perf jevent: fix core dump on software events on s390

Running commands such as
 # ./perf stat -e cs -- true
 Segmentation fault (core dumped)
 # ./perf stat -e cpu-clock-- true
 Segmentation fault (core dumped)
 #

dump core. This should not happen as these events are defined
even when no hardware PMU is available.
Debugging this reveals this call chain:

  perf_pmus__find_by_type(type=1)
  +--> pmu_read_sysfs(core_only=false)
       +--> perf_pmu__find2(dirfd=3, name=0x152a113 "software")
            +--> perf_pmu__lookup(pmus=0x14f0568 <other_pmus>, dirfd=3,
                                  lookup_name=0x152a113 "software")
                 +--> perf_pmu__find_events_table (pmu=0x1532130)

Now the pmu is "software" and it tries to find a proper table
generated by the pmu-event generation process for s390:

 # cd pmu-events/
 # ./jevents.py  s390 all /root/linux/tools/perf/pmu-events/arch |\
        grep -E '^const struct pmu_table_entry'
 const struct pmu_table_entry pmu_events__cf_z10[] = {
 const struct pmu_table_entry pmu_events__cf_z13[] = {
 const struct pmu_table_entry pmu_metrics__cf_z13[] = {
 const struct pmu_table_entry pmu_events__cf_z14[] = {
 const struct pmu_table_entry pmu_metrics__cf_z14[] = {
 const struct pmu_table_entry pmu_events__cf_z15[] = {
 const struct pmu_table_entry pmu_metrics__cf_z15[] = {
 const struct pmu_table_entry pmu_events__cf_z16[] = {
 const struct pmu_table_entry pmu_metrics__cf_z16[] = {
 const struct pmu_table_entry pmu_events__cf_z196[] = {
 const struct pmu_table_entry pmu_events__cf_zec12[] = {
 const struct pmu_table_entry pmu_metrics__cf_zec12[] = {
 const struct pmu_table_entry pmu_events__test_soc_cpu[] = {
 const struct pmu_table_entry pmu_metrics__test_soc_cpu[] = {
 const struct pmu_table_entry pmu_events__test_soc_sys[] = {
 #

However event "software" is not listed, as can be seen in the
generated const struct pmu_events_map pmu_events_map[].
So in function perf_pmu__find_events_table(), the variable
table is initialized to NULL, but never set to a proper
value. The function scans all generated &pmu_events_map[]
tables, but no table matches, because the tables are
s390 CPU Measurement unit specific:

  i = 0;
  for (;;) {
      const struct pmu_events_map *map = &pmu_events_map[i++];
      if (!map->arch)
           break;

      --> the maps are there because the build generated them

           if (!strcmp_cpuid_str(map->cpuid, cpuid)) {
                table = &map->event_table;
                break;
           }
      --> Since no matching CPU string the table var remains 0x0
      }
      free(cpuid);
      if (!pmu)
           return table;

      --> The pmu is "software" so it exists and no return

      --> and here perf dies because table is 0x0
      for (i = 0; i < table->num_pmus; i++) {
	      ...
      }
      return NULL;

Fix this and do not access the table variable. Instead return 0x0
which is the same return code when the for-loop was not successful.

Output after:
 # ./perf stat -e cs -- true

 Performance counter stats for 'true':

                 0      cs

       0.000853105 seconds time elapsed

       0.000061000 seconds user
       0.000827000 seconds sys

 # ./perf stat -e cpu-clock -- true

 Performance counter stats for 'true':

              0.25 msec cpu-clock #    0.341 CPUs utilized

       0.000728383 seconds time elapsed

       0.000055000 seconds user
       0.000706000 seconds sys

 # ./perf stat -e cycles -- true

 Performance counter stats for 'true':

   <not supported>      cycles

       0.000767298 seconds time elapsed

       0.000055000 seconds user
       0.000739000 seconds sys

 #

Fixes: 7c52f10c0d ("perf pmu: Cache JSON events table")
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: dengler@linux.ibm.com
Cc: gor@linux.ibm.com
Cc: hca@linux.ibm.com
Cc: sumanthk@linux.ibm.com
Cc: svens@linux.ibm.com
Link: https://lore.kernel.org/r/20230913125157.2790375-1-tmricht@linux.ibm.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/pmu-events/jevents.py

@@ -991,7 +991,7 @@ const struct pmu_events_table *perf_pmu__find_events_table(struct perf_pmu *pmu)
                 }
         }
         free(cpuid);
-        if (!pmu)
+        if (!pmu || !table)
                 return table;
 
         for (i = 0; i < table->num_pmus; i++) {