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

[kernel@dakr.org]

On 2025-02-26 09:38, Alice Ryhl wrote:
> On Tue, Feb 25, 2025 at 10:31 PM Lyude Paul <lyude@redhat.com> 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@redhat.com>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> ---
>>  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?

I was wondering the same, but it seems that the subsequent device_add() 
call takes care of that:

https://elixir.bootlin.com/linux/v6.14-rc3/source/drivers/base/core.c#L3588

device_del() drops the reference.

This makes device->parent only valid for the duration between 
faux_device_create() and faux_device_remove().

But this detail shouldn’t be relevant for this API.