Seeing crashes likely from negative touch indexes, check for them

ui/events/ozone/evdev/touch_event_converter_evdev.cc

Index: ui/events/ozone/evdev/touch_event_converter_evdev.cc
diff --git a/ui/events/ozone/evdev/touch_event_converter_evdev.cc b/ui/events/ozone/evdev/touch_event_converter_evdev.cc
index 3cca7ac012a268c59fbb9c2e20ccdf2c056c8967..c50a0bc4e1958ca14313887dceca4d17c6f55639 100644
--- a/ui/events/ozone/evdev/touch_event_converter_evdev.cc
+++ b/ui/events/ozone/evdev/touch_event_converter_evdev.cc
@@ -170,8 +170,9 @@ void TouchEventConverterEvdev::ProcessInputEvent(const input_event& input) {
   } else if(syn_dropped_) {
     // Do nothing. This branch indicates we have lost sync with the driver.
   } else if (input.type == EV_ABS) {
-    if (current_slot_ >= MAX_FINGERS) {
-      LOG(ERROR) << "too many touch events: " << current_slot_;
+    if ((current_slot_ >= MAX_FINGERS || current_slot_ < 0) &&
+        input.code != ABS_MT_SLOT) {

[spang, 2015/02/02 19:22:00]

Can't we keep the range check inside "case ABS_MT_SLOT"?

I don't see why we'd do it for every event. It should only be necessary when
updating current_slot_.

+      LOG(ERROR) << "invalid touch event index: " << current_slot_;
       return;
     }
     ProcessAbs(input);
@@ -223,13 +224,7 @@ void TouchEventConverterEvdev::ProcessAbs(const input_event& input) {
       events_[current_slot_].pressure_ /= pressure_max_ - pressure_min_;
       break;
     case ABS_MT_SLOT:
-      if (input.value >= MAX_FINGERS) {
-        LOG(ERROR) << "multi-touch slot " << input.value
-                   << " exceeds MAX_FINGERS";
-        break;
-      }
       current_slot_ = input.value;
-      altered_slots_.set(current_slot_);
       break;
     default:
       DVLOG(5) << "unhandled code for EV_ABS: " << input.code;