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

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Message-Id: <DBIIXC694S6P.18BOBCI4LZ4HA@kernel.org>
Date: Tue, 22 Jul 2025 12:49:24 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alistair Popple" <apopple@...dia.com>
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 Tue Jul 22, 2025 at 7:17 AM CEST, Alistair Popple wrote:
> 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()`.

Makes sense, then I wouldn't recommend changing it.

> However now I'm confused ... what if anything needs changing to get
> these two small patches merged?

I'd like to see `as_raw` get a `Guarantee` section, but that is
independent from this patch series.

---
Cheers,
Benno