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

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Message-ID: <871pra6b7k.fsf@kernel.org>
Date: Mon, 23 Jun 2025 11:47:43 +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 Fri Jun 20, 2025 at 1:29 PM CEST, Andreas Hindborg wrote:
>> "Benno Lossin" <lossin@...nel.org> 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.
>
> Gotcha and there never ever will be custom code that is executed
> before/during parameter setting (so code aside from code in `kernel`)?

Not with the parameter parsers we provide now. In the case of custom
parsing code, I suppose there is nothing preventing the parsing code
from spinning up a thread that could do stuff while more parameters are
initialized by the kernel.


Best regards,
Andreas Hindborg