Re: [PATCH v17 4/6] rust: faux: Allow retrieving a bound Device

["Danilo Krummrich" <dakr@kernel.org>]

On Wed Jun 3, 2026 at 9:42 PM CEST, Lyude Paul wrote:
> When writing up some rust code that used faux devices for unit testing, I
> noticed that we never actually added the Bound device context to
> faux::Registration's AsRef<device::Device> implementation. This being said:
> the Registration object itself is proof that a driver is bound to the
> device - so this should be safe.

Yes, it should be, but the reasons are non-trivial and should be part of the
safety comment below.

  - faux_match() always returns 1, and probe runs synchronously
    (PROBE_FORCE_SYNCHRONOUS)

  - suppress_bind_attrs = true on faux_driver prevents userspace-triggered
    unbind via sysfs

  - mem::forget(Registration) is not a problem; if the Registration is leaked,
    the faux device stays bound forever

> Signed-off-by: Lyude Paul <lyude@redhat.com>

I can pick this through the driver-core tree already, as it seems this it not
needed by this series. Feel free to resend as individual patch for thus purpose.

> ---
>  rust/kernel/faux.rs | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/rust/kernel/faux.rs b/rust/kernel/faux.rs
> index 43b4974f48cd2..e0856b2964a2c 100644
> --- a/rust/kernel/faux.rs
> +++ b/rust/kernel/faux.rs
> @@ -25,7 +25,8 @@
>  ///
>  /// # Invariants
>  ///
> -/// `self.0` always holds a valid pointer to an initialized and registered [`struct faux_device`].
> +/// - `self.0` always holds a valid pointer to an initialized and registered [`struct faux_device`].
> +/// - This object is proof that the object described by this `Registration` is bound to a device.
>  ///
>  /// [`struct faux_device`]: srctree/include/linux/device/faux.h
>  pub struct Registration(NonNull<bindings::faux_device>);
> @@ -59,8 +60,8 @@ fn as_raw(&self) -> *mut bindings::faux_device {
>      }
>  }
>  
> -impl AsRef<device::Device> for Registration {
> -    fn as_ref(&self) -> &device::Device {
> +impl AsRef<device::Device<device::Bound>> for Registration {
> +    fn as_ref(&self) -> &device::Device<device::Bound> {
>          // SAFETY: The underlying `device` in `faux_device` is guaranteed by the C API to be
>          // a valid initialized `device`.
>          unsafe { device::Device::from_raw(addr_of_mut!((*self.as_raw()).dev)) }
> -- 
> 2.54.0