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

[Yang Jihong <yangjihong1@huawei.com>]

Hello,

On 2023/6/15 6:03, Ian Rogers wrote:
> On Wed, Jun 14, 2023 at 9:18 AM Ian Rogers <irogers@google.com> wrote:
>>
>> On Wed, Jun 14, 2023 at 8:18 AM Yang Jihong <yangjihong1@huawei.com> 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@huawei.com>
>>
>> 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.

Thanks,
Yang