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Message-ID: <20161111230750.GP3157@twins.programming.kicks-ass.net>
Date: Sat, 12 Nov 2016 00:07:50 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Mark Rutland <mark.rutland@....com>
Cc: kernel-hardening@...ts.openwall.com, Kees Cook <keescook@...omium.org>,
Greg KH <gregkh@...uxfoundation.org>,
Will Deacon <will.deacon@....com>,
Elena Reshetova <elena.reshetova@...el.com>,
Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <h.peter.anvin@...el.com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: Re: [RFC v4 PATCH 00/13] HARDENED_ATOMIC
On Fri, Nov 11, 2016 at 02:00:34PM +0100, Peter Zijlstra wrote:
> +static inline bool refcount_sub_and_test(int i, refcount_t *r)
> +{
> + unsigned int old, new, val = atomic_read(&r->refs);
> +
> + for (;;) {
regardless of the sub_and_test vs inc_and_test issue, this should
probably also have:
if (val == UINT_MAX)
return false;
such that we stay saturated. If for some reason someone can trigger more
dec's than inc's, we'd be hosed.
> + new = val - i;
> + if (new > val)
> + BUG(); /* underflow */
> +
> + old = atomic_cmpxchg_release(&r->refs, val, new);
> + if (old == val)
> + break;
> +
> + val = old;
> + }
> +
> + return !new;
> +}
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