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

Madhavan T. Venkataraman
2021-10-23T12:51:56Z

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