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

Mark Rutland
2021-10-22T18:52:01Z

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.

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

>
> 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;

Regardles of that cleanup:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>

I'll continue reviewing the series next week.

Thanks,
Mark.