linux-kernel - Re: [PATCH] rust/dma: Take &mut self in CoherentAllocation::field

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Message-ID: <aQE7KliosIU_0Bll@google.com>
Date: Tue, 28 Oct 2025 21:52:42 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Danilo Krummrich <dakr@...nel.org>, 
	Abdiel Janulgue <abdiel.janulgue@...il.com>, Daniel Almeida <daniel.almeida@...labora.com>, 
	Robin Murphy <robin.murphy@....com>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>
Subject: Re: [PATCH] rust/dma: Take &mut self in CoherentAllocation::field_write()

On Tue, Oct 28, 2025 at 05:18:01PM -0400, Lyude Paul wrote:
> At the moment - CoherentAllocation::field_write() only takes an immutable
> reference to self. This means it's possible for a user to mistakenly call
> field_write() while Rust still has a slice taken out for the coherent
> allocation:
> 
>   let alloc: CoherentAllocation<CoolStruct> = /* … */;
> 
>   let evil_slice = unsafe { alloc.as_slice(/* … */)? };
>   dma_write!(alloc[1].cool_field = 42); /* UB! */
> 
> Keep in mind: the above example is technically a violation of the safety
> contract of as_slice(), so luckily this detail shouldn't currently be
> causing any UB in the kernel. But, there's no reason we should be solely
> relying on the safety contract for enforcing this when we can just use a
> mutable reference and already do so in other parts of the API.
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Cc: Danilo Krummrich <dakr@...nel.org>

Didn't we do this intentionally so that it's possible to write to
different parts of the allocation without protecting the entire region
with a lock?

Alice