Date:   Sat, 9 Oct 2021 18:41:46 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     broonie@...nel.org, jpoimboe@...hat.com, ardb@...nel.org,
        nobuta.keiya@...itsu.com, sjitindarsingh@...il.com,
        catalin.marinas@....com, will@...nel.org, jmorris@...ei.org,
        pasha.tatashin@...een.com, jthierry@...hat.com,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v8 1/4] arm64: Make all stack walking functions use
 arch_stack_walk()



On 8/24/21 8:13 AM, Mark Rutland wrote:
> On Thu, Aug 12, 2021 at 02:06:00PM -0500, madvenka@...ux.microsoft.com wrote:
>> From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
>>
>> Currently, there are multiple functions in ARM64 code that walk the
>> stack using start_backtrace() and unwind_frame(). Convert all of
>> them to use arch_stack_walk(). This makes maintenance easier.
> 
> It would be good to split this into a series of patches as Mark Brown
> suggested in v7.
> 
>> Here is the list of functions:
>>
>> 	perf_callchain_kernel()
>> 	get_wchan()
>> 	return_address()
>> 	dump_backtrace()
>> 	profile_pc()
> 
> Note that arch_stack_walk() depends on CONFIG_STACKTRACE (which is not in
> defconfig), so we'll need to reorganise things such that it's always defined,
> or factor out the core of that function and add a wrapper such that we
> can always use it.
> 
>> Signed-off-by: Madhavan T. Venkataraman <madvenka@...ux.microsoft.com>
>> ---
>>  arch/arm64/include/asm/stacktrace.h |  3 ---
>>  arch/arm64/kernel/perf_callchain.c  |  5 +---
>>  arch/arm64/kernel/process.c         | 39 ++++++++++++++++++-----------
>>  arch/arm64/kernel/return_address.c  |  6 +----
>>  arch/arm64/kernel/stacktrace.c      | 38 +++-------------------------
>>  arch/arm64/kernel/time.c            | 22 +++++++++-------
>>  6 files changed, 43 insertions(+), 70 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
>> index 8aebc00c1718..e43dea1c6b41 100644
>> --- a/arch/arm64/include/asm/stacktrace.h
>> +++ b/arch/arm64/include/asm/stacktrace.h
>> @@ -61,9 +61,6 @@ struct stackframe {
>>  #endif
>>  };
>>  
>> -extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
>> -extern void walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
>> -			    bool (*fn)(void *, unsigned long), void *data);
>>  extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>>  			   const char *loglvl);
>>  
>> diff --git a/arch/arm64/kernel/perf_callchain.c b/arch/arm64/kernel/perf_callchain.c
>> index 4a72c2727309..2f289013c9c9 100644
>> --- a/arch/arm64/kernel/perf_callchain.c
>> +++ b/arch/arm64/kernel/perf_callchain.c
>> @@ -147,15 +147,12 @@ static bool callchain_trace(void *data, unsigned long pc)
>>  void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
>>  			   struct pt_regs *regs)
>>  {
>> -	struct stackframe frame;
>> -
>>  	if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
>>  		/* We don't support guest os callchain now */
>>  		return;
>>  	}
>>  
>> -	start_backtrace(&frame, regs->regs[29], regs->pc);
>> -	walk_stackframe(current, &frame, callchain_trace, entry);
>> +	arch_stack_walk(callchain_trace, entry, current, regs);
>>  }
> 
> We can also update callchain_trace take the return value of
> perf_callchain_store into acount, e.g.
> 
> | static bool callchain_trace(void *data, unsigned long pc) 
> | {
> | 	struct perf_callchain_entry_ctx *entry = data;
> | 	return perf_callchain_store(entry, pc) == 0;
> | }
> 
>>  
>>  unsigned long perf_instruction_pointer(struct pt_regs *regs)
>> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
>> index c8989b999250..52c12fd26407 100644
>> --- a/arch/arm64/kernel/process.c
>> +++ b/arch/arm64/kernel/process.c
>> @@ -544,11 +544,28 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
>>  	return last;
>>  }
>>  
>> +struct wchan_info {
>> +	unsigned long	pc;
>> +	int		count;
>> +};
>> +
>> +static bool get_wchan_cb(void *arg, unsigned long pc)
>> +{
>> +	struct wchan_info *wchan_info = arg;
>> +
>> +	if (!in_sched_functions(pc)) {
>> +		wchan_info->pc = pc;
>> +		return false;
>> +	}
>> +	wchan_info->count--;
>> +	return !!wchan_info->count;
>> +}
> 
> This will terminate one entry earlier than the old logic since we used
> to use a post-increment (testing the prior value), and now we're
> effectively using a pre-decrement (testing the new value).
> 
> I don't think that matters all that much in practice, but it might be
> best to keep the original logic, e.g. initialize `count` to 0 and here
> do:
> 
> 	return wchan_info->count++ < 16;
> 
>> +
>>  unsigned long get_wchan(struct task_struct *p)
>>  {
>> -	struct stackframe frame;
>> -	unsigned long stack_page, ret = 0;
>> -	int count = 0;
>> +	unsigned long stack_page;
>> +	struct wchan_info wchan_info;
>> +
>>  	if (!p || p == current || task_is_running(p))
>>  		return 0;
>>  
>> @@ -556,20 +573,12 @@ unsigned long get_wchan(struct task_struct *p)
>>  	if (!stack_page)
>>  		return 0;
>>  
>> -	start_backtrace(&frame, thread_saved_fp(p), thread_saved_pc(p));
>> +	wchan_info.pc = 0;
>> +	wchan_info.count = 16;
>> +	arch_stack_walk(get_wchan_cb, &wchan_info, p, NULL);
>>  
>> -	do {
>> -		if (unwind_frame(p, &frame))
>> -			goto out;
>> -		if (!in_sched_functions(frame.pc)) {
>> -			ret = frame.pc;
>> -			goto out;
>> -		}
>> -	} while (count++ < 16);
>> -
>> -out:
>>  	put_task_stack(p);
>> -	return ret;
>> +	return wchan_info.pc;
>>  }
> 
> Other than the comment above, this looks good to me.
> 
>>  unsigned long arch_align_stack(unsigned long sp)
>> diff --git a/arch/arm64/kernel/return_address.c b/arch/arm64/kernel/return_address.c
>> index a6d18755652f..92a0f4d434e4 100644
>> --- a/arch/arm64/kernel/return_address.c
>> +++ b/arch/arm64/kernel/return_address.c
>> @@ -35,15 +35,11 @@ NOKPROBE_SYMBOL(save_return_addr);
>>  void *return_address(unsigned int level)
>>  {>  	struct return_address_data data;
>> -	struct stackframe frame;
>>  
>>  	data.level = level + 2;
>>  	data.addr = NULL;
>>  
>> -	start_backtrace(&frame,
>> -			(unsigned long)__builtin_frame_address(0),
>> -			(unsigned long)return_address);
>> -	walk_stackframe(current, &frame, save_return_addr, &data);
>> +	arch_stack_walk(save_return_addr, &data, current, NULL);
>>  
>>  	if (!data.level)
>>  		return data.addr;
> 
> Nor that arch_stack_walk() will start with it's caller, so
> return_address() will be included in the trace where it wasn't
> previously, which implies we need to skip an additional level.
> 
> That said, I'm not entirely sure why we need to skip 2 levels today; it
> might be worth checking that's correct.
> 
> We should also mark return_address() as noinline to avoid surprises with
> LTO.
> 
>> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
>> index 8982a2b78acf..1800310f92be 100644
>> --- a/arch/arm64/kernel/stacktrace.c
>> +++ b/arch/arm64/kernel/stacktrace.c
>> @@ -151,23 +151,21 @@ void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
>>  }
>>  NOKPROBE_SYMBOL(walk_stackframe);
>>  
>> -static void dump_backtrace_entry(unsigned long where, const char *loglvl)
>> +static bool dump_backtrace_entry(void *arg, unsigned long where)
>>  {
>> +	char *loglvl = arg;
>>  	printk("%s %pSb\n", loglvl, (void *)where);
>> +	return true;
>>  }
>>  
>>  void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>>  		    const char *loglvl)
>>  {
>> -	struct stackframe frame;
>> -	int skip = 0;
>> -
>>  	pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
>>  
>>  	if (regs) {
>>  		if (user_mode(regs))
>>  			return;
>> -		skip = 1;
>>  	}
> 
> We can simplifiy this to:
> 
> 	if (regs && user_mode(regs))
> 		return;
> 
>>  
>>  	if (!tsk)
>> @@ -176,36 +174,8 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>>  	if (!try_get_task_stack(tsk))
>>  		return;
>>  
>> -	if (tsk == current) {
>> -		start_backtrace(&frame,
>> -				(unsigned long)__builtin_frame_address(0),
>> -				(unsigned long)dump_backtrace);
>> -	} else {
>> -		/*
>> -		 * task blocked in __switch_to
>> -		 */
>> -		start_backtrace(&frame,
>> -				thread_saved_fp(tsk),
>> -				thread_saved_pc(tsk));
>> -	}
>> -
>>  	printk("%sCall trace:\n", loglvl);
>> -	do {
>> -		/* skip until specified stack frame */
>> -		if (!skip) {
>> -			dump_backtrace_entry(frame.pc, loglvl);
>> -		} 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, loglvl);
>> -		}
>> -	} while (!unwind_frame(tsk, &frame));
>> +	arch_stack_walk(dump_backtrace_entry, (void *)loglvl, tsk, regs);
> 
> It turns out we currently need this skipping to get the balance the
> ftrace call stack, and arch_stack_walk() doesn't currently do the right
> thing when starting from regs. That balancing isn't quite right, and
> will be wrong in some case when unwinding across exception boundaries;
> we could implement HAVE_FUNCTION_GRAPH_RET_ADDR_PTR using the FP to
> solve that.
> 

Hi Mark,

It seems that the behavior is the same in the old and new code. Do you
agree?

In the old code when regs is used, the stack trace starts from regs->regs[29]
and the first PC displayed is regs->pc. arch_stack_walk() does the same thing.

Can you elaborate what change you want me to make here?

Madhavan

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