Re: [PATCH v3 4/4] rust: add parameter support to the `module!` macro

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

"Miguel Ojeda" <miguel.ojeda.sandonis@gmail.com> writes:

> On Fri, Dec 13, 2024 at 2:17 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> scheduled for removal. Interior mutability via `SyncUnsafeCell` provides
>> the same functionality and it is my understanding that this feature is
>> on track to be stabilized.
>
> I am not sure about the last bit, but even if it is on track, we do
> not want to start using new language features or APIs that could
> potentially change.
>
> And even if it is a feature that we are sure will not change, we
> should still let upstream Rust know before using it, since we are
> actively discussing with them the remaining unstable items that the
> kernel needs and they are putting the kernel in their roadmap.
>
> So I suggest we mention it next week in the Rust/Rust for Linux meeting.

I don't think we ever discussed this?

I was going to put this in the commit trailer for v4:

---
Version 3 of this patch enabled the unstable feature `sync_unsafe_cell` [1] to
avoid `static mut` variables as suggested by Trevor Tross and Benno Lossin [2].

As we are moving closer to a new edition, it is now clear that `static mut` is
not being deprecated in the 2024 edition as first anticipated [3].

Still, `SyncUnsafeCell` allows us to use safe code when referring to the
parameter value:

```
{param_name}.as_mut_ptr().cast()
```

rather than unsafe code:

```
unsafe { addr_of_mut!(__{name}_{param_name}_value) }.cast()
```

Thus, this version (4) keeps the feature enabled.

[1] https://github.com/rust-lang/rust/issues/95439
[2] https://lore.kernel.org/all/CALNs47sqt==o+hM5M1b0vTayKH177naybg_KurcirXszYAa22A@mail.gmail.com/
[3] https://github.com/rust-lang/rust/issues/53639#issuecomment-2434023115
---

What do you think?

>
>> Not sure. `val` being null not supposed to happen in the current
>> configuration. It should be an unreachable state. So BUG is the right thing?
>
> Since you can easily return an error in this situation, I would say
> ideally a `WARN_ON_ONCE` + returning an error would be the best
> option, and covers you in case the rest changes and someone forgets to
> update this.

Returning an error and `pr_warn!` is doable. As far as I can tell, we do
not have `WARN_ON_ONCE` yet?

>
>> Not in the current configuration. The parameters can only be declared
>> "read only". It should be impossible for anyone to call this function.
>
> What I meant is, can you avoid writing the function to begin with, by
> leaving a null function pointer in the `kernel_param_ops` struct, i.e.
> `None`?
>

It turns out we can!


Best regards,
Andreas Hindborg