linux-kernel - Re: [PATCH v2 1/2] rust: Update PCI binding safety comments and add inline compiler hint

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Message-Id: <DB92OHEUBB06.2VTHW9KQVV52X@kernel.org>
Date: Fri, 11 Jul 2025 10:11:33 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alistair Popple" <apopple@...dia.com>
Cc: <rust-for-linux@...r.kernel.org>, "Danilo Krummrich" <dakr@...nel.org>,
 "Bjorn Helgaas" <bhelgaas@...gle.com>,
 Krzysztof Wilczyński <kwilczynski@...nel.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>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Greg Kroah-Hartman"
 <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>,
 "John Hubbard" <jhubbard@...dia.com>, "Alexandre Courbot"
 <acourbot@...dia.com>, <linux-pci@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] rust: Update PCI binding safety comments and add
 inline compiler hint

On Fri Jul 11, 2025 at 1:22 AM CEST, Alistair Popple wrote:
> On Thu, Jul 10, 2025 at 10:01:05AM +0200, Benno Lossin wrote:
>> On Thu Jul 10, 2025 at 4:24 AM CEST, Alistair Popple wrote:
>> > diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
>> > index 8435f8132e38..5c35a66a5251 100644
>> > --- a/rust/kernel/pci.rs
>> > +++ b/rust/kernel/pci.rs
>> > @@ -371,14 +371,18 @@ fn as_raw(&self) -> *mut bindings::pci_dev {
>> >  
>> >  impl Device {
>> >      /// Returns the PCI vendor ID.
>> > +    #[inline]
>> >      pub fn vendor_id(&self) -> u16 {
>> > -        // SAFETY: `self.as_raw` is a valid pointer to a `struct pci_dev`.
>> > +        // SAFETY: by its type invariant `self.as_raw` is always a valid pointer to a
>> 
>> s/by its type invariant/by the type invariants of `Self`,/
>> s/always//
>> 
>> Also, which invariant does this refer to? The only one that I can see
>> is:
>> 
>>     /// A [`Device`] instance represents a valid `struct device` created by the C portion of the kernel.
>
> Actually isn't that wrong? Shouldn't that read for "a valid `struct pci_dev`"?

Yeah it should probably be changed, I'm not sure what exactly is
required here, but this already would be an improvement:

    /// `self.0` is a valid `struct pci_dev`.

>> And this doesn't say anything about the validity of `self.as_raw()`...
>
> Isn't it up to whatever created this pci::Device to ensure the underlying struct
> pci_dev remains valid for at least the lifetime of `Self`?

Well yes and no. It is up to the creator of this specific `pci::Device`
to ensure that it is valid, but that is true for all creators of
`pci::Device`. In other words this property doesn't change while the
`pci::Device` is alive so we call it an "invariant".

When creating a `pci::Device`, you have to ensure all invariants are met
and then anyone using it can rely on them being true.

Now in this particular instance the `as_raw` function is just calling
`self.0.get()`. I'm not sure that's worth it, since it isn't even
shorter and it makes the safety docs a bit worse. So my suggestion would
be to remove it.

> Sorry I'm quite new to Rust (and especially Rust in the kernel), so
> not sure what the best way to express that in a SAFETY style comment
> would be. Are you saying the list of invariants for pci::Device also
> needs expanding?

No worries, safety documentation is pretty hard :)

---
Cheers,
Benno

>
> Thanks.
>
>> > +        // `struct pci_dev`.
>> >          unsafe { (*self.as_raw()).vendor }
>> >      }
>> >  
>> >      /// Returns the PCI device ID.
>> > +    #[inline]
>> >      pub fn device_id(&self) -> u16 {
>> > -        // SAFETY: `self.as_raw` is a valid pointer to a `struct pci_dev`.
>> > +        // SAFETY: by its type invariant `self.as_raw` is always a valid pointer to a
>> > +        // `struct pci_dev`.
>> 
>> Ditto here.
>> 
>> ---
>> Cheers,
>> Benno
>> 
>> >          unsafe { (*self.as_raw()).device }
>> >      }
>> >  
>>