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

["Benno Lossin" <lossin@kernel.org>]

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