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Date: Tue, 25 Jul 2017 20:03:08 +0800
From: Li Kun <hw.likun@...wei.com>
To: Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...nel.org>
CC: Peter Zijlstra <peterz@...radead.org>,
Josh Poimboeuf
<jpoimboe@...hat.com>,
Christoph Hellwig <hch@...radead.org>,
"Eric W.
Biederman" <ebiederm@...ssion.com>,
Andrew Morton
<akpm@...ux-foundation.org>,
Jann Horn <jannh@...gle.com>, Eric Biggers
<ebiggers3@...il.com>,
Elena Reshetova <elena.reshetova@...el.com>,
"Hans
Liljestrand" <ishkamiel@...il.com>,
Greg KH <gregkh@...uxfoundation.org>,
Alexey Dobriyan <adobriyan@...il.com>,
"Serge E. Hallyn" <serge@...lyn.com>, <arozansk@...hat.com>,
Davidlohr Bueso <dave@...olabs.net>,
Manfred Spraul
<manfred@...orfullife.com>,
"axboe@...nel.dk" <axboe@...nel.dk>,
"James
Bottomley" <James.Bottomley@...senpartnership.com>,
"x86@...nel.org"
<x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
"David S. Miller"
<davem@...emloft.net>,
Rik van Riel <riel@...hat.com>, <linux-kernel@...r.kernel.org>,
linux-arch <linux-arch@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v8 3/3] x86/refcount: Implement fast
refcount overflow protection
Hi Kees,
on 2017/7/25 2:35, Kees Cook wrote:
> +static __always_inline __must_check
> +int __refcount_add_unless(refcount_t *r, int a, int u)
> +{
> + int c, new;
> +
> + c = atomic_read(&(r->refs));
> + do {
> + if (unlikely(c == u))
> + break;
> +
> + asm volatile("addl %2,%0\n\t"
> + REFCOUNT_CHECK_LT_ZERO
> + : "=r" (new)
> + : "0" (c), "ir" (a),
> + [counter] "m" (r->refs.counter)
> + : "cc", "cx");
here when the result LT_ZERO, you will saturate the r->refs.counter and
make the
atomic_try_cmpxchg(&(r->refs), &c, new) bound to fail first time.
maybe we can just saturate the value of variable "new" ?
> +
> + } while (!atomic_try_cmpxchg(&(r->refs), &c, new));
> +
> + return c;
> +}
> +
--
Best Regards
Li Kun
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