dmi data: fix failure of reading dmi data for unset dmi values

it is not uncommon to find dmi data in /sys full of 'ff'. utf-8
decoding of those would fail, causing warning and stacktrace.

Return '.' instead of \xff. This is what dmidecode would return.

$ dmidecode --string system-product-name

3 files changed, 20 insertions, 3 deletions

diff --git a/ChangeLog b/ChangeLog
index da1ca9ee..ebaacf6a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -88,6 +88,7 @@
  - No longer run pollinate in seed_random (LP: #1554152)
  - groups: add defalt user to 'lxd' group.  Create groups listed
    for a user if they do not exist. (LP: #1539317)
+ - dmi data: fix failure of reading dmi data for unset dmi values
 
 0.7.6:
  - open 0.7.6
diff --git a/cloudinit/util.py b/cloudinit/util.py
index e7407ea4..01dc7751 100644
--- a/cloudinit/util.py
+++ b/cloudinit/util.py
@@ -2140,13 +2140,19 @@ def _read_dmi_syspath(key):
             LOG.debug("did not find %s", dmi_key_path)
             return None
 
-        key_data = load_file(dmi_key_path)
+        key_data = load_file(dmi_key_path, decode=False)
         if not key_data:
             LOG.debug("%s did not return any data", dmi_key_path)
             return None
 
-        LOG.debug("dmi data %s returned %s", dmi_key_path, key_data)
-        return key_data.strip()
+        # uninitialized dmi values show as all \xff and /sys appends a '\n'.
+        # in that event, return a string of '.' in the same length.
+        if key_data == b'\xff' * (len(key_data) - 1) + b'\n':
+            key_data = b'.' * (len(key_data) - 1) + b'\n'
+
+        str_data = key_data.decode('utf8').strip()
+        LOG.debug("dmi data %s returned %s", dmi_key_path, str_data)
+        return str_data
 
     except Exception:
         logexc(LOG, "failed read of %s", dmi_key_path)
diff --git a/tests/unittests/test_util.py b/tests/unittests/test_util.py
index 95990165..0a986fec 100644
--- a/tests/unittests/test_util.py
+++ b/tests/unittests/test_util.py
@@ -385,6 +385,16 @@ class TestReadDMIData(helpers.FilesystemMockingTestCase):
         self.patch_mapping({})
         self.assertEqual(None, util.read_dmi_data('expect-fail'))
 
+    def test_dots_returned_instead_of_foxfox(self):
+        # uninitialized dmi values show as \xff, return those as .
+        my_len = 32
+        dmi_value = b'\xff' * my_len + b'\n'
+        expected = '.' * my_len
+        dmi_key = 'system-product-name'
+        sysfs_key = 'product_name'
+        self._create_sysfs_file(sysfs_key, dmi_value)
+        self.assertEqual(expected, util.read_dmi_data(dmi_key))
+
 
 class TestMultiLog(helpers.FilesystemMockingTestCase):