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

[Shi, Yang]

On 11/18/2015 1:41 PM, Z Lim wrote:
> On Wed, Nov 18, 2015 at 1:07 PM, Shi, Yang <yang.shi@xxxxxxxxxx> 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@xxxxxxxxxx>

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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