Re: [PATCH 2/2] arm64: stacktrace: Add skip when task == current

[chenjun (AM)]

在 2021/3/18 3:34, Mark Rutland 写道:
> On Wed, Mar 17, 2021 at 06:36:36PM +0000, Catalin Marinas wrote:
>> On Wed, Mar 17, 2021 at 02:20:50PM +0000, Chen Jun wrote:
>>> On ARM64, cat /sys/kernel/debug/page_owner, all pages return the same
>>> stack:
>>>   stack_trace_save+0x4c/0x78
>>>   register_early_stack+0x34/0x70
>>>   init_page_owner+0x34/0x230
>>>   page_ext_init+0x1bc/0x1dc
>>>
>>> The reason is that:
>>> check_recursive_alloc always return 1 because that
>>> entries[0] is always equal to ip (__set_page_owner+0x3c/0x60).
>>>
>>> The root cause is that:
>>> commit 5fc57df2f6fd ("arm64: stacktrace: Convert to ARCH_STACKWALK")
>>> make the save_trace save 2 more entries.
>>>
>>> Add skip in arch_stack_walk when task == current.
>>>
>>> Fixes: 5fc57df2f6fd ("arm64: stacktrace: Convert to ARCH_STACKWALK")
>>> Signed-off-by: Chen Jun <chenjun102@xxxxxxxxxx>
>>> ---
>>>   arch/arm64/kernel/stacktrace.c | 5 +++--
>>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
>>> index ad20981..c26b0ac 100644
>>> --- a/arch/arm64/kernel/stacktrace.c
>>> +++ b/arch/arm64/kernel/stacktrace.c
>>> @@ -201,11 +201,12 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
>>>   
>>>   	if (regs)
>>>   		start_backtrace(&frame, regs->regs[29], regs->pc);
>>> -	else if (task == current)
>>> +	else if (task == current) {
>>> +		((struct stacktrace_cookie *)cookie)->skip += 2;
>>>   		start_backtrace(&frame,
>>>   				(unsigned long)__builtin_frame_address(0),
>>>   				(unsigned long)arch_stack_walk);
>>> -	else
>>> +	} else
>>>   		start_backtrace(&frame, thread_saved_fp(task),
>>>   				thread_saved_pc(task));
>>
>> I don't like abusing the cookie here. It's void * as it's meant to be an
>> opaque type. I'd rather skip the first two frames in walk_stackframe()
>> instead before invoking fn().
> 
> I agree that we shouldn't touch cookie here.
> 
> I don't think that it's right to bodge this inside walk_stackframe(),
> since that'll add bogus skipping for the case starting with regs in the
> current task. If we need a bodge, it has to live in arch_stack_walk()
> where we set up the initial unwinding state.
> 
> In another thread, we came to the conclusion that arch_stack_walk()
> should start at its parent, and its parent should add any skipping it
> requires.
> 
> Currently, arch_stack_walk() is off-by-one, and we can bodge that by
> using __builtin_frame_address(1), though I'm waiting for some compiler
> folk to confirm that's sound. Otherwise we need to add an assembly
> trampoline to snapshot the FP, which is unfortunastely convoluted.
> 
> This report suggests that a caller of arch_stack_walk() is off-by-one
> too, which suggests a larger cross-architecture semantic issue. I'll try
> to take a look tomorrow.
> 
> Thanks,
> Mark.
> 
>>
>> Prior to the conversion to ARCH_STACKWALK, we were indeed skipping two
>> more entries in __save_stack_trace() if tsk == current. Something like
>> below, completely untested:
>>
>> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
>> index ad20981dfda4..2a9f759aa41a 100644
>> --- a/arch/arm64/kernel/stacktrace.c
>> +++ b/arch/arm64/kernel/stacktrace.c
>> @@ -115,10 +115,15 @@ NOKPROBE_SYMBOL(unwind_frame);
>>   void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
>>   			     bool (*fn)(void *, unsigned long), void *data)
>>   {
>> +	/* for the current task, we don't want this function nor its caller */
>> +	int skip = tsk == current ? 2 : 0;
>> +
>>   	while (1) {
>>   		int ret;
>>   
>> -		if (!fn(data, frame->pc))
>> +		if (skip)
>> +			skip--;
>> +		else if (!fn(data, frame->pc))
>>   			break;
>>   		ret = unwind_frame(tsk, frame);
>>   		if (ret < 0)
>>
>>
>> -- 
>> Catalin
> 

This change will make kmemleak broken.
Maybe the reason is what Mark pointed out. I will try to check out.

-- 
Regards
Chen Jun