Subject: [PATCH] arm64: bpf: fix buffer pointer

From: [email-masked] (Shi, Yang)
Date: Wed, 18 Nov 2015 14:59:17 -0800

On 11/18/2015 1:41 PM, Z Lim wrote:
> On Wed, Nov 18, 2015 at 1:07 PM, Shi, Yang <[email-masked]> 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 <[email-masked]>

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 :)
>