Re: [PATCH v11 2/3] rust: add parameter support to the `module!` macro

[Alice Ryhl <aliceryhl@google.com>]

On Mon, May 05, 2025 at 11:55:33AM +0200, Andreas Hindborg wrote:
> "Alice Ryhl" <aliceryhl@google.com> writes:
> 
> > On Fri, May 02, 2025 at 02:16:35PM +0200, Andreas Hindborg wrote:
> > It would be a use-after-free to
> > access it during module teardown. For example, what if I access this
> > static during its own destructor? Or during the destructor of another
> > module parameter?
> 
> Yes, that is a problem.
> 
> We can get around it for now by just not calling `free` for now. We only
> support simple types that do not need drop. I think we would have to
> seal the `ModuleParam` trait for this.
> 
> For a proper solution, we could
>  - Require a token to read the parameter.
>  - Synchronize on a module private field and return an option from the
>    parameter getter. This would require module exit to run before param
>    free. I think this is the case, but I did not check.
>  - Use a `Revocable` and revoke the parameter in `free`.
> 
> Any other ideas or comments on the outlined solutions?

I think the simplest you can do right now is

trait ModuleParam: Copy

so that it can't contain any non-trivial values. That way you don't need
Drop either.

Long term, I think we need a way to detect whether it's safe to access
module globals. The exact same problem applies to the existing global
for the module itself - except it's worse there because we can't access
that one during init either.

Alice