[PATCH v4 1/2] function_graph: Support recording and printing the return value of function

[Donglin Peng]

On 2023/3/16 20:39, Masami Hiramatsu (Google) wrote:
> On Thu, 16 Mar 2023 11:18:23 +0800
> Donglin Peng <pengdonglin at sangfor.com.cn> wrote:
> 
>> On 2023/3/15 22:57, Peter Zijlstra wrote:
>>> On Wed, Mar 15, 2023 at 10:13:48AM -0400, Steven Rostedt wrote:
>>>> On Wed, 15 Mar 2023 14:49:11 +0100
>>>> Peter Zijlstra <peterz at infradead.org> wrote:
>>>>
>>>>>> diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
>>>>>> index 1265ad519249..35ac9c58dc77 100644
>>>>>> --- a/arch/x86/kernel/ftrace_64.S
>>>>>> +++ b/arch/x86/kernel/ftrace_64.S
>>>>>> @@ -348,6 +348,10 @@ SYM_CODE_START(return_to_handler)
>>>>>>    	movq %rax, (%rsp)
>>>>>>    	movq %rdx, 8(%rsp)
>>>>>>    	movq %rbp, %rdi
>>>>>> +#ifdef CONFIG_FUNCTION_GRAPH_RETVAL
>>>>>> +	/* Pass the function return value to ftrace_return_to_handler */
>>>>>> +	movq %rax, %rsi
>>>>>> +#endif
>>>>>>    
>>>>>>    	call ftrace_return_to_handler
>>>>>
>>>>> What about the case of double register return values (when the value
>>>>> is returned in the A,D pair) ?
>>>>
>>>> Is there anything that does that in 64 bit kernels?
>>>
>>> Note sure; but I have a patch series that introduces cmpxchg128 and
>>> friends. Most of the actual functions are __always_inline, but still,
>>> the moment a compiler decides to break out a subfunction on a u128
>>> boundary we're in luck.
>> I have reviewed the kretprobe implementation and noticed that $retval
>> only retrieves the value of pt_regs.ax, which is an unsigned long data
>> type. I wrote a demo and tested it on an x86 machine, and found that
>> $retval only shows the least significant 32 bits of retval.Therefore,I
>> think it can be consistent with kretprobe.
> 
> Yeah, because it doesn't support 128bit type (u128/long long).
> It is possible to support it, but I think we can just document this
> as a limitation for fgraph return value. It is just for quickly trace
> most of the return values.
> 
Yeah,I will add this limitation to the documents in v5.

> Thank you,
> 
>>
>> static noinline unsigned long long test_retval_func(void)
>> {
>> 	unsigned long long value = 0x1234567887654321;
>> 	return value;
>> }
>>
>> add a kretprobe event:
>> echo 'r:myretprobe test_retval_func $retval:x64' > kprobe_events
>>
>> the trace log:
>> myretprobe: (retval_open+0x1c/0x2c [test_retval] <- test_retval_func)
>> arg1=0x87654321
> 
>