Re: [PATCH 1/4] locking/atomic/x86: Silence intentional wrapping addition

From: Kees Cook <keescook@chromium.org>
To: David Howells <dhowells@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	"David S. Miller" <davem@davemloft.net>,
	David Ahern <dsahern@kernel.org>,
	Eric Dumazet <edumazet@google.com>,
	Paolo Abeni <pabeni@redhat.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Uros Bizjak <ubizjak@gmail.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
	netdev@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH 1/4] locking/atomic/x86: Silence intentional wrapping addition
Date: Thu, 2 May 2024 07:57:02 -0700	[thread overview]
Message-ID: <202405020755.0101DBCB@keescook> (raw)
In-Reply-To: <2100617.1714117250@warthog.procyon.org.uk>

On Fri, Apr 26, 2024 at 08:40:50AM +0100, David Howells wrote:
> Kees Cook <keescook@chromium.org> wrote:
> 
> > -	return i + xadd(&v->counter, i);
> > +	return wrapping_add(int, i, xadd(&v->counter, i));
> 
> Ewww.  Can't you just mark the variable as wrapping in some way, either by:
> 
> 	unsigned int __cyclic counter;

Yeah, that's the plan now. Justin is currently working on the "wraps"
attribute for Clang:
https://github.com/llvm/llvm-project/pull/86618

-- 
Kees Cook