Re: [PATCH v2] arm64: add the printing of tpidr_elx in __show_regs()

From: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
To: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>,
	"James Morse" <james.morse@arm.com>
Subject: Re: [PATCH v2] arm64: add the printing of tpidr_elx in __show_regs()
Date: Thu, 28 Apr 2022 20:10:42 +0800	[thread overview]
Message-ID: <367e3bda-1541-2f9c-5b63-9820f03041e6@huawei.com> (raw)
In-Reply-To: <d3570761-1273-831f-dfbe-aefbadfa37f7@huawei.com>

On 2022/4/28 20:03, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/4/28 19:07, Leizhen (ThunderTown) wrote:
>>
>>
>> On 2022/4/28 18:21, Will Deacon wrote:
>>> On Wed, Mar 16, 2022 at 02:24:08PM +0800, Zhen Lei wrote:
>>>> Commit 7158627686f0 ("arm64: percpu: implement optimised pcpu access
>>>> using tpidr_el1") and commit 6d99b68933fb ("arm64: alternatives: use
>>>> tpidr_el2 on VHE hosts") use tpidr_elx to cache my_cpu_offset to optimize
>>>> pcpu access. However, when performing reverse execution based on the
>>>> registers and the memory contents in kdump, this information is sometimes
>>>> required if there is a pcpu access.
>>>>
>>>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>>>> ---
>>>>  arch/arm64/kernel/process.c | 11 +++++++++++
>>>>  1 file changed, 11 insertions(+)
>>>>
>>>> v1 --> v2:
>>>> Directly print the tpidr_elx register of the current exception level.
>>>> Avoid coupling with the implementation of 'my_cpu_offset'.
>>>>
>>>> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
>>>> index 5369e649fa79ff8..738932e6fa4e947 100644
>>>> --- a/arch/arm64/kernel/process.c
>>>> +++ b/arch/arm64/kernel/process.c
>>>> @@ -216,6 +216,17 @@ void __show_regs(struct pt_regs *regs)
>>>>  	show_regs_print_info(KERN_DEFAULT);
>>>>  	print_pstate(regs);
>>>>  
>>>> +	switch (read_sysreg(CurrentEL)) {
>>>
>>> This should use is_kernel_in_hyp_mode() to detect if we're running at El2.
> 
> static inline bool is_kernel_in_hyp_mode(void)
> {
>         return read_sysreg(CurrentEL) == CurrentEL_EL2;
> }
> 
> I think it's more intuitive to use "switch (read_sysreg(CurrentEL))".
> 
>>>
>>>> +	case CurrentEL_EL1:
>>>> +		printk("tpidr_el1 : %016llx\n", read_sysreg(TPIDR_EL1));
>>>> +		break;
>>>> +	case CurrentEL_EL2:
>>>> +		printk("tpidr_el2 : %016llx\n", read_sysreg(TPIDR_EL2));
>>>> +		break;
>>>> +	default:
>>>> +		break;
>>>> +	}
>>>
>>> I think this path can be triggered directly from usermode, so we really
>>> shouldn't be printing raw kernel virtual addresses here.
>>
>> I run echo c > /proc/sysrq-trigger and didn't trigger this path, but maybe
>> there's another way. Analysis from the other side, except for the instruction
>> address, all generic registers r0-r31 is output as raw. There's also an
>> opportunity to contain the instruction address.
> 
> On second thought, there seemed to be nothing wrong with it. The user need
> to have capable() first. Then the address of the perpcu memory is not static,

Sorry, __my_cpu_offset is just an offset.

> the memory is dynamically allocated, exposing it is no different than exposing sp.
> 
>>
>> So how about:
>> +       if (oops_in_progress)
>> +               printk("tpidr : %016lx\n", __my_cpu_offset);
>>
>>>
>>> Will
>>> .
>>>
>>
> 

-- 
Regards,
  Zhen Lei

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