Re: [PATCH 2/2] rust/faux: Add missing parent argument to Registration::new()

[Alice Ryhl]

On Tue, Feb 25, 2025 at 10:31 PM Lyude Paul <lyude@xxxxxxxxxx> wrote:
>
> A little late in the review of the faux device interface, we added the
> ability to specify a parent device when creating new faux devices - but
> this never got ported over to the rust bindings. So, let's add the missing
> argument now so we don't have to convert other users later down the line.
>
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
>  rust/kernel/faux.rs              | 10 ++++++++--
>  samples/rust/rust_driver_faux.rs |  2 +-
>  2 files changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/rust/kernel/faux.rs b/rust/kernel/faux.rs
> index 41751403cd868..ae99ea3d114ef 100644
> --- a/rust/kernel/faux.rs
> +++ b/rust/kernel/faux.rs
> @@ -23,11 +23,17 @@
>
>  impl Registration {
>      /// Create and register a new faux device with the given name.
> -    pub fn new(name: &CStr) -> Result<Self> {
> +    pub fn new(name: &CStr, parent: Option<&device::Device>) -> Result<Self> {
>          // SAFETY:
>          // - `name` is copied by this function into its own storage
>          // - `faux_ops` is safe to leave NULL according to the C API
> -        let dev = unsafe { bindings::faux_device_create(name.as_char_ptr(), null_mut(), null()) };
> +        let dev = unsafe {
> +            bindings::faux_device_create(
> +                name.as_char_ptr(),
> +                parent.map_or(null_mut(), |p| p.as_raw()),
> +                null(),

This function signature only requires that `parent` is valid for the
duration of this call to `new`, but `faux_device_create` stashes a
pointer without touching the refcount. How do you ensure that the
`parent` pointer does not become dangling?

Alice