Re: [RFC v2 4/4] arm64: ftrace: add a stack frame for exception handler

[Jungseok Lee <jungseoklee85@gmail.com>]

On Aug 4, 2015, at 4:44 PM, AKASHI Takahiro wrote:

Hi Akashi,

> On arm64, an exception handler use the same stack as in non-exception
> contexts, but doesn't create a stack frame for elx_xx entry, only updating
> sp register. This behavior results in save_stace_trace() missing a function
> that is the one when an exception happens.
> 
> This patch creates a stack frame for this case, and puts an additional
> entry for the function  in a stack trace list.
> 
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> ---
> arch/arm64/kernel/entry.S      |    4 ++++
> arch/arm64/kernel/stacktrace.c |   17 +++++++++++++++++
> 2 files changed, 21 insertions(+)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index f860bfd..aacb6c6 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -107,6 +107,8 @@
> 	str	x21, [sp, #S_SYSCALLNO]
> 	.endif
> 
> +	/* create a stack frame for stack tracer */
> +	mov	x29, sp
> 	/*
> 	 * Registers that may be useful after this macro is invoked:
> 	 *
> @@ -737,3 +739,5 @@ ENTRY(sys_rt_sigreturn_wrapper)
> 	mov	x0, sp
> 	b	sys_rt_sigreturn
> ENDPROC(sys_rt_sigreturn_wrapper)
> +
> +ENTRY(end_of_vectors)
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index d1790eb..22ce7c9 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -25,6 +25,10 @@
> #include <asm/stacktrace.h>
> 
> #define S_FRAME_SIZE sizeof(struct pt_regs) /* asm-offsets.h */
> +#define S_FP offsetof(struct pt_regs, regs[29])
> +#define S_LR offsetof(struct pt_regs, regs[30])
> +
> +extern unsigned int *vectors, *end_of_vectors;
> 
> /*
>  * AArch64 PCS assigns the frame pointer to x29.
> @@ -50,6 +54,19 @@ int notrace unwind_frame(struct stackframe *frame)
> 	if (fp < low || fp > high - 0x18 || fp & 0xf)
> 		return -EINVAL;
> 
> +	if ((frame->pc >= (unsigned long)&vectors) &&
> +			(frame->pc < (unsigned long)&end_of_vectors)) {
> +		/*
> +		 * Expection handler does not use a normal format of
> +		 * stack frame, but allocates struct pt_regs.
> +		 */
> +		frame->sp = frame->sp + S_FRAME_SIZE;
> +		frame->fp = *(unsigned long *)(fp + S_FP);
> +		frame->pc = *(unsigned long *)(fp + S_LR);

Not frame->pc = *(unsigned long *)(fp + S_PC)? Don't we need to look up elr_el1
since this is an exception?

> +
> +		return 0;
> +	}
> +
> 	frame->sp = fp + 0x10;

I'm just curious about this constant, 0x10. Do you have an idea on this value?
As reviewing objdump of vmlinux, it looks needed to analyze the first store-pair
instruction of each function.

Please correct me if I'm wrong.

Best Regards
Jungseok Lee