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

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Date: Fri, 26 Apr 2024 08:40:50 +0100
From: David Howells <dhowells@...hat.com>
To: Kees Cook <keescook@...omium.org>
Cc: dhowells@...hat.com, Mark Rutland <mark.rutland@....com>,
    Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
    Boqun Feng <boqun.feng@...il.com>,
    Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
    Borislav Petkov <bp@...en8.de>,
    Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
    "H. Peter Anvin" <hpa@...or.com>, Jakub Kicinski <kuba@...nel.org>,
    Catalin Marinas <catalin.marinas@....com>,
    Arnd Bergmann <arnd@...db.de>,
    Andrew Morton <akpm@...ux-foundation.org>,
    "David S. Miller" <davem@...emloft.net>,
    David Ahern <dsahern@...nel.org>, Eric Dumazet <edumazet@...gle.com>,
    Paolo Abeni <pabeni@...hat.com>,
    "Paul E. McKenney" <paulmck@...nel.org>,
    Uros Bizjak <ubizjak@...il.com>, linux-kernel@...r.kernel.org,
    linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
    netdev@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/4] locking/atomic/x86: Silence intentional wrapping addition

Kees Cook <keescook@...omium.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;

or, to use rxrpc packet sequence numbers as an example:

	typedef unsigned int __cyclic rxrpc_seq_t;

	rxrpc_seq_t	tx_top;

Then you can get the compiler to spit out a warning if you use <, <=, > or >=
on the numbers as an added bonus.  (You should use before() and after()
instead).

David