Re: [PATCH 1/3] rust: sync: return `Result<&T, T>` from `SetOnce::populate()`

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] `ModuleParamAccess` lacks `#[repr(C)]`, leading to potential Undefined Behavior and memory corruption when its pointer is cast to `SetOnce<T>`.
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--- Patch [1]: [PATCH 1/3] rust: sync: return `Result<&T, T>` from `SetOnce::populate()` ---

commit e0d10f7a2c42bcda24ee9851cc2fda3d4c4bc480
Author: Alice Ryhl [off-list ref]

rust: sync: return Result<&T, T> from SetOnce::populate()

This commit changes the return value of SetOnce::populate() from a boolean
to a Result to allow infallible access to the inserted value on success.
It also ensures the inserted value is not dropped under a lock if populate()
fails, and updates ModuleParam and Binder to handle the new return type.

[Severity: High]
This is a pre-existing issue, but since we are modifying this code block, is
it safe to cast this void pointer directly to SetOnce<T>?

Looking at the definition of ModuleParamAccess<T>, it lacks a #[repr(C)]
annotation:

rust/kernel/module_param.rs:ModuleParamAccess {
    value: SetOnce<T>,
    default: T,
}

Since it uses the default Rust layout, the compiler is free to reorder these
fields. When as_void_ptr() passes a pointer to the entire struct into the
C kernel parameters API:

rust/kernel/module_param.rs:ModuleParamAccess::as_void_ptr() {
    core::ptr::from_ref(self).cast_mut().cast()
}

And set_param casts it back directly to SetOnce<T> (assuming it sits at
offset 0), could this lead to memory corruption if the compiler decides to
place default before value?

-        container
-            .populate(new_value)
-            .then_some(0)
-            .ok_or(kernel::error::code::EEXIST)
+        match container.populate(new_value) {
+            Ok(_) => Ok(0),
+            Err(_) => Err(EEXIST),
+        }
     })
 }

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Sashiko AI review · https://sashiko.dev/#/patchset/20260722-setonce-populate-v1-0-fa7455c26c42@google.com?part=1