Re: [PATCH 2/2] arm64: bpf: make BPF prologue and epilogue align with ARM64 AAPCS

[Shi, Yang]

On 11/12/2015 7:28 PM, Z Lim wrote:
> On Thu, Nov 12, 2015 at 1:57 PM, Yang Shi <yang.shi@xxxxxxxxxx> wrote:
> > Save and restore FP/LR in BPF prog prologue and epilogue, save SP to FP
> > in prologue in order to get the correct stack backtrace.
> >
> > However, ARM64 JIT used FP (x29) as eBPF fp register, FP is subjected to
> > change during function call so it may cause the BPF prog stack base address
> > change too.
> >
> > Use x25 to replace FP as BPF stack base register (fp). Since x25 is callee
> > saved register, so it will keep intact during function call.
> > It is initialized in BPF prog prologue when BPF prog is started to run
> > everytime. When BPF prog exits, it could be just tossed.
> >
> > So, the BPF stack layout looks like:
> >
> >                                   high
> >           original A64_SP =>   0:+-----+ BPF prologue
> >                                  |     | FP/LR and callee saved registers
> >           BPF fp register => -64:+-----+
> >                                  |     |
> >                                  | ... | BPF prog stack
> >                                  |     |
> >                                  |     |
> >           current A64_SP/FP =>   +-----+
> >                                  |     |
> >                                  | ... | Function call stack
> >                                  |     |
> >                                  +-----+
> >                                    low
>
> Yang, for stack unwinding to work, shouldn't it be something like the following?

Yes, thanks for catching this. v3 will be post soon.

Yang

>            | LR |
> A64_FP => | FP |
>            | .. |

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