--- a/js/src/perf/pm_linux.cpp
+++ b/js/src/perf/pm_linux.cpp
@@ -134,23 +134,26 @@ Impl::~Impl()
 EventMask
 Impl::init(EventMask toMeasure)
 {
     MOZ_ASSERT(group_leader == -1);
     if (!toMeasure)
         return EventMask(0);
 
     EventMask measured = EventMask(0);
-    struct perf_event_attr attr;
+    union {
+      struct perf_event_attr attr;
+      char dummy[256];
+    };
     for (const auto& slot : kSlots) {
         if (!(toMeasure & slot.bit))
             continue;
 
-        memset(&attr, 0, sizeof(attr));
-        attr.size = sizeof(attr);
+        memset(&dummy, 0, sizeof(dummy));
+        attr.size = sizeof(dummy);
 
         // Set the type and config fields to indicate the counter we
         // want to enable.  We want read format 0, and we're not using
         // sampling, so leave those fields unset.
         attr.type = slot.type;
         attr.config = slot.config;
 
         // If this will be the group leader it should start off
@@ -288,19 +291,22 @@ PerfMeasurement::canMeasureSomething()
     // API.  If it doesn't, syscall(__NR_perf_event_open, ...) is
     // guaranteed to return -1 and set errno to ENOSYS.
     //
     // We set up input parameters that should provoke an EINVAL error
     // from a kernel that does implement perf_event_open, but we can't
     // be sure it will (newer kernels might add more event types), so
     // we have to take care to close any valid fd it might return.
 
-    struct perf_event_attr attr;
-    memset(&attr, 0, sizeof(attr));
-    attr.size = sizeof(attr);
+    union {
+      struct perf_event_attr attr;
+      char dummy[256];
+    };
+    memset(&dummy, 0, sizeof(dummy));
+    attr.size = sizeof(dummy);
     attr.type = PERF_TYPE_MAX;
 
     int fd = sys_perf_event_open(&attr, 0, -1, -1, 0);
     if (fd >= 0) {
         close(fd);
         return true;
     }
     return errno != ENOSYS;