| Message ID | 56554740.4070904@linaro.org |
|---|---|
| State | New |
| Headers | show |
Jungseok, On 11/25/2015 08:48 PM, Jungseok Lee wrote: > On Nov 25, 2015, at 2:29 PM, AKASHI Takahiro wrote: >> 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. > > Yes, __do_kernel_fault. > >> 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.) > > Thanks for clear explanation! > > Since I'm not the only person to experience the above case, I report one more and > the last observation ;) > > PC and LR information is printed out when function_graph is set. > > PC is at sysrq_handle_crash+0x24/0x30 > LR is at sysrq_handle_crash+0x10/0x30 > > The logs are as follows when function_graph turns off. > > PC is at sysrq_handle_crash+0x24/0x30 > LR is at __handle_sysrq+0x128/0x19c > > I think __show_regs() might have a similar problem when retrieving LR according to > the below informative patch. Thoughts? Well, I believe that it is normal. Sysrq_handle_crash() is basically a leaf function, but once function or function graph tracer is turned on, ftrace_caller() is called before its function body and so LR has been modified when panic/show_regs(). Thanks, -Takahiro AKASHI >>>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org> >>>> --- >>>> 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 <takahiro.akashi@linaro.org> >> 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 <takahiro.akashi@linaro.org> >> --- >> 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 > > This fixes up the issue I mentioned. > > Best Regards > Jungseok Lee > _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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;