Messages in this thread | | | Date | Thu, 16 Mar 2023 21:13:56 +0800 | Subject | Re: [PATCH v4 1/2] function_graph: Support recording and printing the return value of function | From | Donglin Peng <> |
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On 2023/3/16 20:39, Masami Hiramatsu (Google) wrote: > 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. > 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 > >
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