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

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Message-ID: <bwbern2t7k5fcj6zxze6bjpasu3t26n6dmfptlmhbhd7qmligs@3fgwifsw7qai>
Date: Tue, 22 Jul 2025 15:17:09 +1000
From: Alistair Popple <apopple@...dia.com>
To: Benno Lossin <lossin@...nel.org>
Cc: Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.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 10:46:13PM +0200, Benno Lossin wrote:
> On Fri Jul 11, 2025 at 9:33 PM CEST, Danilo Krummrich wrote:
> > On Fri Jul 11, 2025 at 8:30 PM CEST, Benno Lossin wrote:
> >> On Fri Jul 11, 2025 at 5:02 PM CEST, Danilo Krummrich wrote:
> >>> On Thu Jul 10, 2025 at 10:01 AM CEST, 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.
> >>>>
> >>>> And this doesn't say anything about the validity of `self.as_raw()`...
> >>>
> >>> Hm...why not? If an instance of Self always represents a valid struct pci_dev,
> >>> then consequently self.as_raw() can only be a valid pointer to a struct pci_dev,
> >>> no?
> >>
> >> While it's true, you need to look into the implementation of `as_raw`.
> >> It could very well return a null pointer...
> >>
> >> This is where we can use a `Guarantee` on that function. But since it's
> >> not shorter than `.0.get()`, I would just remove it.
> >
> > We have 15 to 20 as_raw() methods of this kind in the tree. If this really needs
> > a `Guarantee` to be clean, we should probably fix it up in a treewide change.
> >
> > as_raw() is a common pattern and everyone knows what it does, `.0.get()` seems
> > much less obvious.

Coming from a C kernel programming background I agree `.as_raw()` is more
obvious than `.0.get()`. However now I'm confused ... what if anything needs
changing to get these two small patches merged?

Thanks.

 - Alistair

> Yeah I guess then we need to do the treewide change... I don't have the
> bandwidth for that, we can probably make this a good-first-issue.
> 
> ---
> Cheers,
> Benno