--- a/testing/mozharness/mozharness/mozilla/testing/testbase.py
+++ b/testing/mozharness/mozharness/mozilla/testing/testbase.py
@@ -8,16 +8,17 @@
 import copy
 import os
 import platform
 import pprint
 import re
 import urllib2
 import json
 import socket
+import errno
 
 from mozharness.base.errors import BaseErrorList
 from mozharness.base.log import FATAL, WARNING
 from mozharness.base.python import (
     ResourceMonitoringMixin,
     VirtualenvMixin,
     virtualenv_config_options,
 )
@@ -752,17 +753,25 @@ Did you run with --create-virtualenv? Is
                     cache=c.get('tooltool_cache')
                 )
             except KeyError:
                 self.error('missing a required key.')
 
             abs_minidump_path = os.path.join(dirs['abs_work_dir'],
                                              minidump_stackwalk_path)
             if os.path.exists(abs_minidump_path):
-                self.chmod(abs_minidump_path, 0755)
+                try:
+                    self.chmod(abs_minidump_path, 0755)
+                except OSError as e:
+                    # docker image for Linux tests on Taskcluster already
+                    # ship minidump stackwalk. If mozharness finds it, it
+                    # won't have permission to modify bits. It is harmless,
+                    # however, because it already has executable permission.
+                    if e.errno != errno.EPERM:
+                        raise
                 self.minidump_stackwalk_path = abs_minidump_path
             else:
                 self.warning("minidump stackwalk path was given but couldn't be found. "
                              "Tried looking in '%s'" % abs_minidump_path)
                 # don't burn the job but we should at least turn them orange so it is caught
                 self.buildbot_status(TBPL_WARNING, WARNING)
 
         return self.minidump_stackwalk_path