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

[Z Lim]

On Thu, Nov 12, 2015 at 1:57 PM, Yang Shi <yang.shi at linaro.org> 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?

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