linux-kernel - Re: [PATCH 1/4] rust: device: implement Device::parent()

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Message-ID: <Z9vTDUnr-G4vmUqS@google.com>
Date: Thu, 20 Mar 2025 08:34:21 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: bhelgaas@...gle.com, gregkh@...uxfoundation.org, rafael@...nel.org, 
	ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, 
	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me, 
	a.hindborg@...nel.org, tmgross@...ch.edu, linux-pci@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] rust: device: implement Device::parent()

On Wed, Mar 19, 2025 at 09:30:25PM +0100, Danilo Krummrich wrote:
> Device::parent() returns a reference to the device' parent device, if
> any.
> 
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> ---
>  rust/kernel/device.rs | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index 21b343a1dc4d..76b341441f3f 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -65,6 +65,19 @@ pub(crate) fn as_raw(&self) -> *mut bindings::device {
>          self.0.get()
>      }
>  
> +    /// Returns a reference to the parent device, if any.
> +    pub fn parent<'a>(&self) -> Option<&'a Self> {
> +        // SAFETY: By the type invariant `self.as_raw()` is always valid.
> +        let parent = unsafe { *self.as_raw() }.parent;

This means:
1. Copy the entire `struct device` onto the stack.
2. Read the `parent` field of the copy.

Please write this instead to only read the `parent` field:
let parent = unsafe { *self.as_raw().parent };

> +        if parent.is_null() {
> +            None
> +        } else {
> +            // SAFETY: Since `parent` is not NULL, it must be a valid pointer to a `struct device`.
> +            Some(unsafe { Self::as_ref(parent) })
> +        }
> +    }
> +
>      /// Convert a raw C `struct device` pointer to a `&'a Device`.
>      ///
>      /// # Safety
> -- 
> 2.48.1
>