[PATCH 2/3] perf script: Exception handling when the print fmt is empty

[Taeung Song]

After collecting samples for events 'syscalls:',
perf-script with python script doesn't occasionally
work generating a segmentation fault.

The reason is that the print fmt is empty and
a value of event->print_fmt.args is NULL, so
dereferencing the null pointer results in
a segmentation fault i.e.

    # perf record -e syscalls:*
    # perf script -g python
    # perf script -s perf-script.py

    in trace_begin
    syscalls__sys_enter_brk  3 79841.832099154  3777 test.sh  syscall_nr=12, 
brk=0

    ... (omitted) ...

    Segmentation fault (core dumped)

For example, a format of sys_enter_getuid() hasn't
print fmt as below.

    # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_getuid/format
    name: sys_enter_getuid
    ID: 188
    format:
            field:unsigned short common_type;         offset:0; size:2; 
signed:0;
            field:unsigned char common_flags;         offset:2; size:1; 
signed:0;
            field:unsigned char common_preempt_count; offset:3; size:1; 
signed:0;
            field:int common_pid;                     offset:4; size:4; 
signed:1;
            field:int syscall_nr;                     offset:8; size:4; 
signed:1;

    print fmt: ""

So add exception handling to avoid this problem.

Cc: Jiri Olsa <jo...@kernel.org>
Cc: Wang Nan <wangn...@huawei.com>
Signed-off-by: Taeung Song <treeze.tae...@gmail.com>
---
 tools/perf/util/scripting-engines/trace-event-perl.c   | 3 +++
 tools/perf/util/scripting-engines/trace-event-python.c | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/tools/perf/util/scripting-engines/trace-event-perl.c 
b/tools/perf/util/scripting-engines/trace-event-perl.c
index 544509c..b3aabc0 100644
--- a/tools/perf/util/scripting-engines/trace-event-perl.c
+++ b/tools/perf/util/scripting-engines/trace-event-perl.c
@@ -187,6 +187,9 @@ static void define_event_symbols(struct event_format *event,
                                 const char *ev_name,
                                 struct print_arg *args)
 {
+       if (args == NULL)
+               return;
+
        switch (args->type) {
        case PRINT_NULL:
                break;
diff --git a/tools/perf/util/scripting-engines/trace-event-python.c 
b/tools/perf/util/scripting-engines/trace-event-python.c
index d72fafc..309d90f 100644
--- a/tools/perf/util/scripting-engines/trace-event-python.c
+++ b/tools/perf/util/scripting-engines/trace-event-python.c
@@ -205,6 +205,9 @@ static void define_event_symbols(struct event_format *event,
                                 const char *ev_name,
                                 struct print_arg *args)
 {
+       if (args == NULL)
+               return;
+
        switch (args->type) {
        case PRINT_NULL:
                break;
-- 
2.5.0