Re: [PATCH v3 1/7] arm64: ftrace: adjust callsite addresses examined by stack tracer

[Jungseok Lee <jungseoklee85@gmail.com>]

On Oct 8, 2015, at 7:01 PM, AKASHI Takahiro wrote:

Hi Akashi,

> On arm64, no PC values returned by save_stack_trace() will match to LR
> values saved in stack frames on a stack after the following commit:
>    commit e306dfd06fcb ("ARM64: unwind: Fix PC calculation")
> As a result, the output from stack tracer will be messed up.
> 
> This patch introduces an arch-defined macro, FTRACE_STACK_FRAME_OFFSET,
> so that check_stack() can handle this case correctly.
> 
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> ---
> arch/arm64/include/asm/ftrace.h |    5 +++--
> arch/arm64/kernel/stacktrace.c  |    7 ++++---
> include/linux/ftrace.h          |    7 +++++++
> kernel/trace/trace_stack.c      |    5 +++--
> 4 files changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index c5534fa..2b43e20 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -13,8 +13,9 @@
> 
> #include <asm/insn.h>
> 
> -#define MCOUNT_ADDR		((unsigned long)_mcount)
> -#define MCOUNT_INSN_SIZE	AARCH64_INSN_SIZE
> +#define MCOUNT_ADDR			((unsigned long)_mcount)
> +#define MCOUNT_INSN_SIZE		AARCH64_INSN_SIZE
> +#define FTRACE_STACK_FRAME_OFFSET	AARCH64_INSN_SIZE
> 
> #ifndef __ASSEMBLY__
> #include <linux/compat.h>
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index 407991b..bc0689a 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -20,6 +20,7 @@
> #include <linux/sched.h>
> #include <linux/stacktrace.h>
> 
> +#include <asm/insn.h>
> #include <asm/stacktrace.h>
> 
> /*
> @@ -49,10 +50,10 @@ int notrace unwind_frame(struct stackframe *frame)
> 	frame->sp = fp + 0x10;
> 	frame->fp = *(unsigned long *)(fp);
> 	/*
> -	 * -4 here because we care about the PC at time of bl,
> -	 * not where the return will go.
> +	 * decrement PC by AARCH64_INSN_SIZE here because we care about
> +	 * the PC at time of bl, not where the return will go.
> 	 */
> -	frame->pc = *(unsigned long *)(fp + 8) - 4;
> +	frame->pc = *(unsigned long *)(fp + 8) - AARCH64_INSN_SIZE;
> 
> 	return 0;
> }
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 6cd8c0e..d77b195 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -263,6 +263,13 @@ static inline void ftrace_kill(void) { }
> #endif /* CONFIG_FUNCTION_TRACER */
> 
> #ifdef CONFIG_STACK_TRACER
> +/*
> + * the offset value to add to return address from save_stack_trace()
> + */
> +#ifndef FTRACE_STACK_FRAME_OFFSET
> +#define FTRACE_STACK_FRAME_OFFSET 0
> +#endif
> +
> extern int stack_tracer_enabled;
> int
> stack_trace_sysctl(struct ctl_table *table, int write,
> diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> index b746399..30521ea 100644
> --- a/kernel/trace/trace_stack.c
> +++ b/kernel/trace/trace_stack.c
> @@ -105,7 +105,7 @@ check_stack(unsigned long ip, unsigned long *stack)
> 
> 	/* Skip over the overhead of the stack tracer itself */
> 	for (i = 0; i < max_stack_trace.nr_entries; i++) {
> -		if (stack_dump_trace[i] == ip)
> +		if ((stack_dump_trace[i] + FTRACE_STACK_FRAME_OFFSET) == ip)
> 			break;
> 	}
> 
> @@ -133,7 +133,8 @@ check_stack(unsigned long ip, unsigned long *stack)
> 		for (; p < top && i < max_stack_trace.nr_entries; p++) {
> 			if (stack_dump_trace[i] == ULONG_MAX)
> 				break;
> -			if (*p == stack_dump_trace[i]) {
> +			if (*p == (stack_dump_trace[i]
> +					+ FTRACE_STACK_FRAME_OFFSET)) {
> 				stack_dump_trace[x] = stack_dump_trace[i++];
> 				this_size = stack_dump_index[x++] =
> 					(top - p) * sizeof(unsigned long);
> -- 

This change is always on my tree for IRQ stack feature development. It makes
stack_trace prints out useful info although it can't give an accurate data.
I believe this hunk helps to figure out max stack depth context.

Acked-by: Jungseok Lee <jungseoklee85@gmail.com>

Thanks!

Best Regards
Jungseok Lee