Revert "perf/core: Drop kernel samples even though :u is specified"

This reverts commit cc1582c.

This commit introduced a regression that broke rr-project, which uses sampling
events to receive a signal on overflow (but does not care about the contents
of the sample). These signals are critical to the correct operation of rr.

There's been some back and forth about how to fix it - but to not keep
applications in limbo queue up a revert.

Reported-by: Kyle Huey <[email protected]>
Acked-by: Kyle Huey <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Cc: Jin Yao <[email protected]>
Cc: Vince Weaver <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: <[email protected]>
Link: http://lkml.kernel.org/r/20170628105600.GC5981@leverpostej
Signed-off-by: Ingo Molnar <[email protected]>

Author: Ingo Molnar

kernel/events/core.c

@@ -7308,21 +7308,6 @@ int perf_event_account_interrupt(struct perf_event *event)
 	return __perf_event_account_interrupt(event, 1);
 }
 
-static bool sample_is_allowed(struct perf_event *event, struct pt_regs *regs)
-{
-	/*
-	 * Due to interrupt latency (AKA "skid"), we may enter the
-	 * kernel before taking an overflow, even if the PMU is only
-	 * counting user events.
-	 * To avoid leaking information to userspace, we must always
-	 * reject kernel samples when exclude_kernel is set.
-	 */
-	if (event->attr.exclude_kernel && !user_mode(regs))
-		return false;
-
-	return true;
-}
-
 /*
  * Generic event overflow handling, sampling.
  */
@@ -7343,12 +7328,6 @@ static int __perf_event_overflow(struct perf_event *event,
 
 	ret = __perf_event_account_interrupt(event, throttle);
 
-	/*
-	 * For security, drop the skid kernel samples if necessary.
-	 */
-	if (!sample_is_allowed(event, regs))
-		return ret;
-
 	/*
 	 * XXX event_limit might not quite work as expected on inherited
 	 * events