Re: [PATCH] perf/core: Fix perf_sample_data not properly initialized for different swevents in perf_tp_event()

[Yang Jihong <yangjihong1@huawei.com>]

Hello,

On 2023/4/17 19:45, Peter Zijlstra wrote:
> On Wed, Apr 12, 2023 at 09:52:40AM +0000, Yang Jihong wrote:
>> data->sample_flags may be modified in perf_prepare_sample(),
>> in perf_tp_event(), different swevents use the same on-stack
>> perf_sample_data, the previous swevent may change sample_flags in
>> perf_prepare_sample(), as a result, some members of perf_sample_data are
>> not correctly initialized when next swevent_event preparing sample
>> (for example data->id, the value varies according to swevent).
>>
>> A simple scenario triggers this problem is as follows:
>>
>>    # perf record -e sched:sched_switch --switch-output-event sched:sched_switch -a sleep 1
>>    [ perf record: dump data: Woken up 0 times ]
>>    [ perf record: Dump perf.data.2023041209014396 ]
>>    [ perf record: dump data: Woken up 0 times ]
>>    [ perf record: Dump perf.data.2023041209014662 ]
>>    [ perf record: dump data: Woken up 0 times ]
>>    [ perf record: Dump perf.data.2023041209014910 ]
>>    [ perf record: Woken up 0 times to write data ]
>>    [ perf record: Dump perf.data.2023041209015164 ]
>>    [ perf record: Captured and wrote 0.069 MB perf.data.<timestamp> ]
>>    # ls -l
>>    total 860
>>    -rw------- 1 root root  95694 Apr 12 09:01 perf.data.2023041209014396
>>    -rw------- 1 root root 606430 Apr 12 09:01 perf.data.2023041209014662
>>    -rw------- 1 root root  82246 Apr 12 09:01 perf.data.2023041209014910
>>    -rw------- 1 root root  82342 Apr 12 09:01 perf.data.2023041209015164
>>    # perf script -i perf.data.2023041209014396
>>    0x11d58 [0x80]: failed to process type: 9 [Bad address]
>>
>> Solution: Add perf_sample_data_flags_{save, restore} helpers to save and
>> restore sample_flags when processing different swevents
>>
>> After fix:
>>
>>    # perf record -e sched:sched_switch --switch-output-event sched:sched_switch -a sleep 1
>>    [ perf record: dump data: Woken up 0 times ]
>>    [ perf record: Dump perf.data.2023041209442259 ]
>>    [ perf record: dump data: Woken up 0 times ]
>>    [ perf record: Dump perf.data.2023041209442514 ]
>>    [ perf record: dump data: Woken up 0 times ]
>>    [ perf record: Dump perf.data.2023041209442760 ]
>>    [ perf record: Woken up 0 times to write data ]
>>    [ perf record: Dump perf.data.2023041209443003 ]
>>    [ perf record: Captured and wrote 0.069 MB perf.data.<timestamp> ]
>>    # ls -l
>>    total 864
>>    -rw------- 1 root root 100166 Apr 12 09:44 perf.data.2023041209442259
>>    -rw------- 1 root root 606438 Apr 12 09:44 perf.data.2023041209442514
>>    -rw------- 1 root root  82246 Apr 12 09:44 perf.data.2023041209442760
>>    -rw------- 1 root root  82342 Apr 12 09:44 perf.data.2023041209443003
>>    # perf script -i perf.data.2023041209442259 | head -n 5
>>                perf   232 [000]    66.846217: sched:sched_switch: prev_comm=perf prev_pid=232 prev_prio=120 prev_state=D ==> next_comm=perf next_pid=234 next_prio=120
>>                perf   234 [000]    66.846449: sched:sched_switch: prev_comm=perf prev_pid=234 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=232 next_prio=120
>>                perf   232 [000]    66.846546: sched:sched_switch: prev_comm=perf prev_pid=232 prev_prio=120 prev_state=R ==> next_comm=perf next_pid=234 next_prio=120
>>                perf   234 [000]    66.846606: sched:sched_switch: prev_comm=perf prev_pid=234 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=232 next_prio=120
>>                perf   232 [000]    66.846646: sched:sched_switch: prev_comm=perf prev_pid=232 prev_prio=120 prev_state=R ==> next_comm=perf next_pid=234 next_prio=120
> 
> This seems a little bit short on analysis; what actual flags are the
> problem? Much of the data will in fact be identical between these
> invocations and endlessly re-computing that is wasteful.
> 
In the preceding example, data->id is incorrectly initialized,
that is, PERF_SAMPLE_ID | PERF_SAMPLE_IDENTIFIER
The detailed process is as follows:
1. In the example of perf record, create two swevents evt1 and evt2,
and attach to sched:sched_switch, assume that the IDs are 10 and 11
2. In perf_tp_event():
struct perf_sample_data data // on-stack value
hlist_for_each_entry_rcu
  -> perf_swevent_event(evt1, &data)
   -> perf_swevent_overflow(evt1, &data)
    -> ...
     -> perf_prepare_sample(evt1, &data)
      -> __perf_event_header__init_id(evt1, &data)
       -> data->sample_flags |= data->type & PERF_SAMPLE_ID_ALL
          // data->sample_flags set PERF_SAMPLE_ID | PERF_SAMPLE_IDENTIFIER
       -> if (sample_type & (PERF_SAMPLE_ID | PERF_SAMPLE_IDENTIFIER))
                   data->id = primary_event_id(event);
          // for evt1, the if condition is met,
          // and data->id can be correctly initialized.
...
  -> perf_swevent_event(evt2, &data)
   -> perf_swevent_overflow(evt2, &data)
    -> ...
     -> perf_prepare_sample(evt2, &data)
      -> __perf_event_header__init_id(evt2, &data)
       -> if (sample_type & (PERF_SAMPLE_ID | PERF_SAMPLE_IDENTIFIER))
                   data->id = primary_event_id(event);
          // data->sample_flags set PERF_SAMPLE_ID | 
PERF_SAMPLE_IDENTIFIER in evt1,
          // therefore, the condition is not met for evt2.
          // As a result, the ID of evt2 is not assigned to data-id.

> I'm thinking perhaps those flags that update ->dyn_size are the problem?
> At the same time, Should you not also then clear dyn_size?
Yes, according to the code, dyn_size should also be cleared.
Maybe we need to change it to the following, which would be more 
appropriate?

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -10144,14 +10144,14 @@ void perf_tp_event(u16 event_type, u64 count, 
void *record, int entry_size,
                 },
         };

-       perf_sample_data_init(&data, 0, 0);
-       perf_sample_save_raw_data(&data, &raw);
-
         perf_trace_buf_update(record, event_type);

         hlist_for_each_entry_rcu(event, head, hlist_entry) {
-               if (perf_tp_event_match(event, &data, regs))
+               if (perf_tp_event_match(event, &data, regs)) {
+                       perf_sample_data_init(&data, 0, 0);
+                       perf_sample_save_raw_data(&data, &raw);
                         perf_swevent_event(event, count, &data, regs);
+               }
         }

Thanks,
Yang.


> 
> .
>