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

[AKASHI Takahiro]

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@xxxxxxxxxx>
> > ---
> > 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/