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Message-ID: <b3c2f9b4-8cf2-6290-ad2b-2acbbb3b28cd@huawei.com>
Date: Wed, 26 Jul 2017 11:32:55 +0800
From: Li Kun <hw.likun@...wei.com>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC: <linux-arm-kernel@...ts.infradead.org>,
<kernel-hardening@...ts.openwall.com>, <will.deacon@....com>,
<keescook@...omium.org>, <mark.rutland@....com>,
<labbott@...oraproject.org>
Subject: Re: [PATCH v2] arm64: kernel: implement fast refcount checking
Hi Ard,
on 2017/7/26 2:15, Ard Biesheuvel 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(
> + "adds %0, %0, %2 ;"
> + REFCOUNT_CHECK(lt)
> + : "=r" (new)
> + : "0" (c), "Ir" (a),
> + [counter] "r" (&r->refs.counter),
> + [brk_imm] "i" (REFCOUNT_BRK_IMM)
> + : "cc", "x16", "x17");
> +
> + } while (!atomic_try_cmpxchg(&(r->refs), &c, new));
> +
> + return c;
> +}
> +
As my reply to Kee's v8 patch, the logic here is a little incorrect, if
we saturate the
r->refs.counter and return from the exception, the value of "new" is not changed
and will be written back to the refcount.
I implement the logic like this, maybe you can take some reference if you find it usefull.
int __refcount_add_unless(refcount_t *r, int a, int u)
{
int c;
volatile int new;
c = atomic_read(&(r->refs));
do {
if (unlikely(c == u))
break;
asm volatile("adds %w0,%w1,%w2\n"
: "=&r" (new)
: "r" (c), "Ir" (a)
: "memory");
asm volatile(REFCOUNT_CHECK_LT_ZERO
::[imm] "i"(REFCOUNT_BRK_IMM),[counter] "r" (&new)
: "cc", "x19");
} while (!atomic_try_cmpxchg(&(r->refs), &c, new));
return c;
}
> /* This registration must happen early, before debug_traps_init(). */
> void __init trap_init(void)
> {
> register_break_hook(&bug_break_hook);
> + register_break_hook(&refcount_break_hook);
> }
> diff --git a/arch/arm64/lib/atomic_ll_sc.c b/arch/arm64/lib/atomic_ll_sc.c
> index b0c538b0da28..5f038abdc635 100644
> --- a/arch/arm64/lib/atomic_ll_sc.c
> +++ b/arch/arm64/lib/atomic_ll_sc.c
> @@ -1,3 +1,9 @@
> #include <asm/atomic.h>
> #define __ARM64_IN_ATOMIC_IMPL
> +#undef REFCOUNT_CHECK
> +#undef REFCOUNT_INPUTS
> +#undef REFCOUNT_CLOBBERS
> +#define REFCOUNT_CHECK(cond)
> +#define REFCOUNT_INPUTS(r)
> +#define REFCOUNT_CLOBBERS : "cc"
> #include <asm/atomic_ll_sc.h>
--
Best Regards
Li Kun
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