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

On 11/12/2015 7:28 PM, Z Lim wrote:

On Thu, Nov 12, 2015 at 1:57 PM, Yang Shi [off-list ref] wrote:

quoted

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 |
           | .. |