Re: [PATCH v13 2/6] rust: introduce module_param module

[Andreas Hindborg <a.hindborg@kernel.org>]

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

> On Thu Jun 19, 2025 at 2:20 PM CEST, Andreas Hindborg wrote:
>> "Benno Lossin" <lossin@kernel.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