linux-kernel - Re: [PATCH][V3] arm64: perf: Get the wrong PC value in REGS_ABI

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Date:   Wed, 27 May 2020 09:30:01 +0800
From:   Jiping Ma <Jiping.Ma2@...driver.com>
To:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>
Cc:     zhe.he@...driver.com, bruce.ashfield@...il.com,
        yue.tao@...driver.com, will.deacon@....com,
        linux-kernel@...r.kernel.org, paul.gortmaker@...driver.com,
        catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH][V3] arm64: perf: Get the wrong PC value in REGS_ABI_32
 mode



On 05/27/2020 03:54 AM, Will Deacon wrote:
> On Tue, May 26, 2020 at 11:26:11AM +0100, Mark Rutland wrote:
>> On Mon, May 11, 2020 at 10:52:07AM +0800, Jiping Ma wrote:
>>> Modified the patch subject and the change description.
>>>
>>> PC value is get from regs[15] in REGS_ABI_32 mode, but correct PC
>>> is regs->pc(regs[PERF_REG_ARM64_PC]) in arm64 kernel, which caused
>>> that perf can not parser the backtrace of app with dwarf mode in the
>>> 32bit system and 64bit kernel.
>>>
>>> Signed-off-by: Jiping Ma <jiping.ma2@...driver.com>
>> Thanks for this.
>>
>>
>>> ---
>>>   arch/arm64/kernel/perf_regs.c | 4 ++++
>>>   1 file changed, 4 insertions(+)
>>>
>>> diff --git a/arch/arm64/kernel/perf_regs.c b/arch/arm64/kernel/perf_regs.c
>>> index 0bbac61..0ef2880 100644
>>> --- a/arch/arm64/kernel/perf_regs.c
>>> +++ b/arch/arm64/kernel/perf_regs.c
>>> @@ -32,6 +32,10 @@ u64 perf_reg_value(struct pt_regs *regs, int idx)
>>>   	if ((u32)idx == PERF_REG_ARM64_PC)
>>>   		return regs->pc;
>>>   
>>> +	if (perf_reg_abi(current) == PERF_SAMPLE_REGS_ABI_32
>>> +		&& idx == 15)
>>> +		return regs->pc;
>> I think there are some more issues here, and we may need a more
>> substantial rework. For a compat thread, we always expose
>> PERF_SAMPLE_REGS_ABI_32 via per_reg_abi(), but for some reason
>> perf_reg_value() also munges the compat SP/LR into their ARM64
>> equivalents, which don't exist in the 32-bit sample ABI. We also don't
>> zero the regs that don't exist in 32-bit (including the aliasing PC).
> I think this was for the case where you have a 64-bit perf profiling a
> 32-bit task, and it was passing the registers off to libunwind. Won't that
> break if we follow your suggestion?
Yes, it is for 64-bit perf profiling a 32-bit task, not for a compat thread.

>
> Will
>