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From: David Howells <dhowells@redhat.com>
In-Reply-To: <20240424191740.3088894-1-keescook@chromium.org>
References: <20240424191740.3088894-1-keescook@chromium.org> <20240424191225.work.780-kees@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: dhowells@redhat.com, Mark Rutland <mark.rutland@arm.com>,
    Will Deacon <will@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [PATCH 1/4] locking/atomic/x86: Silence intentional wrapping addition
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Content-ID: <2100616.1714117250.1@warthog.procyon.org.uk>
Date: Fri, 26 Apr 2024 08:40:50 +0100
Message-ID: <2100617.1714117250@warthog.procyon.org.uk>

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;

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