linux-kernel - Re: [PATCH v12 1/3] rust: io: add resource abstraction

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Message-ID: <aG9pPW-cjnPQouBp@google.com>
Date: Thu, 10 Jul 2025 07:18:21 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Daniel Almeida <daniel.almeida@...labora.com>
Cc: 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>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, 
	"Ilpo Järvinen" <ilpo.jarvinen@...ux.intel.com>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	Mika Westerberg <mika.westerberg@...ux.intel.com>, Ying Huang <huang.ying.caritas@...il.com>, 
	Benno Lossin <lossin@...nel.org>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, Fiona Behrens <me@...enk.dev>
Subject: Re: [PATCH v12 1/3] rust: io: add resource abstraction

On Tue, Jul 08, 2025 at 02:43:33PM -0300, Daniel Almeida wrote:
> > What you wrote is not equivalent. (*inner) is a place expression, but
> > when you write `unsafe { <place expr> }` that turns it into a value
> > expression by reading the value. So this code copies the entire struct
> > to the stack, and then you read `start` from the copy on the stack.
> 
> To be honest, I've seen a bug where the opposite was going on, a field was
> being written on the value that was copied to the stack, leaving the original unchanged.
> 
> In any case, I thought this would be optimized away by the compiler. Also, IMHO
> there should be a lint for this, because it does not make sense to copy a struct
> just to read a field if we can just read the original location.
> 
> Thanks for catching that though, I will fix it on the next iteration :)

Yes, it is probably optimized out. However, the main problem is that
when you read the entire struct, that could be a data race of another
field which may be UB.

Alice