linux-kernel - Re: [PATCH 3/4] rust: pci: fix unrestricted &mut pci::Device

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Message-ID: <D8F7L7ZGQW9J.3Q8MVXYPTJIT@proton.me>
Date: Thu, 13 Mar 2025 14:30:32 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
Cc: gregkh@...uxfoundation.org, rafael@...nel.org, bhelgaas@...gle.com, ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu, linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 3/4] rust: pci: fix unrestricted &mut pci::Device

On Thu Mar 13, 2025 at 3:25 PM CET, Danilo Krummrich wrote:
> On Thu, Mar 13, 2025 at 10:44:38AM +0000, Benno Lossin wrote:
>> On Thu Mar 13, 2025 at 3:13 AM CET, Danilo Krummrich wrote:
>> > As by now, pci::Device is implemented as:
>> >
>> > 	#[derive(Clone)]
>> > 	pub struct Device(ARef<device::Device>);
>> >
>> > This may be convenient, but has the implication that drivers can call
>> > device methods that require a mutable reference concurrently at any
>> > point of time.
>> 
>> Which methods take mutable references? The `set_master` method you
>> mentioned also took a shared reference before this patch.
>
> Yeah, that's basically a bug that I never fixed (until now), since making it
> take a mutable reference would not have changed anything in terms of
> accessibility.

Gotcha.

>> >  impl AsRef<device::Device> for Device {
>> >      fn as_ref(&self) -> &device::Device {
>> > -        &self.0
>> > +        // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
>> > +        // `struct pci_dev`.
>> > +        let dev = unsafe { addr_of_mut!((*self.as_raw()).dev) };
>> > +
>> > +        // SAFETY: `dev` points to a valid `struct device`.
>> > +        unsafe { device::Device::as_ref(dev) }
>> 
>> Why not use `&**self` instead (ie go through the `Deref` impl)?
>
> `&**self` gives us a `Device` (i.e. `pci::Device`), not a `device::Device`.

Ah, yeah then you'll have to use `unsafe`.

---
Cheers,
Benno