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

[Leo Yan]

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@xxxxxxx>
> > 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@xxxxxxx/T/#u

Thanks for review!

Leo