linux-kernel - Re: C aggregate passing (Rust kernel policy)

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Message-ID: <m4cbniqfsr5xpb2m7k53e7plc6he5ioyl2efiiftdmzod56usd@htwdppje6re5>
Date: Wed, 26 Feb 2025 17:27:05 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Martin Uecker <uecker@...raz.at>, Ralf Jung <post@...fj.de>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	Ventura Jack <venturajack85@...il.com>, Gary Guo <gary@...yguo.net>, airlied@...il.com, 
	boqun.feng@...il.com, david.laight.linux@...il.com, ej@...i.de, 
	gregkh@...uxfoundation.org, hch@...radead.org, hpa@...or.com, ksummit@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, miguel.ojeda.sandonis@...il.com, rust-for-linux@...r.kernel.org
Subject: Re: C aggregate passing (Rust kernel policy)

On Wed, Feb 26, 2025 at 01:14:30PM -0800, Linus Torvalds wrote:
> But dammit, doing things like "read the same variable twice even
> though the programmer only read it once" *IS* observable! It's
> observable as an actual security issue when it causes TOCTOU behavior
> that was introduced into the program by the compiler.

This is another one that's entirely eliminated due to W^X references.

IOW: if you're writing code where rematerializing reads is even a
_concern_ in Rust, then you had to drop to unsafe {} to do it - and your
code is broken, and yes it will have UB.