linux-kernel - Re: [PATCH 2/8] rust: device: add drvdata accessors

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Message-Id: <DB4G2QJ8LA5W.384ECLNXUM0CY@kernel.org>
Date: Sat, 05 Jul 2025 23:38:04 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Danilo Krummrich" <dakr@...nel.org>
Cc: <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>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <david.m.ertman@...el.com>, <ira.weiny@...el.com>, <leon@...nel.org>,
 <kwilczynski@...nel.org>, <bhelgaas@...gle.com>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-pci@...r.kernel.org>
Subject: Re: [PATCH 2/8] rust: device: add drvdata accessors

On Sat Jul 5, 2025 at 5:06 PM CEST, Danilo Krummrich wrote:
> On Sat, Jul 05, 2025 at 01:15:06PM +0200, Benno Lossin wrote:
>> On Sat Jun 21, 2025 at 9:43 PM CEST, Danilo Krummrich wrote:
>> > +impl Device<Internal> {
>> > +    /// Store a pointer to the bound driver's private data.
>> > +    pub fn set_drvdata(&self, data: impl ForeignOwnable) {
>> > +        // SAFETY: By the type invariants, `self.as_raw()` is a valid pointer to a `struct device`.
>> > +        unsafe { bindings::dev_set_drvdata(self.as_raw(), data.into_foreign().cast()) }
>> > +    }
>> > +
>> > +    /// Take ownership of the private data stored in this [`Device`].
>> > +    ///
>> > +    /// # Safety
>> > +    ///
>> > +    /// - Must only be called once after a preceding call to [`Device::set_drvdata`].
>> > +    /// - The type `T` must match the type of the `ForeignOwnable` previously stored by
>> > +    ///   [`Device::set_drvdata`].
>> > +    pub unsafe fn drvdata_obtain<T: ForeignOwnable>(&self) -> T {
>> > +        // SAFETY: By the type invariants, `self.as_raw()` is a valid pointer to a `struct device`.
>> > +        let ptr = unsafe { bindings::dev_get_drvdata(self.as_raw()) };
>> > +
>> > +        // SAFETY: By the safety requirements of this function, `ptr` comes from a previous call to
>> > +        // `into_foreign()`.
>> 
>> Well, you're also relying on `dev_get_drvdata` to return the same
>> pointer that was given to `dev_set_drvdata`.
>> 
>> Otherwise the safety docs look fine.
>
> Great! What do you think about:
>
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index 146eba147d2f..b01cb8e8dab3 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -80,8 +80,11 @@ pub unsafe fn drvdata_obtain<T: ForeignOwnable>(&self) -> T {
>          // SAFETY: By the type invariants, `self.as_raw()` is a valid pointer to a `struct device`.
>          let ptr = unsafe { bindings::dev_get_drvdata(self.as_raw()) };
>
> -        // SAFETY: By the safety requirements of this function, `ptr` comes from a previous call to
> -        // `into_foreign()`.
> +        // SAFETY:
> +        // - By the safety requirements of this function, `ptr` comes from a previous call to
> +        //   `into_foreign()`.
> +        // - `dev_get_drvdata()` guarantees to return the same pointer given to `dev_set_drvdata()`
> +        //   in `into_foreign()`.

Looks good, though I haven't done a full review, but you can have my:

Acked-by: Benno Lossin <lossin@...nel.org>

---
Cheers,
Benno

>          unsafe { T::from_foreign(ptr.cast()) }
>      }