Re: [PATCH] perf/core: generate overflow signal when samples are dropped (WAS: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region)

[Alexey Budankov]

On 29.06.2017 11:13, Alexey Budankov wrote:
> Hi Folks,
> 
> On 28.06.2017 16:07, Mark Rutland wrote:
>> On Wed, Jun 28, 2017 at 08:40:30AM -0400, Vince Weaver wrote:
>>> On Wed, 28 Jun 2017, Mark Rutland wrote:
>>>
>>>> On Wed, Jun 28, 2017 at 11:12:48AM +0100, Mark Rutland wrote:
>>>
>>>> Instead of bailing out early in perf_event_overflow, we can bail prior
>>>> to performing the actual sampling in __perf_event_output(). This avoids
>>>> the information leak, but preserves the generation of the signal.
>>>>
>>>> Since we don't place any sample data into the ring buffer, the signal is
>>>> arguably spurious. However, a userspace ringbuffer consumer can already
>>>> consume data prior to taking the associated signals, and therefore must
>>>> handle spurious signals to operate correctly. Thus, this signal
>>>> shouldn't be harmful.
>>>
>>> this could still break some of my perf_event validation tests.
>>>
>>> Ones that set up a sampling event for every 1M instructions, run for 100M 
>>> instructions, and expect there to be 100 samples received.
>>
>> Is that test reliable today?
>>
>> I'd expect that at least on ARM it's not, given that events can be
>> counted imprecisely, and mode filters can be applied imprecisely. So you
>> might get fewer (or more) samples. I'd imagine similar is true on other
>> archtiectures.
>>
>> If sampling took long enough, the existing ratelimiting could come into
>> effect, too.
>>
>> Surely that already has some error margin?
> 
> FYI.
> 
>>From my recent experience and observation (on Intel Xeon Phi) 
> wakeup_events_overflow and overflow_poll tests may fail if 
> /proc/sys/kernel/perf_event_max_sample_rate is low enough:
> 
> # cat /proc/sys/kernel/perf_event_max_sample_rate
> 6000
> # abudanko/perf_event_tests/tests/overflow/wakeup_events_overflow
> This tests wakeup event overflows.
> Testing with wakeup_events=1.
> Counts, using mmap buffer 0x7f707b0dc000
> 	POLL_IN : 10
> 	POLL_OUT: 0
> 	POLL_MSG: 0
> 	POLL_ERR: 0
> 	POLL_PRI: 0
> 	POLL_HUP: 0
> 	UNKNOWN : 0
> Testing wakeup events overflow...                            PASSED
> # abudanko/perf_event_tests/tests/overflow/overflow_poll
> This tests using poll() to catch overflow.
> Monitoring pid 131412 status 1407
> Child has stopped due to signal 5 (Trace/breakpoint trap)
> Continuing child
> Returned HUP!
> Counts, using mmap buffer 0x7fb3bfe04000
> 	POLL_IN : 10
> 	POLL_OUT: 0
> 	POLL_MSG: 0
> 	POLL_ERR: 0
> 	POLL_PRI: 0
> 	POLL_HUP: 1
> 	UNKNOWN : 0
> Testing catching overflow with poll()...                     PASSED
> 
> # echo 1000 > /proc/sys/kernel/perf_event_max_sample_rate
> # abudanko/perf_event_tests/tests/overflow/overflow_poll
> This tests using poll() to catch overflow.
> Monitoring pid 131551 status 1407
> Child has stopped due to signal 5 (Trace/breakpoint trap)
> Continuing child
> Returned HUP!
> Counts, using mmap buffer 0x7f80532df000
> 	POLL_IN : 9
> 	POLL_OUT: 0
> 	POLL_MSG: 0
> 	POLL_ERR: 0
> 	POLL_PRI: 0
> 	POLL_HUP: 1
> 	UNKNOWN : 0
> Unexpected POLL_IN interrupt.
> Testing catching overflow with poll()...                     FAILED
> [root@nntpdsd52-210 ~]# abudanko/perf_event_tests/tests/overflow/overflow_poll
> This tests using poll() to catch overflow.
> Monitoring pid 131553 status 1407
> Child has stopped due to signal 5 (Trace/breakpoint trap)
> Continuing child
> Returned HUP!
> Counts, using mmap buffer 0x7f650952c000
> 	POLL_IN : 9
> 	POLL_OUT: 0
> 	POLL_MSG: 0
> 	POLL_ERR: 0
> 	POLL_PRI: 0
> 	POLL_HUP: 1
> 	UNKNOWN : 0
> Unexpected POLL_IN interrupt.
> Testing catching overflow with poll()...                     FAILED

More of the other test:

# echo 1000 > /proc/sys/kernel/perf_event_max_sample_rate
[ ~]# abudanko/perf_event_tests/tests/overflow/wakeup_events_overflow
This tests wakeup event overflows.
Testing with wakeup_events=1.
Counts, using mmap buffer 0x7fe65deff000
	POLL_IN : 4
	POLL_OUT: 0
	POLL_MSG: 0
	POLL_ERR: 0
	POLL_PRI: 0
	POLL_HUP: 0
	UNKNOWN : 0
POLL_IN value 4, expected 10.
Testing wakeup events overflow...                            FAILED
[ ~]# abudanko/perf_event_tests/tests/overflow/wakeup_events_overflow
This tests wakeup event overflows.
Testing with wakeup_events=1.
Counts, using mmap buffer 0x7f818a732000
	POLL_IN : 2
	POLL_OUT: 0
	POLL_MSG: 0
	POLL_ERR: 0
	POLL_PRI: 0
	POLL_HUP: 0
	UNKNOWN : 0
POLL_IN value 2, expected 10.
Testing wakeup events overflow...                            FAILED
[ ~]# abudanko/perf_event_tests/tests/overflow/wakeup_events_overflow
This tests wakeup event overflows.
Testing with wakeup_events=1.
Counts, using mmap buffer 0x7fb3675c5000
	POLL_IN : 4
	POLL_OUT: 0
	POLL_MSG: 0
	POLL_ERR: 0
	POLL_PRI: 0
	POLL_HUP: 0
	UNKNOWN : 0
POLL_IN value 4, expected 10.
Testing wakeup events overflow...                            FAILED

> 
>>
>>> If we're so worried about info leakage, can't we just zero-out the problem 
>>> address (or randomize the kernel address) rather than just pretending the 
>>> interrupt didn't happen?
>>
>> Making up zeroed or randomized data is going to confuse users. I can't
>> imagine that real users are going to want bogus samples that they have
>> to identify (somehow) in order to skip when processing the data.
>>
>> I can see merit in signalling "lost" samples to userspace, so long as
>> they're easily distinguished from real samples.
>>
>> One option is to fake up a sample using the user regs regardless, but
>> that's both fragile and surprising in other cases.
>>
>> Thanks,
>> Mark.
>>
> 
> Thanks,
> Alexey
> 
> 
>