linux-kernel - Re: [PATCH v13 3/7] rust: pci: impl AsMut<Device> for pci::Device

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Message-ID: <Z8rcS4hEHIYNzPll@cassiopeiae>
Date: Fri, 7 Mar 2025 12:45:15 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>,
	rust-for-linux@...r.kernel.org, daniel.almeida@...labora.com,
	robin.murphy@....com, 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>,
	Valentin Obst <kernel@...entinobst.de>,
	open list <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com,
	"open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v13 3/7] rust: pci: impl AsMut<Device> for pci::Device

On Fri, Mar 07, 2025 at 11:18:03AM +0000, Alice Ryhl wrote:
> On Fri, Mar 07, 2025 at 01:06:20PM +0200, Abdiel Janulgue wrote:
> > From: Danilo Krummrich <dakr@...nel.org>
> > 
> > Some device methods require mutable references, since they change the
> > underlying struct device without lock protection.
> > 
> > Hence, make it possible to retrieve a mutable reference to a Device from
> > a mutable pci::Device.
> > 
> > Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> > ---
> >  rust/kernel/pci.rs | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
> > index 4c98b5b9aa1e..141430dac2d5 100644
> > --- a/rust/kernel/pci.rs
> > +++ b/rust/kernel/pci.rs
> > @@ -432,3 +432,14 @@ fn as_ref(&self) -> &device::Device {
> >          &self.0
> >      }
> >  }
> > +
> > +impl AsMut<device::Device> for Device {
> > +    fn as_mut(&mut self) -> &mut device::Device {
> > +        // SAFETY:
> > +        // - `self.0.as_raw()` is valid by the type invariant of `device::Device`,
> > +        // - `struct device` is embedded in `struct pci_dev`, hence it is safe to give out a
> > +        //    mutable reference for `device::Device` if we have a mutable reference to the
> > +        //    corresponding `pci::Device`.
> > +        unsafe { &mut *self.0.as_raw().cast() }
> > +    }
> > +}
> 
> This makes it possible to call `mem::swap` on two devices, which I doubt
> is allowed.

Good catch, that's of course not allowed.

Obviously, this is for Device::dma_set_mask(), introduced in a subsequent patch.

Thinking about it, maybe it should be implemented by bus devices instead (e.g.
pci::Device) -- dma_set_mask() doesn't make sense for every device.