Re: [PATCH] perf top & record: Fix segfault when default cycles event is not supported

[Ian Rogers]

On Wed, Jun 14, 2023 at 8:18 AM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
>
> The perf-record and perf-top call parse_event() to add a cycles event to
> an empty evlist. For the system that does not support hardware cycles
> event, such as QEMU, the evlist is empty due to the following code process:
>
>   parse_event(evlist, "cycles:P" or ""cycles:Pu")
>     parse_events(evlist, "cycles:P")
>       __parse_events
>         ...
>         ret = parse_events__scanner(str, &parse_state);
>         // ret = 0
>         ...
>         ret2 = parse_events__sort_events_and_fix_groups()
>         if (ret2 < 0)
>           return ret;
>         // The cycles event is not supported, here ret2 = -EINVAL,
>         // Here return 0.
>         ...
>         evlist__splice_list_tail(evlist)
>         // The code here does not execute to, so the evlist is still empty.
>
> A null pointer occurs when the content in the evlist is accessed later.
>
> Before:
>
>   # perf list hw
>
>   List of pre-defined events (to be used in -e or -M):
>
>   # perf record true
>   libperf: Miscounted nr_mmaps 0 vs 1
>   WARNING: No sample_id_all support, falling back to unordered processing
>   perf: Segmentation fault
>   Obtained 1 stack frames.
>   [0xc5beff]
>   Segmentation fault
>
> Solution:
>   If cycles event is not supported, try to fall back to cpu-clock event.
>
> After:
>   # perf record true
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 0.006 MB perf.data ]
>   #
>
> Fixes: 7b100989b4f6 ("perf evlist: Remove __evlist__add_default")
> Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>

Thanks, useful addition. The cpu-clock fall back wasn't present before
7b100989b4f6 so is the fixes tag correct?

Wrt segv, I'm beginning to think that we should always forcibly create
a core PMU even if we can't find one one in sysfs, my guess is that is
what triggers the segv.

evlist__add_default doesn't really explain what the function is doing
and default can have >1 meaning. Perhaps, evlist__add_cycles.

Thanks,
Ian

> ---
>  tools/perf/builtin-record.c |  4 +---
>  tools/perf/builtin-top.c    |  3 +--
>  tools/perf/util/evlist.c    | 18 ++++++++++++++++++
>  tools/perf/util/evlist.h    |  1 +
>  4 files changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index aec18db7ff23..29ae2b84a63a 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -4161,9 +4161,7 @@ int cmd_record(int argc, const char **argv)
>                 record.opts.tail_synthesize = true;
>
>         if (rec->evlist->core.nr_entries == 0) {
> -               bool can_profile_kernel = perf_event_paranoid_check(1);
> -
> -               err = parse_event(rec->evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
> +               err = evlist__add_default(rec->evlist);
>                 if (err)
>                         goto out;
>         }
> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> index c363c04e16df..798cb9252a5f 100644
> --- a/tools/perf/builtin-top.c
> +++ b/tools/perf/builtin-top.c
> @@ -1665,8 +1665,7 @@ int cmd_top(int argc, const char **argv)
>                 goto out_delete_evlist;
>
>         if (!top.evlist->core.nr_entries) {
> -               bool can_profile_kernel = perf_event_paranoid_check(1);
> -               int err = parse_event(top.evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
> +               int err = evlist__add_default(top.evlist);
>
>                 if (err)
>                         goto out_delete_evlist;
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 7ef43f72098e..60efa762405e 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -287,6 +287,24 @@ struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide)
>         return evsel;
>  }
>
> +int evlist__add_default(struct evlist *evlist)
> +{
> +       bool can_profile_kernel;
> +       int err;
> +
> +       can_profile_kernel = perf_event_paranoid_check(1);
> +       err = parse_event(evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
> +       if (err)
> +               return err;
> +
> +       if (!evlist->core.nr_entries) {
> +               pr_debug("The cycles event is not supported, trying to fall back to cpu-clock event\n");
> +               return parse_event(evlist, "cpu-clock");
> +       }
> +
> +       return 0;
> +}
> +
>  #ifdef HAVE_LIBTRACEEVENT
>  struct evsel *evlist__add_sched_switch(struct evlist *evlist, bool system_wide)
>  {
> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
> index 664c6bf7b3e0..47eea809ee91 100644
> --- a/tools/perf/util/evlist.h
> +++ b/tools/perf/util/evlist.h
> @@ -116,6 +116,7 @@ int arch_evlist__cmp(const struct evsel *lhs, const struct evsel *rhs);
>
>  int evlist__add_dummy(struct evlist *evlist);
>  struct evsel *evlist__add_aux_dummy(struct evlist *evlist, bool system_wide);
> +int evlist__add_default(struct evlist *evlist);
>  static inline struct evsel *evlist__add_dummy_on_all_cpus(struct evlist *evlist)
>  {
>         return evlist__add_aux_dummy(evlist, true);
> --
> 2.30.GIT
>