Emir wrote:
> +    let mut bitmap = Bitmap::new(BITMAP_LEN, GFP_KERNEL).expect("alloc sparse bitmap failed");
> +    let nbits = BITMAP_LEN / SPARSENESS;
> +    for _i in 0..nbits {
> +        // SAFETY: BITMAP_LEN fits in 32 bits.
> +        let bit: usize =
> +            unsafe { bindings::__get_random_u32_below(BITMAP_LEN.try_into().unwrap()) as _ };

This safety comment argues why the .unwrap() will not result in a panic,
but it should instead argue why the call to __get_random_u32_below() is
okay. I guess that it's because __get_random_u32_below() is always safe
to call?

I'm not a big fan of these .try_into().unwrap() conversions. I would
probably just have written `BITMAP_LEN as u32`. But I know that this is
a point of disagreement with other Rust maintainers.

Alice