[PATCH 3/3] perf/x86/intel: Disallow precise_ip on BTS event

[Jiri Olsa]

Vince reported crash in bts flush code when touching the callchain
data, which was supposed to be initialized as an 'early' callchain,
but intel_pmu_drain_bts_buffer does not do that.

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
  ...
  Call Trace:
   <IRQ>
   intel_pmu_drain_bts_buffer+0x151/0x220
   ? intel_get_event_constraints+0x219/0x360
   ? perf_assign_events+0xe2/0x2a0
   ? select_idle_sibling+0x22/0x3a0
   ? __update_load_avg_se+0x1ec/0x270
   ? enqueue_task_fair+0x377/0xdd0
   ? cpumask_next_and+0x19/0x20
   ? load_balance+0x134/0x950
   ? check_preempt_curr+0x7a/0x90
   ? ttwu_do_wakeup+0x19/0x140
   x86_pmu_stop+0x3b/0x90
   x86_pmu_del+0x57/0x160
   event_sched_out.isra.106+0x81/0x170
   group_sched_out.part.108+0x51/0xc0
   __perf_event_disable+0x7f/0x160
   event_function+0x8c/0xd0
   remote_function+0x3c/0x50
   flush_smp_call_function_queue+0x35/0xe0
   smp_call_function_single_interrupt+0x3a/0xd0
   call_function_single_interrupt+0xf/0x20
   </IRQ>

It was triggered by fuzzer but can be easily reproduced by:
  # perf record -e cpu/branch-instructions/pu -g -c 1

Peter suggested not to allow branch tracing for precise events:

> Now arguably, this is really stupid behaviour. Who in his right mind
> wants callchain output on BTS entries. And even if they do, BTS +
> precise_ip is nonsensical.
>
> So in my mind disallowing precise_ip on BTS would be the simplest fix.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Fixes: 6cbc304f2f36 ("perf/x86/intel: Fix unwind errors from PEBS entries (mk-II)")
Reported-by: Vince Weaver <vincent.weaver@xxxxxxxxx>
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 arch/x86/events/intel/core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 3fd3842571e8..284893373d69 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -3102,6 +3102,10 @@ static int intel_pmu_bts_config(struct perf_event *event)
 		if (!attr->exclude_kernel)
 			return -EOPNOTSUPP;
 
+		/* BTS is not allowed for precise events. */
+		if (attr->precise_ip)
+			return -EOPNOTSUPP;
+
 		/* disallow bts if conflicting events are present */
 		if (x86_add_exclusive(x86_lbr_exclusive_lbr))
 			return -EBUSY;
-- 
2.17.2