Message-ID: <564D02C5.1020200@linaro.org>
Date: Wed, 18 Nov 2015 14:59:17 -0800
From: "Shi, Yang" <yang.shi@linaro.org>
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To: Z Lim <zlim.lnx@gmail.com>
CC: "David S. Miller" <davem@davemloft.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Daniel Borkmann <daniel@iogearbox.net>, Xi Wang <xi.wang@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        Network Development <netdev@vger.kernel.org>
Subject: Re: [PATCH] arm64: bpf: fix buffer pointer
References: <1447836962-4086-1-git-send-email-zlim.lnx@gmail.com>	<564CE886.5010109@linaro.org> <CABg9mcud=quxDbAGwXo0CyHjDa4aOL_yuk9hwRp1asu-z+fOAQ@mail.gmail.com>
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On 11/18/2015 1:41 PM, Z Lim wrote:
> On Wed, Nov 18, 2015 at 1:07 PM, Shi, Yang <yang.shi@linaro.org> wrote:
>> On 11/18/2015 12:56 AM, Zi Shen Lim wrote:
>>>                  emit_a64_mov_i64(r3, size, ctx);
>>> -               emit(A64_ADD_I(1, r4, fp, MAX_BPF_STACK), ctx);
>>> +               emit(A64_SUB_I(1, r4, fp, STACK_SIZE), ctx);
>>
>>
>> Should not it sub MAX_BPF_STACK?
>
> No, if it's at (BPF_FP - MAX_BPF_STACK), we'll be writing into the BPF
> stack area, which should only be used by the BPF program.
>
>> If you sub STACK_SIZE here, the buffer pointer will point to bottom of the
>> reserved area.
>
> Yes, that's the idea. The buffer is allocated in here. Right now we're
> using this "reserved" space for this buffer only.

OK, I see. The buffer grows from low to high.

Thanks for the elaboration.

Acked-by: Yang Shi <yang.shi@linaro.org>

Yang

>
>>
>> You stack layout change also shows this:
>>
>> +        *                        +-----+ <= (BPF_FP - MAX_BPF_STACK)
>> +        *                        |RSVD | JIT scratchpad
>> +        * current A64_SP =>      +-----+ <= (BPF_FP - STACK_SIZE)
>
> Yes, this diagram reflects the code and intention.
>
>
> Thanks for reviewing, we definitely need more of these :)
>

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