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SubjectRe: [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 <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?

Yes, thanks for catching this. v3 will be post soon.

Yang

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



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