Re: [PATCH] perf jevent: fix core dump on software events on s390

[Ian Rogers]

On Fri, Sep 15, 2023 at 4:40 PM Namhyung Kim <namhyung@xxxxxxxxx> wrote:
>
> Hello,
>
> On Thu, Sep 14, 2023 at 6:14 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> >
> > On Wed, Sep 13, 2023 at 5:52 AM Thomas Richter <tmricht@xxxxxxxxxxxxx> wrote:
> > >
> > > 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
> > >
> > >  #
> > >
> > > Signed-off-by: Thomas Richter <tmricht@xxxxxxxxxxxxx>
> >
> > Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
>
> I'll add this too, ok?
>
> Fixes: 7c52f10c0d4d8 ("perf pmu: Cache JSON events table")

Looks good, thanks!
Ian

> Thanks,
> Namhyung