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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 16 Mar 2023 11:18:23 +0800
Donglin Peng <pengdonglin@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@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.

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


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>