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

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Date:   Wed, 13 Sep 2023 09:31:29 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Thomas Richter <tmricht@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        acme@...nel.org, sumanthk@...ux.ibm.com, dengler@...ux.ibm.com,
        svens@...ux.ibm.com, gor@...ux.ibm.com, hca@...ux.ibm.com
Subject: Re: [PATCH] perf jevent: fix core dump on software events on s390

On Wed, Sep 13, 2023 at 5:52 AM Thomas Richter <tmricht@...ux.ibm.com> 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@...ux.ibm.com>

Reviewed-by: Ian Rogers <irogers@...gle.com>

Thanks!
Ian

> ---
>  tools/perf/pmu-events/jevents.py | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/perf/pmu-events/jevents.py b/tools/perf/pmu-events/jevents.py
> index a7e88332276d..72ba4a9239c6 100755
> --- a/tools/perf/pmu-events/jevents.py
> +++ b/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++) {
> --
> 2.41.0
>