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 2:49 PM CEST, Danilo Krummrich wrote:
> On Tue Jul 22, 2025 at 2:08 PM CEST, Benno Lossin wrote:
>> On Tue Jul 22, 2025 at 1:35 PM CEST, Alice Ryhl wrote:
>>> On Tue, Jul 22, 2025 at 12:57 PM Benno Lossin <lossin@kernel.org> wrote:
>>>>
>>>> On Tue Jul 22, 2025 at 11:51 AM CEST, Danilo Krummrich wrote:
>>>> > I think they're good, but we're pretty late in the cycle now. That should be
>>>> > fine though, we can probably take them through the nova tree, or in the worst
>>>> > case share a tag, if needed.
>>>> >
>>>> > Given that, it would probably be good to add the Guarantee section on as_raw(),
>>>> > as proposed by Benno, right away.
>>>> >
>>>> > @Benno: Any proposal on what this section should say?
>>>>
>>>> At a minimum I'd say "The returned pointer is valid.", but that doesn't
>>>> really say for what it's valid... AFAIK you're mostly using this pointer
>>>> to pass it to the C side, in that case, how about:
>>>>
>>>>     /// # Guarantees
>>>>     ///
>>>>     /// The returned pointer is valid for reads and writes from the C side for as long as `self` exists.
>>>>
>>>> Maybe we need to change it a bit more, but let's just start with this.
>>>>
>>>> (If you're also using the pointer from Rust, then we need to make
>>>> changes)
>>>
>>> Honestly I think this is a bit over the top. I wouldn't bother adding
>>> a section like that to every single as_raw() method out there.
>>
>> Hmm. And then just assume that these kinds of functions return valid
>> pointers? I get that this is annoying to put on every function...
>>
>> Another option would be to have a `Ptr<'a, T>` type that is a valid
>> pointer, but doesn't allow writing/reading safely (you need to justify
>> why it's not a data race). And for FFI there could be an `as_ptr`
>> function.
>
> I don't understand where's the difference between the two. For FFI calls we'd
> also have to justify it's not a data race, no?

Yes, but there you need a raw pointer.

> The only guarantee we take as granted from as_raw() is that it returns a raw
> pointer to the wrapped FFI type in Self, i.e. it points to valid memory. Any
> additional guarantees may come from the context where the pointer is used and
> which specific fields it is used to access.

Sure you need additional guarantees from the context, but you also need
the fact that the pointer coming from `as_raw` isn't just a random
pointer, but that it is derived from the reference...

I don't have any good plan forward for this, so maybe we should revisit
this in the future...

---
Cheers,
Benno