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

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Message-Id: <DBIKM7U4TSB8.17MTNSR81W8F3@kernel.org>
Date: Tue, 22 Jul 2025 14:08:55 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Danilo Krummrich" <dakr@...nel.org>, "Alistair Popple"
 <apopple@...dia.com>, <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>,
 "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 1:35 PM CEST, Alice Ryhl wrote:
> On Tue, Jul 22, 2025 at 12:57 PM Benno Lossin <lossin@...nel.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.

---
Cheers,
Benno