Subject: [PATCH v6 3/6] arm64: ftrace: fix a stack tracer's output under function graph tracer

From: [email-masked] (AKASHI Takahiro)
Date: Wed, 25 Nov 2015 14:29:36 +0900

On 11/24/2015 10:37 PM, Jungseok Lee wrote:
> On Nov 18, 2015, at 3:43 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) a stack tracer's output
>> b) perf call graph (with perf record -g)
>> c) dump_backtrace (at panic et al.)
>>
>> For example, in case of a),
>>   $ echo function_graph > /sys/kernel/debug/tracing/current_tracer
>>   $ echo 1 > /proc/sys/kernel/stack_trace_enabled
>>   $ cat /sys/kernel/debug/tracing/stack_trace
>>         Depth    Size   Location    (54 entries)
>>         -----    ----   --------
>>   0)     4504      16   gic_raise_softirq+0x28/0x150
>>   1)     4488      80   smp_cross_call+0x38/0xb8
>>   2)     4408      48   return_to_handler+0x0/0x40
>>   3)     4360      32   return_to_handler+0x0/0x40
>>   ...
>>
>> In case of b),
>>   $ echo function_graph > /sys/kernel/debug/tracing/current_tracer
>>   $ perf record -e mem:XXX:x -ag -- sleep 10
>>   $ perf report
>>                   ...
>>                   |          |          |--0.22%-- 0x550f8
>>                   |          |          |          0x10888
>>                   |          |          |          el0_svc_naked
>>                   |          |          |          sys_openat
>>                   |          |          |          return_to_handler
>>                   |          |          |          return_to_handler
>>                   ...
>>
>> In case of c),
>>   $ echo function_graph > /sys/kernel/debug/tracing/current_tracer
>>   $ echo c > /proc/sysrq-trigger
>>   ...
>>   Call trace:
>>   [<ffffffc00044d3ac>] sysrq_handle_crash+0x24/0x30
>>   [<ffffffc000092250>] return_to_handler+0x0/0x40
>>   [<ffffffc000092250>] return_to_handler+0x0/0x40
>>   ...
>>
>> This patch replaces such entries with real addresses preserved in
>> current->ret_stack[] at unwind_frame(). This way, we can cover all
>> the cases.
>
> I've observed a strange behavior when playing with case c). Call trace
> is as follows when function_graph is not used.
>
> Call trace:
>   [<fffffe00003dc738>] sysrq_handle_crash+0x24/0x30          <- (1)
>   [<fffffe00003dd2ac>] __handle_sysrq+0x128/0x19c            <- (2)
>   [<fffffe00003dd730>] write_sysrq_trigger+0x60/0x74
>   [<fffffe0000249fc4>] proc_reg_write+0x84/0xc0
>   [<fffffe00001f2638>] __vfs_write+0x44/0x104                <- (3)
>   [<fffffe00001f2e60>] vfs_write+0x98/0x1a8
>   [<fffffe00001f3730>] SyS_write+0x50/0xb0
>   [<fffffe00000939ec>] el0_svc_naked+0x20/0x28               <- (4)
>
> When function_graph is set, some entries, such as do_mem_abort, are added
> between (1) and (2). In addition, entries from (3) to (4) are not printed
> out. As tracking down elements of ret_stack[], I realize dump_backtrace()
> has been terminated before reaching to ret_stack[0]. Have you seen this
> kind of behavior? I believe push & pop operations work correctly.
>
> Please correct me if I'm wrong.

Oops, I mis-interpreted the result output.
You are right.
This can happen because the original dump_backtrace() wants to trace a stack
from a function where an exception has taken place, sysrq_handle_crash(), while
ret_stack[curr_ret_stack] doesn't point to that function, but the very top of
traced functions in callchains, that is, __do_kernel_fault in your case, probably.
So it results in replacing entries of return_to_handler to wrong function addresses.

A fixup! patch attached below (informative only) fixes this issue by once tracing
all the functions on a stack, but preventing a top few ones from being printed.
But there is a tricky thing here: we have to use 'regs->pc' instead of frame.pc
as a trapped function because, as I've already mentioned before, we always miss
the function when walking through a stack from an exception handler to functions
in a thread context.
(Please note that we will introduce a dummy stack frame at interrupt, but not
at exception.)

>> Signed-off-by: AKASHI Takahiro <[email-masked]>
>> ---
>> arch/arm64/include/asm/ftrace.h     |    2 ++
>> arch/arm64/include/asm/stacktrace.h |    3 +++
>> arch/arm64/kernel/perf_callchain.c  |    3 +++
>> arch/arm64/kernel/process.c         |    3 +++
>> arch/arm64/kernel/return_address.c  |    3 +++
>> arch/arm64/kernel/stacktrace.c      |   17 +++++++++++++++++
>> arch/arm64/kernel/time.c            |    3 +++
>> arch/arm64/kernel/traps.c           |    3 +++
>> 8 files changed, 37 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/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
>> index 6fb61c5..801a16db 100644
>> --- a/arch/arm64/include/asm/stacktrace.h
>> +++ b/arch/arm64/include/asm/stacktrace.h
>> @@ -22,6 +22,9 @@ struct stackframe {
>> 	unsigned long fp;
>> 	unsigned long sp;
>> 	unsigned long pc;
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	unsigned int graph;
>> +#endif
>> };
>
> How about using int instead of unsigned int to align with cure_ret_stack
> of struct task_struct?
>
>> extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
>> diff --git a/arch/arm64/kernel/perf_callchain.c b/arch/arm64/kernel/perf_callchain.c
>> index 797220d..ff46654 100644
>> --- a/arch/arm64/kernel/perf_callchain.c
>> +++ b/arch/arm64/kernel/perf_callchain.c
>> @@ -164,6 +164,9 @@ void perf_callchain_kernel(struct perf_callchain_entry *entry,
>> 	frame.fp = regs->regs[29];
>> 	frame.sp = regs->sp;
>> 	frame.pc = regs->pc;
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	frame.graph = current->curr_ret_stack;
>> +#endif
>>
>> 	walk_stackframe(current, &frame, callchain_trace, entry);
>> }
>> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
>> index 98bf546..88d742b 100644
>> --- a/arch/arm64/kernel/process.c
>> +++ b/arch/arm64/kernel/process.c
>> @@ -344,6 +344,9 @@ unsigned long get_wchan(struct task_struct *p)
>> 	frame.fp = thread_saved_fp(p);
>> 	frame.sp = thread_saved_sp(p);
>> 	frame.pc = thread_saved_pc(p);
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	frame.graph = p->curr_ret_stack;
>> +#endif
>> 	stack_page = (unsigned long)task_stack_page(p);
>> 	do {
>> 		if (frame.sp < stack_page ||
>> diff --git a/arch/arm64/kernel/return_address.c b/arch/arm64/kernel/return_address.c
>> index 07b37ac..1718706 100644
>> --- a/arch/arm64/kernel/return_address.c
>> +++ b/arch/arm64/kernel/return_address.c
>> @@ -43,6 +43,9 @@ void *return_address(unsigned int level)
>> 	frame.fp = (unsigned long)__builtin_frame_address(0);
>> 	frame.sp = current_stack_pointer;
>> 	frame.pc = (unsigned long)return_address; /* dummy */
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	frame.graph = current->curr_ret_stack;
>> +#endif
>>
>> 	walk_stackframe(current, &frame, save_return_addr, &data);
>>
>> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
>> index 9c7acf8..0a39049 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>
>>
>> @@ -66,6 +67,19 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>> 	frame->fp = *(unsigned long *)(fp);
>> 	frame->pc = *(unsigned long *)(fp + 8);
>>
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	if (tsk && tsk->ret_stack &&
>> +			(frame->pc == (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 = tsk->ret_stack[frame->graph--].ret;
>> +	}
>> +#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
>
> There is an index check of ret_stack[] in case of x86 [1]. Even though
> graph is unsigned int, I think we need to check the value of frame->graph
> before accessing ret_stack[].

I'm not sure that the checking is very useful because, if it happens,
it is a bug. It might make sense to avoid a possible panic though.

>> +
>> 	/*
>> 	 * Check whether we are going to walk through from interrupt stack
>> 	 * to task stack.
>> @@ -137,6 +151,9 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
>> 		frame.sp = current_stack_pointer;
>> 		frame.pc = (unsigned long)save_stack_trace_tsk;
>> 	}
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	frame.graph = tsk->curr_ret_stack;
>> +#endif
>>
>> 	walk_stackframe(tsk, &frame, save_trace, &data);
>> 	if (trace->nr_entries < trace->max_entries)
>> diff --git a/arch/arm64/kernel/time.c b/arch/arm64/kernel/time.c
>> index 6e5c521..5977969 100644
>> --- a/arch/arm64/kernel/time.c
>> +++ b/arch/arm64/kernel/time.c
>> @@ -52,6 +52,9 @@ unsigned long profile_pc(struct pt_regs *regs)
>> 	frame.fp = regs->regs[29];
>> 	frame.sp = regs->sp;
>> 	frame.pc = regs->pc;
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	frame.graph = -1; /* no task info */
>> +#endif
>
> graph is unsigned int type. Is this intentional?

No. This initialization is, I believe, redundant as it is not checked anywhere,
but I will re-think of it along with the checking above.

Thanks,
-Takahiro AKASHI

> Best Regards
> Jungseok Lee
>
> [1] arch/x86/kernel/dumpstack.c
>
----8<----
 From 9ebba7167f7838daf68d8231f04141d2f4d4b7b5 Mon Sep 17 00:00:00 2001
From: AKASHI Takahiro <[email-masked]>
Date: Wed, 25 Nov 2015 13:29:54 +0900
Subject: [PATCH] fixup! arm64: ftrace: fix a stack tracer's output under
  function graph tracer

Signed-off-by: AKASHI Takahiro <[email-masked]>
---
  arch/arm64/kernel/traps.c |   23 +++++++++++++++++------
  1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 46053c2..f140029 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -147,17 +147,14 @@ static void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
  {
  	struct stackframe frame;
  	unsigned long _irq_stack_ptr = per_cpu(irq_stack_ptr, smp_processor_id());
+	int skip;

  	pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);

  	if (!tsk)
  		tsk = current;

-	if (regs) {
-		frame.fp = regs->regs[29];
-		frame.sp = regs->sp;
-		frame.pc = regs->pc;
-	} else if (tsk == current) {
+	if (tsk == current) {
  		frame.fp = (unsigned long)__builtin_frame_address(0);
  		frame.sp = current_stack_pointer;
  		frame.pc = (unsigned long)dump_backtrace;
@@ -173,13 +170,27 @@ static void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
  	frame.graph = tsk->curr_ret_stack;
  #endif

+	skip = (regs ? 1 : 0);
  	pr_emerg("Call trace:\n");
  	while (1) {
  		unsigned long where = frame.pc;
  		unsigned long stack;
  		int ret;

-		dump_backtrace_entry(where);
+		/* skip until specified stack frame */
+		if (!skip)
+			dump_backtrace_entry(where);
+		else if (frame.fp == regs->regs[29]) {
+			skip = 0;
+			/*
+			 * Mostly, this is the case where this function is
+			 * called in panic/abort. As exception handler's
+			 * stack frame does not contain the corresponding pc
+			 * at which an exception has taken place, use regs->pc
+			 * instead.
+			 */
+			dump_backtrace_entry(regs->pc);
+		}
  		ret = unwind_frame(tsk, &frame);
  		if (ret < 0)
  			break;
-- 
1.7.9.5