Re: perf list dumps core using 5.13.0.rc2

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, May 19, 2021 at 04:57:05PM +0200, Thomas Richter escreveu:
> Using kernel 5.13.0.rc2 command perf list dumps core on my x86
> virtual machine:
> 
> [root@f34 perf]# ./perf list
> Segmentation fault (core dumped)
> ^C
> [root@f34 perf]#
> 
> The root case this this change in file ../include/uapi/linux/perf_event.h:
> enum perf_sw_ids {
>         PERF_COUNT_SW_CPU_CLOCK                 = 0,
>         ...
>  --->   PERF_COUNT_SW_CGROUP_SWITCHES           = 11,
> 
>         PERF_COUNT_SW_MAX,                      /* non-ABI */
> };
> 
> This change increases PERF_COUNT_SW_MAX to 12. However this
> change is not reflected in file util/parse-events.c where data structure
> 
> struct event_symbol event_symbols_sw[PERF_COUNT_SW_MAX] = {
>         ....
> }
> 
> is defined and it misses the symbol name and alias for this new software

Sure, it is not there, but then PERF_COUNT_SW_MAX is used in the array
definition, so it ends up having
event_symbols_sw[PERF_COUNT_SW_MAX].symbol == NULL

> event. So when command 'perf list' is called, the call chain sequence
> is
>   cmd_list()
>     print_events()
>       print_symbol_events(event_glob, PERF_TYPE_SOFTWARE,
>                           event_symbols_sw, PERF_COUNT_SW_MAX, name_only);
> 
> where PERF_COUNT_SW_MAX is 12 and  structure event_symbols_sw[] contains
> only 11 elements. This ends up with the last element of the array being
> all zeroes and the line:
> 
>     if (!name_only && strlen(syms->alias))

But before this we have:

                if (event_glob != NULL && syms->symbol != NULL &&
                    !(strglobmatch(syms->symbol, event_glob) ||
                      (syms->alias && strglobmatch(syms->alias, event_glob))))
                        continue;

We need to move that syms->symbol != NULL part to be the first one and
continue if that array entry isn't populated, I'll add a patch with your
Reported-by:

I was scratching my head looking why I hadn't hit this, it doesn't
support PERF_COUNT_SW_CGROUP_SWITCHES so it bails out at
!is_event_supported(type, i):

        for (i = 0; i < max; i++, syms++) {

                if (event_glob != NULL && syms->symbol != NULL &&
                    !(strglobmatch(syms->symbol, event_glob) ||
                      (syms->alias && strglobmatch(syms->alias, event_glob))))
                        continue;

                if (!is_event_supported(type, i))
                        continue;

                if (!evt_num_known) {
                        evt_num++;
                        continue;
                }

                if (!name_only && strlen(syms->alias))

> 
> dumps core because syms->alias is a NULL pointer.
> 
> Is this dummy event PERF_COUNT_SW_CGROUP_SWITCHES simply missing in the
> event_symbols_sw[] array or is there more to it (which I am missing).
> 
> Thanks. 
> 
> PS: I can sent a patch if needed....
> 
> -- 
> Thomas Richter, Dept 3303, IBM s390 Linux Development, Boeblingen, Germany
> --
> Vorsitzender des Aufsichtsrats: Gregor Pillen
> Geschäftsführung: Dirk Wittkopp
> Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294

-- 

- Arnaldo