Re: [PATCH v1 3/3] perf probe: Generate hash event for long symbol

[Leo Yan <leo.yan@arm.com>]

On 10/12/24 06:30, Masami Hiramatsu (Google) wrote:

[...]

>> ---8<---
>>
>>  From 3b09a6f89c7e383c6b1d2b7e6bd80c6bfa658d5b Mon Sep 17 00:00:00 2001
>> From: Leo Yan <leo.yan@arm.com>
>> Date: Fri, 11 Oct 2024 07:58:08 +0000
>> Subject: [PATCH] perf probe: Correct demangled symbols in C++ program
>>
>> An issue can be observed when probe C++ demangled symbol with steps:
>>
>>    # nm test_cpp_mangle | grep print_data
>>      0000000000000c94 t _GLOBAL__sub_I__Z10print_datai
>>      0000000000000afc T _Z10print_datai
>>      0000000000000b38 T _Z10print_dataR5Point
>>
>>    # ./perf probe -x /home/niayan01/test_cpp_mangle -F --demangle
>>      ...
>>      print_data(Point&)
>>      print_data(int)
>>      ...
>>
>>    # ./perf --debug verbose=3 probe -x test_cpp_mangle --add "test=print_data(int)"
>>      probe-definition(0): test=print_data(int)
>>      symbol:print_data(int) file:(null) line:0 offset:0 return:0 lazy:(null)
>>      0 arguments
>>      Open Debuginfo file: /home/niayan01/test_cpp_mangle
>>      Try to find probe point from debuginfo.
>>      Symbol print_data(int) address found : afc
>>      Matched function: print_data [2ccf]
>>      Probe point found: print_data+0
>>      Found 1 probe_trace_events.
>>      Opening /sys/kernel/tracing//uprobe_events write=1
>>      Opening /sys/kernel/tracing//README write=0
>>      Writing event: p:probe_test_cpp_mangle/test /home/niayan01/test_cpp_mangle:0xb38
>>      ...
>>
>> When tried to probe symbol "print_data(int)", the log show:
>>
>>      Symbol print_data(int) address found : afc
>>
>> The found address is 0xafc - which is right if we connect with the
>> output result from nm. Afterwards when write event, the command uses
>> offset 0xb38 in the last log, which is a wrong address.
>>
>> The dwarf_diename() gets a common function name, in above case, it
>> returns string "print_data". As a result, the tool parses the offset
>> based on the common name. This leads to probe at the wrong symbol
>> "print_data(Point&)".
>>
>> To fix the issue, use the die_get_linkage_name() function to retrieve
>> the distinct linkage name - this is the mangled name for the C++ case.
>> Based on this unique name, the tool can get a correct offset for
>> probing. Based on DWARF doc, it is possible the linkage name is missed
>> in the DIE, it rolls back to use dwarf_diename().
> 
> Can you add the result after applying this patch here?

Sure. The result with this patch is:

   # perf --debug verbose=3 probe -x test_cpp_mangle --add "test=print_data(int)"
     probe-definition(0): test=print_data(int)
     symbol:print_data(int) file:(null) line:0 offset:0 return:0 lazy:(null)
     0 arguments
     Open Debuginfo file: /home/niayan01/test_cpp_mangle
     Try to find probe point from debuginfo.
     Symbol print_data(int) address found : afc
     Matched function: print_data [2d06]
     Probe point found: print_data+0
     Found 1 probe_trace_events.
     Opening /sys/kernel/tracing//uprobe_events write=1
     Opening /sys/kernel/tracing//README write=0
     Writing event: p:probe_test_cpp_mangle/test /home/niayan01/test_cpp_mangle:0xafc
     Added new event:
       probe_test_cpp_mangle:test (on print_data(int) in /home/niayan01/test_cpp_mangle)
                                                                                                                                                                                                                                                        
     You can now use it in all perf tools, such as:
                                                                                                                                                                                                                                                        
             perf record -e probe_test_cpp_mangle:test -aR sleep 1
                                                                                                                                                                                                                                                        
   # perf --debug verbose=3 probe -x test_cpp_mangle --add "test2=print_data(Point&)"
     probe-definition(0): test2=print_data(Point&)
     symbol:print_data(Point&) file:(null) line:0 offset:0 return:0 lazy:(null)
     0 arguments
     Open Debuginfo file: /home/niayan01/test_cpp_mangle
     Try to find probe point from debuginfo.
     Symbol print_data(Point&) address found : b38
     Matched function: print_data [2ccf]
     Probe point found: print_data+0
     Found 1 probe_trace_events.
     Opening /sys/kernel/tracing//uprobe_events write=1
     Parsing probe_events: p:probe_test_cpp_mangle/test /home/niayan01/test_cpp_mangle:0x0000000000000afc
     Group:probe_test_cpp_mangle Event:test probe:p
     Opening /sys/kernel/tracing//README write=0
     Writing event: p:probe_test_cpp_mangle/test2 /home/niayan01/test_cpp_mangle:0xb38
     Added new event:
       probe_test_cpp_mangle:test2 (on print_data(Point&) in /home/niayan01/test_cpp_mangle)
                                                                                                                                                                                                                                                        
     You can now use it in all perf tools, such as:
                                                                                                                                                                                                                                                        
             perf record -e probe_test_cpp_mangle:test2 -aR sleep 1

I have sent out a formal patch (with fixed tag):
https://lore.kernel.org/linux-perf-users/20241012141432.877894-1-leo.yan@arm.com/T/#u

Thanks for review!

Leo