linux-kernel - Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics

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Message-Id: <DAUAW2Y0HYLY.3CDC9ZW0BUKI4@kernel.org>
Date: Tue, 24 Jun 2025 01:27:38 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <lkmm@...ts.linux.dev>, <linux-arch@...r.kernel.org>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>,
 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>,
 "Will Deacon" <will@...nel.org>, "Peter Zijlstra" <peterz@...radead.org>,
 "Mark Rutland" <mark.rutland@....com>, "Wedson Almeida Filho"
 <wedsonaf@...il.com>, "Viresh Kumar" <viresh.kumar@...aro.org>, "Lyude
 Paul" <lyude@...hat.com>, "Ingo Molnar" <mingo@...nel.org>, "Mitchell Levy"
 <levymitchell0@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Linus Torvalds"
 <torvalds@...ux-foundation.org>, "Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics

On Mon Jun 23, 2025 at 9:09 PM CEST, Boqun Feng wrote:
> On Mon, Jun 23, 2025 at 07:30:19PM +0100, Gary Guo wrote:
>> cannot just transmute between from pointers to usize (which is its
>> Repr):
>> * Transmuting from pointer to usize discards provenance
>> * Transmuting from usize to pointer gives invalid provenance
>> 
>> We want neither behaviour, so we must store `usize` directly and
>> always call into repr functions.
>> 
>
> If we store `usize`, how can we support the `get_mut()` then? E.g.
>
>     static V: i32 = 32;
>
>     let mut x = Atomic::new(&V as *const i32 as *mut i32);
>     // ^ assume we expose_provenance() in new().
>
>     let ptr: &mut *mut i32 = x.get_mut(); // which is `&mut self.0.get()`.
>
>     let ptr_val = *ptr; // Does `ptr_val` have the proper provenance?

If `get_mut` transmutes the integer into a pointer, then it will have
the wrong provenance (it will just have plain invalid provenance).

---
Cheers,
Benno