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 > > >