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

[Donglin Peng]

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@xxxxxxxxxxxxx> 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.

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