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

[Yang Jihong]

Hello,

On 2023/6/15 10:04, Ian Rogers wrote:
> On Wed, Jun 14, 2023 at 6:55 PM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
> > Hello,
> >
> > On 2023/6/15 6:03, Ian Rogers wrote:
> > > On Wed, Jun 14, 2023 at 9:18 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> > > > 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?
> > >
> > > Hmm... it should be coming from evsel__fallback:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/util/evsel.c?h=tmp.perf-tools-next#n2840
> > > so we shouldn't duplicate that logic. The question is why we're not
> > > doing the fallback.
> >
> > Yes, it's a bit of the same logic as evsel__fallback, or we can call
> > evlist__add_default() as before, simply create an evsel of hardware
> > cycles and add it directly to evlist.
> >
> > Please confirm whether this solution is feasible. If it is feasible, I
> > will send a v2 version.
>
> The previous evlist__add_default logic didn't handle wildcard PMUs for
> cycles, hence wanting to reuse the parse events logic. The error is
> that the logic now isn't doing the fallback properly. I think an
> evlist__add_cycles which uses evsel__fallback makes sense matching the
> previous logic. I'd be happy if you took a look. I'll write a patch so
> that the perf_pmus list of core PMUs is never empty.

Do you mean to do fallback when parsing events? Instead of doing it at 
evsel__open?

Okay, I'll re-patch it with this solution, and it looks like it should work.

Thanks,
Yang