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

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Message-ID: <56461DCD.4020701@linaro.org>
Date:	Fri, 13 Nov 2015 09:28:45 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Z Lim <zlim.lnx@...il.com>
CC:	Alexei Starovoitov <ast@...nel.org>, daniel@...earbox.net,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>, Xi Wang <xi.wang@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linaro-kernel@...ts.linaro.org
Subject: 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 <yang.shi@...aro.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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