Messages in this thread | | | Date | Thu, 16 Mar 2023 21:39:25 +0900 | From | Masami Hiramatsu (Google) <> | Subject | Re: [PATCH v4 1/2] function_graph: Support recording and printing the return value of function |
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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>
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