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