linux-kernel - Re: [PATCH v13 2/6] rust: introduce module

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Message-ID: <87o6ui66xi.fsf@kernel.org>
Date: Fri, 20 Jun 2025 12:31:05 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Benno Lossin" <lossin@...nel.org>
Cc: "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>,  "Alice
 Ryhl" <aliceryhl@...gle.com>,  "Masahiro Yamada" <masahiroy@...nel.org>,
  "Nathan Chancellor" <nathan@...nel.org>,  "Luis Chamberlain"
 <mcgrof@...nel.org>,  "Danilo Krummrich" <dakr@...nel.org>,  "Nicolas
 Schier" <nicolas.schier@...ux.dev>,  "Trevor Gross" <tmgross@...ch.edu>,
  "Adam Bratschi-Kaye" <ark.email@...il.com>,
  <rust-for-linux@...r.kernel.org>,  <linux-kernel@...r.kernel.org>,
  <linux-kbuild@...r.kernel.org>,  "Petr Pavlu" <petr.pavlu@...e.com>,
  "Sami Tolvanen" <samitolvanen@...gle.com>,  "Daniel Gomez"
 <da.gomez@...sung.com>,  "Simona Vetter" <simona.vetter@...ll.ch>,  "Greg
 KH" <gregkh@...uxfoundation.org>,  "Fiona Behrens" <me@...enk.dev>,
  "Daniel Almeida" <daniel.almeida@...labora.com>,
  <linux-modules@...r.kernel.org>
Subject: Re: [PATCH v13 2/6] rust: introduce module_param module

"Benno Lossin" <lossin@...nel.org> writes:

> On Thu Jun 19, 2025 at 2:20 PM CEST, Andreas Hindborg wrote:
>> "Benno Lossin" <lossin@...nel.org> writes:
>>> On Thu Jun 12, 2025 at 3:40 PM CEST, Andreas Hindborg wrote:

[...]

>>>> +    crate::error::from_result(|| {
>>>> +        let new_value = T::try_from_param_arg(arg)?;
>>>> +
>>>> +        // SAFETY: By function safety requirements `param` is be valid for reads.
>>>> +        let old_value = unsafe { (*param).__bindgen_anon_1.arg as *mut T };
>>>> +
>>>> +        // SAFETY: By function safety requirements, the target of `old_value` is valid for writes
>>>> +        // and is initialized.
>>>> +        unsafe { *old_value = new_value };
>>>
>>> So if we keep the `ModuleParam: Copy` bound from above, then we don't
>>> need to drop the type here (as `Copy` implies `!Drop`). So we could also
>>> remove the requirement for initialized memory and use `ptr::write` here
>>> instead. Thoughts?
>>
>> Yes, that is the rationale for the `Copy` bound. What would be the
>> benefit of using `ptr::write`? They should be equivalent for `Copy`
>> types, right.
>
> They should be equivalent, but if we drop the requirement that the value
> is initialized to begin with, then removing `Copy` will result in UB
> here.
>
>> I was using `ptr::replace`, but Alice suggested the pace expression
>> assignment instead, since I was not using the old value.
>
> That makes sense, but if we also remove the initialized requirement,
> then I would prefer `ptr::write`.

OK, we can do that.


Best regards,
Andreas Hindborg