[PATCH] perf probe: Correct demangled symbols in C++ program

[Leo Yan]

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().

Signed-off-by: Leo Yan <leo.yan@xxxxxxx>
---
 tools/perf/util/probe-finder.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
index 630e16c54ed5..28a14005fc1f 100644
--- a/tools/perf/util/probe-finder.c
+++ b/tools/perf/util/probe-finder.c
@@ -1583,8 +1583,21 @@ int debuginfo__find_probe_point(struct debuginfo *dbg, u64 addr,

        /* Find a corresponding function (name, baseline and baseaddr) */
        if (die_find_realfunc(&cudie, (Dwarf_Addr)addr, &spdie)) {
-               /* Get function entry information */
-               func = basefunc = dwarf_diename(&spdie);
+               /*
+                * Get function entry information.
+                *
+                * As described in the document DWARF Debugging Information
+                * Format Version 5, section 2.22 Linkage Names, "mangled names,
+                * are used in various ways, ... to distinguish multiple
+                * entities that have the same name".
+                *
+                * Firstly try to get distinct linkage name, if fail then
+                * rollback to get associated name in DIE.
+                */
+               func = basefunc = die_get_linkage_name(&spdie);
+               if (!func)
+                        func = basefunc = dwarf_diename(&spdie);
+
                if (!func ||
                    die_entrypc(&spdie, &baseaddr) != 0 ||
                    dwarf_decl_line(&spdie, &baseline) != 0) {
--
2.25.1