Re: [PATCH v10 04/11] arm64: Make return_address() use arch_stack_walk()

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: broonie@kernel.org, jpoimboe@redhat.com, ardb@kernel.org,
	nobuta.keiya@fujitsu.com, sjitindarsingh@gmail.com,
	catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org,
	linux-arm-kernel@lists.infradead.org,
	live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v10 04/11] arm64: Make return_address() use arch_stack_walk()
Date: Sat, 23 Oct 2021 07:51:50 -0500	[thread overview]
Message-ID: <9986ddf4-d992-ffd8-7032-a18fdbdb7bb1@linux.microsoft.com> (raw)
In-Reply-To: <20211022185148.GA91654@C02TD0UTHF1T.local>

On 10/22/21 1:51 PM, Mark Rutland wrote:
> On Thu, Oct 14, 2021 at 09:58:40PM -0500, madvenka@linux.microsoft.com wrote:
>> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>>
>> Currently, return_address() in ARM64 code walks the stack using
>> start_backtrace() and walk_stackframe(). Make it use arch_stack_walk()
>> instead. This makes maintenance easier.
>>
>> Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
>> ---
>>  arch/arm64/kernel/return_address.c | 6 +-----
>>  1 file changed, 1 insertion(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/return_address.c b/arch/arm64/kernel/return_address.c
>> index a6d18755652f..92a0f4d434e4 100644
>> --- a/arch/arm64/kernel/return_address.c
>> +++ b/arch/arm64/kernel/return_address.c
>> @@ -35,15 +35,11 @@ NOKPROBE_SYMBOL(save_return_addr);
>>  void *return_address(unsigned int level)
>>  {
>>  	struct return_address_data data;
>> -	struct stackframe frame;
>>  
>>  	data.level = level + 2;
>>  	data.addr = NULL;
>>  
>> -	start_backtrace(&frame,
>> -			(unsigned long)__builtin_frame_address(0),
>> -			(unsigned long)return_address);
>> -	walk_stackframe(current, &frame, save_return_addr, &data);
>> +	arch_stack_walk(save_return_addr, &data, current, NULL);
> 
> This looks equivalent to me. Previously the arguments to
> start_backtrace() meant that walk_stackframe would report
> return_address(), then the caller of return_address(), and so on. As
> arch_stack_walk() starts from its immediate caller (i.e.
> return_address()), that should result in the same trace.
> 
> It would be nice if we could note something to that effect in the commit
> message.
> 

Will do.

> I had a play with ftrace, which uses return_address(), and that all
> looks sound.
> 

Thanks a lot!

>>  
>>  	if (!data.level)
>>  		return data.addr;
> 
> The end of this function currently does:
> 
> 	if (!data.level)
> 		return data.addr;
> 	else
> 		return NULL;
> 
> ... but since we initialize data.addr to NULL, and save_return_addr()
> only writes to data.addr when called at the correct level, we can
> simplify that to:
> 
> 	return data.addr;
> 

OK. I will make this change.

> Regardles of that cleanup:
> 
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>
> Tested-by: Mark Rutland <mark.rutland@arm.com>
> 

Thanks a lot!

> I'll continue reviewing the series next week.
> 

Great!

Madhavan