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

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Message-ID: <20250226092339.989767-1-aliceryhl@google.com>
Date: Wed, 26 Feb 2025 09:23:39 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: kernel@...r.org
Cc: a.hindborg@...nel.org, alex.gaynor@...il.com, aliceryhl@...gle.com, 
	benno.lossin@...ton.me, bjorn3_gh@...tonmail.com, boqun.feng@...il.com, 
	dakr@...nel.org, gary@...yguo.net, gregkh@...uxfoundation.org, 
	linux-kernel@...r.kernel.org, lyude@...hat.com, mairacanal@...eup.net, 
	ojeda@...nel.org, rafael@...nel.org, rust-for-linux@...r.kernel.org, 
	tmgross@...ch.edu
Subject: Re: [PATCH 2/2] rust/faux: Add missing parent argument to Registration::new()

On Wed, Feb 26, 2025 at 10:06 AM <kernel@...r.org> wrote:
>
> On 2025-02-26 09:38, Alice Ryhl wrote:
> > On Tue, Feb 25, 2025 at 10:31 PM Lyude Paul <lyude@...hat.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@...hat.com>
> >> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.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.

I think this could use a few more comments to explain it. E.g.:

diff --git a/drivers/base/faux.c b/drivers/base/faux.c
index 531e9d789ee0..674db8863d96 100644
--- a/drivers/base/faux.c
+++ b/drivers/base/faux.c
@@ -131,6 +131,7 @@ struct faux_device *faux_device_create_with_groups(const char *name,
 
        device_initialize(dev);
        dev->release = faux_device_release;
+       /* The refcount of dev->parent is incremented in device_add. */
        if (parent)
                dev->parent = parent;
        else
diff --git a/rust/kernel/faux.rs b/rust/kernel/faux.rs
index 7673501ebe37..713ee6842e3f 100644
--- a/rust/kernel/faux.rs
+++ b/rust/kernel/faux.rs
@@ -28,6 +28,7 @@ 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
+        // - `faux_device_create` ensures that `parent` stays alive until `faux_device_destroy`.
         let dev = unsafe {
             bindings::faux_device_create(
                 name.as_char_ptr(),