libvirt - Add log if libguestfs can't read host kernel

Host's kernel only allows a root user to have read/write permission in
ubuntu. If compute-service didn't have read permission then libguestfs
will launch image fail.

In libguestfs offical FAQ site had point out this issue, following is
the link
http://libguestfs.org/guestfs-faq.1.html#binaries
It had suggested users to change host's kernel permission. But this
action should be handled by other patch. Here only give a hint what's
going wrong.

Change-Id: I36c93610831e2935d46f7ee37f95619fe6dc1481
Related-Bug: 1413142
Closes-Bug: 1491216

nova/exception.py

@@ -2107,6 +2107,10 @@ class UnsupportedHostCPUControlPolicy(Invalid):
     msg_fmt = _("Requested CPU control policy not supported by host")
 
 
+class LibguestfsCannotReadKernel(Invalid):
+    msg_fmt = _("Libguestfs does not have permission to read host kernel.")
+
+
 class RealtimePolicyNotSupported(Invalid):
     msg_fmt = _("Realtime policy not supported by hypervisor")
 

nova/tests/unit/virt/disk/vfs/test_guestfs.py

@@ -310,3 +310,21 @@ class VirtDiskVFSGuestFSTest(test.NoDBTestCase):
         vfs = vfsimpl.VFSGuestFS(self.qcowfile)
         vfs.setup(mount=False)
         self.assertFalse(setup_os.called)
+
+    @mock.patch('os.access')
+    @mock.patch('os.uname', return_value=('Linux', '', 'kernel_name'))
+    def test_appliance_setup_inspect_capabilties_fail_with_ubuntu(self,
+                                                                  mock_uname,
+                                                                  mock_access):
+        # In ubuntu os will default host kernel as 600 permission
+        m = mock.MagicMock()
+        m.launch.side_effect = Exception
+        vfs = vfsimpl.VFSGuestFS(self.qcowfile)
+        with mock.patch('eventlet.tpool.Proxy', return_value=m):
+            self.assertRaises(exception.LibguestfsCannotReadKernel,
+                                    vfs.inspect_capabilities)
+            m.add_drive.assert_called_once_with('/dev/null')
+            m.launch.assert_called_once_with()
+            mock_access.assert_called_once_with('/boot/vmlinuz-kernel_name',
+                                                mock.ANY)
+            mock_uname.assert_called_once_with()

nova/virt/disk/vfs/guestfs.py

@@ -13,6 +13,7 @@
 # under the License.
 
 from eventlet import tpool
+import os
 from oslo_log import log as logging
 from oslo_utils import importutils
 import six
@@ -78,6 +79,8 @@ class VFSGuestFS(vfs.VFS):
             g.add_drive("/dev/null")  # sic
             g.launch()
         except Exception as e:
+            if os.access("/boot/vmlinuz-%s" % os.uname()[2], os.R_OK):
+                raise exception.LibguestfsCannotReadKernel()
             raise exception.NovaException(
                 _("libguestfs installed but not usable (%s)") % e)