libvirt: Log breadcrumb for known encryption bug

The initial implementation of native LUKS support within Libvirt
introduced a small issue when using a passphrase that is a multiple of
16 bytes in size. This is documented in the following bug and associated
patch posted to the Libvirt development list:

Unable to use LUKS passphrase that is exactly 16 bytes long
https://bugzilla.redhat.com/show_bug.cgi?id=1447297

[libvirt] [PATCH] Fix padding of encrypted data
https://www.redhat.com/archives/libvir-list/2017-May/msg00030.html

This change introduces a known issue release note and logs an additional
breadcrumb when we appear to hit this with pointers to the above.

Closes-Bug: #1778044
Change-Id: Id346bce6e47431988cce7001abcf29a9faf2936a

nova/tests/unit/virt/libvirt/test_driver.py

@@ -7434,6 +7434,46 @@ class LibvirtConnTestCase(test.NoDBTestCase,
         self._test_check_discard(mock_log, driver_discard=None,
                                  bus='virtio', should_log=False)
 
+    @mock.patch('nova.virt.libvirt.blockinfo.get_info_from_bdm')
+    def test_attach_volume_with_libvirt_bug_breadcrumb(self, mock_get_info):
+        drvr = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), False)
+        instance = objects.Instance(**self.test_instance)
+        connection_info = {"driver_volume_type": "fake",
+                           "data": {"device_path": "/fake",
+                                    "access_mode": "rw"}}
+        bdm = {'device_name': 'vdb',
+               'disk_bus': 'fake-bus',
+               'device_type': 'fake-type'}
+        disk_info = {'bus': bdm['disk_bus'], 'type': bdm['device_type'],
+                     'dev': 'vdb'}
+        libvirt_exc = fakelibvirt.make_libvirtError(fakelibvirt.libvirtError,
+            "unable to execute QEMU command 'object-add': Incorrect number"
+            " of padding bytes (56) found on decrypted data",
+            error_code=fakelibvirt.VIR_ERR_INTERNAL_ERROR)
+
+        with test.nested(
+            mock.patch.object(drvr._host, 'get_guest'),
+            mock.patch('nova.virt.libvirt.driver.LOG'),
+            mock.patch.object(drvr, '_connect_volume'),
+            mock.patch.object(drvr, '_get_volume_config'),
+            mock.patch.object(drvr, '_check_discard_for_attach_volume'),
+            mock.patch.object(drvr, '_build_device_metadata'),
+        ) as (mock_get_guest, mock_log, mock_connect_volume,
+              mock_get_volume_config, mock_check_discard, mock_build_metadata):
+
+            mock_conf = mock.MagicMock()
+            mock_guest = mock.MagicMock()
+            mock_guest.attach_device.side_effect = libvirt_exc
+            mock_get_volume_config.return_value = mock_conf
+            mock_get_guest.return_value = mock_guest
+            mock_get_info.return_value = disk_info
+            mock_build_metadata.return_value = objects.InstanceDeviceMetadata()
+
+            self.assertRaises(fakelibvirt.libvirtError, drvr.attach_volume,
+                self.context, connection_info, instance, "/dev/vdb",
+                disk_bus=bdm['disk_bus'], device_type=bdm['device_type'])
+            mock_log.warning.assert_called_once()
+
     @mock.patch('nova.utils.get_image_from_system_metadata')
     @mock.patch('nova.virt.libvirt.blockinfo.get_info_from_bdm')
     @mock.patch('nova.virt.libvirt.host.Host._get_domain')

nova/virt/libvirt/driver.py

@@ -1415,6 +1415,16 @@ class LibvirtDriver(driver.ComputeDriver):
             instance.device_metadata = self._build_device_metadata(
                 context, instance)
             instance.save()
+
+        # TODO(lyarwood) Remove the following breadcrumb once all supported
+        # distributions provide Libvirt 3.3.0 or earlier with
+        # https://libvirt.org/git/?p=libvirt.git;a=commit;h=7189099 applied.
+        except libvirt.libvirtError as ex:
+            if 'Incorrect number of padding bytes' in six.text_type(ex):
+                LOG.warning(_('Failed to attach encrypted volume due to a '
+                    'known Libvirt issue, see the following bug for details: '
+                    'https://bugzilla.redhat.com/show_bug.cgi?id=1447297'))
+                raise
         except Exception:
             LOG.exception(_('Failed to attach volume at mountpoint: %s'),
                           mountpoint, instance=instance)

releasenotes/notes/bug-1778044-f498ee2f2cfb35ea.yaml

@@ -0,0 +1,15 @@
+---
+issues:
+  - |
+    The initial implementation of native LUKS decryption within Libvirt 2.2.0
+    had a `known issue`_ with the use of passphrases that were a multiple of 16
+    bytes in size. This was `resolved`_ in the upstream 3.3.0 release of
+    Libvirt and has been backported to various downstream distribution specific
+    versions.
+
+    A simple warning will reference the above if this issue is encountered by
+    Nova however operators of the environment will still need to update
+    Libvirt to a version where this issue has been fixed to resolve the issue.
+
+    .. _known issue: https://bugzilla.redhat.com/show_bug.cgi?id=1447297
+    .. _resolved: https://libvirt.org/git/?p=libvirt.git;a=commit;h=7189099