Re: [PATCH v4 4/5] rust: auxiliary: add auxiliary registration

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Apr 14, 2025 at 03:18:07PM +0200, Danilo Krummrich wrote:
> Implement the `auxiliary::Registration` type, which provides an API to
> create and register new auxiliary devices in the system.
> 
> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/auxiliary.rs | 88 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 87 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/auxiliary.rs b/rust/kernel/auxiliary.rs
> index 75423737032a..b40d663b42eb 100644
> --- a/rust/kernel/auxiliary.rs
> +++ b/rust/kernel/auxiliary.rs
> @@ -5,7 +5,7 @@
>  //! C header: [`include/linux/auxiliary_bus.h`](srctree/include/linux/auxiliary_bus.h)
>  
>  use crate::{
> -    bindings, device,
> +    bindings, container_of, device,
>      device_id::RawDeviceId,
>      driver,
>      error::{to_result, Result},
> @@ -230,6 +230,18 @@ pub fn parent(&self) -> Option<&device::Device> {
>      }
>  }
>  
> +impl Device {
> +    extern "C" fn release(dev: *mut bindings::device) {
> +        // SAFETY: By the type invariant `self.0.as_raw` is a pointer to the `struct device`
> +        // embedded in `struct auxiliary_device`.
> +        let adev = unsafe { container_of!(dev, bindings::auxiliary_device, dev) }.cast_mut();

FYI, Tamir's patch makes this cast_mut() unnecessary.

> +        // SAFETY: `adev` points to the memory that has been allocated in `Registration::new`, via
> +        // `KBox::new(Opaque::<bindings::auxiliary_device>::zeroed(), GFP_KERNEL)`.
> +        let _ = unsafe { KBox::<Opaque<bindings::auxiliary_device>>::from_raw(adev.cast()) };
> +    }
> +}
> +
>  // SAFETY: `Device` is a transparent wrapper of a type that doesn't depend on `Device`'s generic
>  // argument.
>  kernel::impl_device_context_deref!(unsafe { Device });
> @@ -272,3 +284,77 @@ unsafe impl Send for Device {}
>  // SAFETY: `Device` can be shared among threads because all methods of `Device`
>  // (i.e. `Device<Normal>) are thread safe.
>  unsafe impl Sync for Device {}
> +
> +/// The registration of an auxiliary device.
> +///
> +/// This type represents the registration of a [`struct auxiliary_device`]. When an instance of this
> +/// type is dropped, its respective auxiliary device will be unregistered from the system.
> +///
> +/// # Invariants
> +///
> +/// `self.0` always holds a valid pointer to an initialized and registered
> +/// [`struct auxiliary_device`].
> +pub struct Registration(NonNull<bindings::auxiliary_device>);
> +
> +impl Registration {
> +    /// Create and register a new auxiliary device.
> +    pub fn new(parent: &device::Device, name: &CStr, id: u32, modname: &CStr) -> Result<Self> {
> +        let boxed = KBox::new(Opaque::<bindings::auxiliary_device>::zeroed(), GFP_KERNEL)?;
> +        let adev = boxed.get();
> +
> +        // SAFETY: It's safe to set the fields of `struct auxiliary_device` on initialization.
> +        unsafe {
> +            (*adev).dev.parent = parent.as_raw();
> +            (*adev).dev.release = Some(Device::release);
> +            (*adev).name = name.as_char_ptr();
> +            (*adev).id = id;
> +        }
> +
> +        // SAFETY: `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`,
> +        // which has not been initialized yet.
> +        unsafe { bindings::auxiliary_device_init(adev) };
> +
> +        // Now that `adev` is initialized, leak the `Box`; the corresponding memory will be freed
> +        // by `Device::release` when the last reference to the `struct auxiliary_device` is dropped.
> +        let _ = KBox::into_raw(boxed);
> +
> +        // SAFETY:
> +        // - `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`, which has
> +        //   been initialialized,
> +        // - `modname.as_char_ptr()` is a NULL terminated string.
> +        let ret = unsafe { bindings::__auxiliary_device_add(adev, modname.as_char_ptr()) };
> +        if ret != 0 {
> +            // SAFETY: `adev` is guaranteed to be a valid pointer to a `struct auxiliary_device`,
> +            // which has been initialialized.
> +            unsafe { bindings::auxiliary_device_uninit(adev) };

Does this error-path actually free the box?

Alice

> +            return Err(Error::from_errno(ret));
> +        }
> +
> +        // SAFETY: `adev` is guaranteed to be non-null, since the `KBox` was allocated successfully.
> +        //
> +        // INVARIANT: The device will remain registered until `auxiliary_device_delete()` is called,
> +        // which happens in `Self::drop()`.
> +        Ok(Self(unsafe { NonNull::new_unchecked(adev) }))
> +    }
> +}
> +
> +impl Drop for Registration {
> +    fn drop(&mut self) {
> +        // SAFETY: By the type invariant of `Self`, `self.0.as_ptr()` is a valid registered
> +        // `struct auxiliary_device`.
> +        unsafe { bindings::auxiliary_device_delete(self.0.as_ptr()) };
> +
> +        // This drops the reference we acquired through `auxiliary_device_init()`.
> +        //
> +        // SAFETY: By the type invariant of `Self`, `self.0.as_ptr()` is a valid registered
> +        // `struct auxiliary_device`.
> +        unsafe { bindings::auxiliary_device_uninit(self.0.as_ptr()) };
> +    }
> +}
> +
> +// SAFETY: A `Registration` of a `struct auxiliary_device` can be released from any thread.
> +unsafe impl Send for Registration {}
> +
> +// SAFETY: `Registration` does not expose any methods or fields that need synchronization.
> +unsafe impl Sync for Registration {}
> -- 
> 2.49.0
>