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

On Thu, Nov 12, 2015 at 1:57 PM, Yang Shi yang.shi@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 | | .. |