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Message-ID: <7e7918c6-7186-47f4-4b37-2fc55db1e678@huawei.com> 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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