linux-kernel - Re: [PATCH v5 2/6] arm64: ftrace: fix a stack tracer's output under function graph tracer

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Message-ID: <56415997.1040207@linaro.org>
Date:	Tue, 10 Nov 2015 11:42:31 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Jungseok Lee <jungseoklee85@...il.com>
Cc:	catalin.marinas@....com, will.deacon@....com, rostedt@...dmis.org,
	broonie@...nel.org, david.griego@...aro.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/6] arm64: ftrace: fix a stack tracer's output under
 function graph tracer

On 11/09/2015 11:04 PM, Jungseok Lee wrote:
> On Nov 6, 2015, at 3:44 PM, AKASHI Takahiro wrote:
>
> Hi Akashi,
>
>> Function graph tracer modifies a return address (LR) in a stack frame
>> to hook a function return. This will result in many useless entries
>> (return_to_handler) showing up in a stack tracer's output.
>>
>> This patch replaces such entries with originals values preserved in
>> current->ret_stack[].
>>
>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
>> ---
>> arch/arm64/include/asm/ftrace.h |    2 ++
>> arch/arm64/kernel/stacktrace.c  |   21 +++++++++++++++++++++
>> 2 files changed, 23 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
>> index c5534fa..3c60f37 100644
>> --- a/arch/arm64/include/asm/ftrace.h
>> +++ b/arch/arm64/include/asm/ftrace.h
>> @@ -28,6 +28,8 @@ struct dyn_arch_ftrace {
>>
>> extern unsigned long ftrace_graph_call;
>>
>> +extern void return_to_handler(void);
>> +
>> static inline unsigned long ftrace_call_adjust(unsigned long addr)
>> {
>> 	/*
>> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
>> index ccb6078..5fd3477 100644
>> --- a/arch/arm64/kernel/stacktrace.c
>> +++ b/arch/arm64/kernel/stacktrace.c
>> @@ -17,6 +17,7 @@
>>   */
>> #include <linux/kernel.h>
>> #include <linux/export.h>
>> +#include <linux/ftrace.h>
>> #include <linux/sched.h>
>> #include <linux/stacktrace.h>
>>
>> @@ -73,6 +74,9 @@ struct stack_trace_data {
>> 	struct stack_trace *trace;
>> 	unsigned int no_sched_functions;
>> 	unsigned int skip;
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	unsigned int ret_stack_index;
>> +#endif
>> };
>>
>> static int save_trace(struct stackframe *frame, void *d)
>> @@ -81,6 +85,20 @@ static int save_trace(struct stackframe *frame, void *d)
>> 	struct stack_trace *trace = data->trace;
>> 	unsigned long addr = frame->pc;
>>
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	if (addr == (unsigned long)return_to_handler - AARCH64_INSN_SIZE) {
>
> not if (adds == (unsigned long)return_to_handler)?
>
>> +		/*
>> +		 * This is a case where function graph tracer has
>> +		 * modified a return address (LR) in a stack frame
>> +		 * to hook a function return.
>> +		 * So replace it to an original value.
>> +		 */
>> +		frame->pc = addr =
>> +			current->ret_stack[data->ret_stack_index--].ret
>> +							- AARCH64_INSN_SIZE;
>
> Ditto. not without AARCH64_INSN_SIZE?
>
> I've observed many return_to_handler without the changes.
> Am I missing something?

You're right!
I thought I had tested the patches, but...

>> +	}
>> +#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
>> +
>> 	if (data->no_sched_functions && in_sched_functions(addr))
>> 		return 0;
>> 	if (data->skip) {
>> @@ -100,6 +118,9 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
>>
>> 	data.trace = trace;
>> 	data.skip = trace->skip;
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	data.ret_stack_index = current->curr_ret_stack;
>
> Can I get an idea on why current->curr_ret_stack is used instead of
> tsk->curr_ret_stack?

Thanks for pointing this out.
Will fix it although it works without a change since save_stack_trace_sp() is
called only in a 'current task' context.

-Takahiro AKASHI

> Best Regards
> Jungseok Lee
>
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