linux-kernel - Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z7iZevQkYVGDoeTa@Mac.home>
Date: Fri, 21 Feb 2025 07:19:22 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Tamir Duberstein <tamird@...il.com>
Cc: Andreas Hindborg <a.hindborg@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Danilo Krummrich <dakr@...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>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Lyude Paul <lyude@...hat.com>, Guangbo Cui <2407018371@...com>,
	Dirk Behme <dirk.behme@...il.com>,
	Daniel Almeida <daniel.almeida@...labora.com>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

On Fri, Feb 21, 2025 at 09:46:08AM -0500, Tamir Duberstein wrote:
> On Fri, Feb 21, 2025 at 9:40 AM Boqun Feng <boqun.feng@...il.com> wrote:
> >
> > Hmm... if you mean:
> >
> > trait HasHrTimer {
> >     unsafe fn start(&self, expires: Ktime) {
> >         ...
> >     }
> > }
> >
> > Then it'll be problematic because the pointer derived from `&self`
> > doesn't have write provenance, therefore in a timer callback, the
> > pointer cannot be used for write, which means for example you cannot
> > convert the pointer back into a `Pin<Box<HasTimer>>`.
> >
> > To answer Tamir's question, pointers are heavily used here because we
> > need to preserve the provenance.
> 
> Wouldn't the natural implication be that &mut self is needed? Maybe

For an `Arc<HasTimer>`, you cannot get `&mut self`.

> you can help me understand why pointers can express a contract that
> references can't?

I assume you already know what a pointer provenance is?

	http://doc.rust-lang.org/std/ptr/index.html#provenance

Passing a pointer (including offset operation on it) preserves the
provenance (determined as derive time), however, deriving a pointer from
a reference gives the pointer a provenance based on the reference type.
For example, let's say we have an `Arc<i32>` and a clone:

	let arc = Arc::new(42);
	let clone = arc.clone();

you can obviously do a into_raw() + from_raw() pair:

	let ptr = Arc::into_raw(arc);
	let arc = unsafe { Arc::from_raw(arc) };

however, if you create a reference based on `Arc::into_raw()`, and then
derive a pointer from that, you change the provenance, therefore the
below code would generate UB:

	// cannot mutably borrow because of clone.
	let ptr = unsafe { &*Arc::into_raw(arc) } as *const i32;
    
	let arc = unsafe { Arc::from_raw(ptr) };


(playground code snippet for this example)

	https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=15e051db46c3886b29ed02e579562278

As you already know, the whole thing about pointers/references here is
passing the value to the callback and the callback can "reconstruct" the
data, in such a case, reborrowing in the middle of the chain into a
reference is not necessary, and as the above shows, it can be
problematic.

Hope this could be helpful.

Regards,
Boqun