linux-kernel - Re: [PATCH v2 6/9] rust: device: implement Bound device context

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Message-ID: <D96AXNJRUAA0.3E5KYNM5PZZPG@proton.me>
Date: Mon, 14 Apr 2025 10:49:49 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>, bhelgaas@...gle.com, kwilczynski@...nel.org, gregkh@...uxfoundation.org, rafael@...nel.org, abdiel.janulgue@...il.com
Cc: ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu, daniel.almeida@...labora.com, robin.murphy@....com, linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/9] rust: device: implement Bound device context

On Sun Apr 13, 2025 at 7:37 PM CEST, Danilo Krummrich wrote:
> The Bound device context indicates that a device is bound to a driver.
> It must be used for APIs that require the device to be bound, such as
> Devres or dma::CoherentAllocation.
>
> Implement Bound and add the corresponding Deref hierarchy, as well as the
> corresponding ARef conversion for this device context.
>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> ---
>  rust/kernel/device.rs | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> index 487211842f77..585a3fcfeea3 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -232,13 +232,19 @@ pub trait DeviceContext: private::Sealed {}
>  /// any of the bus callbacks, such as `probe()`.
>  pub struct Core;
>  
> +/// The [`Bound`] context is the context of a bus specific device reference when it is guranteed to
> +/// be bound for the duration of its lifetime.
> +pub struct Bound;

One question about this: is it possible for me to
1. have access to a `ARef<Device<Bound>>` (or `Core`) via some callback,
2. store a clone of the `ARef` in some datastructure,
3. wait for the device to become unbound,
4. use a `Bound`-only context function and blow something up?

Depending on the severity of the "blow something up" we probably need to
change the design. If it's "only a bug" (and not a memory
vulnerability), then this is fine, since people should then "just not do
that" (and I think this design makes that painfully obvious when someone
tries to do something funny with a `Device<Bound>`).

---
Cheers,
Benno