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

From: "Shi, Yang" <[email-masked]>
Date: Fri, 13 Nov 2015 09:28:45 -0800

On 11/12/2015 7:28 PM, Z Lim wrote:
> On Thu, Nov 12, 2015 at 1:57 PM, Yang Shi <[email-masked]> 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 |
>            | .. |