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

[Andreas Hindborg]

"Benno Lossin" <lossin@xxxxxxxxxx> writes:

> On Thu Jun 12, 2025 at 3:40 PM CEST, Andreas Hindborg wrote:
>> +/// A wrapper for kernel parameters.
>> +///
>> +/// This type is instantiated by the [`module!`] macro when module parameters are
>> +/// defined. You should never need to instantiate this type directly.
>> +///
>> +/// Note: This type is `pub` because it is used by module crates to access
>> +/// parameter values.
>> +#[repr(transparent)]
>> +pub struct ModuleParamAccess<T> {
>> +    data: core::cell::UnsafeCell<T>,
>> +}
>> +
>> +// SAFETY: We only create shared references to the contents of this container,
>> +// so if `T` is `Sync`, so is `ModuleParamAccess`.
>> +unsafe impl<T: Sync> Sync for ModuleParamAccess<T> {}
>> +
>> +impl<T> ModuleParamAccess<T> {
>> +    #[doc(hidden)]
>> +    pub const fn new(value: T) -> Self {
>> +        Self {
>> +            data: core::cell::UnsafeCell::new(value),
>> +        }
>> +    }
>> +
>> +    /// Get a shared reference to the parameter value.
>> +    // Note: When sysfs access to parameters are enabled, we have to pass in a
>> +    // held lock guard here.
>> +    pub fn get(&self) -> &T {
>> +        // SAFETY: As we only support read only parameters with no sysfs
>> +        // exposure, the kernel will not touch the parameter data after module
>> +        // initialization.
>
> This should be a type invariant. But I'm having difficulty defining one
> that's actually correct: after parsing the parameter, this is written
> to, but when is that actually?

For built-in modules it is during kernel initialization. For loadable
modules, it during module load. No code from the module will execute
before parameters are set.

> Would we eventually execute other Rust
> code during that time? (for example when we allow custom parameter
> parsing)

I don't think we will need to synchronize because of custom parameter
parsing. Parameters are initialized sequentially. It is not a problem if
the custom parameter parsing code name other parameters, because they
are all initialized to valid values (as they are statics).

>
> This function also must never be `const` because of the following:
>
>     module! {
>         // ...
>         params: {
>             my_param: i64 {
>                 default: 0,
>                 description: "",
>             },
>         },
>     }
>
>     static BAD: &'static i64 = module_parameters::my_param.get();
>
> AFAIK, this static will be executed before loading module parameters and
> thus it makes writing to the parameter UB.

As I understand, the static will be initialized by a constant expression
evaluated at compile time. I am not sure what happens when this is
evaluated in const context:

    pub fn get(&self) -> &T {
        // SAFETY: As we only support read only parameters with no sysfs
        // exposure, the kernel will not touch the parameter data after module
        // initialization.
        unsafe { &*self.data.get() }
    }

Why would that not be OK? I would assume the compiler builds a dependency graph
when initializing statics?


Best regards,
Andreas Hindborg