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

From: Will Deacon <will@kernel.org>
To: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
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 14:13:00 +0100	[thread overview]
Message-ID: <20220428131259.GA14810@willie-the-truck> (raw)
In-Reply-To: <d3570761-1273-831f-dfbe-aefbadfa37f7@huawei.com>

On Thu, Apr 28, 2022 at 08:03:50PM +0800, 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))".

No, I disagree with you here, sorry.

> >>> +	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,
> the memory is dynamically allocated, exposing it is no different than exposing sp.

If show_unhandled_signals is set, then I think any fatal signal takes this
path, no?

Will

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