From c2044d4bd0919860aa2d49687ba9c6ef6f7d37e8 Mon Sep 17 00:00:00 2001
From: Balazs Gibizer <balazs.gibizer@est.tech>
Date: Thu, 2 Jul 2020 12:13:29 +0200
Subject: [PATCH] Use absolute path during qemu img rebase

During an assisted volume snapshot delete request from Cinder nova
removes the snapshot from the backing file chain. During that nova
checks the existence of such file. However in some cases (see the bug
report) the path is relative and therefore os.path.exists fails.

This patch makes sure that nova uses the volume absolute path to make
the backing file path absolute as well.

Closes-Bug #1885528

Change-Id: I58dca95251b607eaff602783fee2fc38e2421944
(cherry picked from commit b9333125790682f9d60bc74fdbb12a098565e7c2)
(cherry picked from commit 831abc9f83a2d3f517030f881e7da724417fea93)
---
 nova/tests/unit/virt/libvirt/test_driver.py | 28 +++++++++++++++++----
 nova/virt/libvirt/driver.py                 | 11 +++++++-
 2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/nova/tests/unit/virt/libvirt/test_driver.py b/nova/tests/unit/virt/libvirt/test_driver.py
index 05480a96ca7d..32d7723b5a00 100644
--- a/nova/tests/unit/virt/libvirt/test_driver.py
+++ b/nova/tests/unit/virt/libvirt/test_driver.py
@@ -25779,8 +25779,23 @@ class LibvirtVolumeSnapshotTestCase(test.NoDBTestCase):
         not running should trigger a blockRebase using qemu-img not libvirt.
         In this test, we rebase the image with another image as backing file.
         """
+        dom_xml = """
+              <domain type='kvm'>
+                <devices>
+                  <disk type='file'>
+                     <source file='/var/lib/nova/instances/%s/disk1_file'/>
+                     <target dev='vda' bus='virtio'/>
+                     <serial>0e38683e-f0af-418f-a3f1-6b67ea0f919d</serial>
+                  </disk>
+                  <disk type='block'>
+                    <source dev='/path/to/dev/1'/>
+                    <target dev='vdb' bus='virtio' serial='1234'/>
+                  </disk>
+                </devices>
+              </domain>""" % self.inst['uuid']
+
         mock_domain, guest = self._setup_block_rebase_domain_and_guest_mocks(
-                                self.dom_xml)
+                                dom_xml)
 
         instance = objects.Instance(**self.inst)
         snapshot_id = 'snapshot-1234'
@@ -25791,10 +25806,13 @@ class LibvirtVolumeSnapshotTestCase(test.NoDBTestCase):
                                               self.delete_info_1)
 
         mock_disk_op_sema.__enter__.assert_called_once()
-        mock_qemu_img_info.assert_called_once_with("snap.img")
-        mock_execute.assert_called_once_with('qemu-img', 'rebase',
-                                             '-b', 'snap.img', '-F',
-                                             'fake_fmt', 'disk1_file')
+        mock_qemu_img_info.assert_called_once_with(
+            "/var/lib/nova/instances/%s/snap.img" % instance.uuid)
+        mock_execute.assert_called_once_with(
+            'qemu-img', 'rebase',
+            '-b', '/var/lib/nova/instances/%s/snap.img' % instance.uuid,
+            '-F', 'fake_fmt',
+            '/var/lib/nova/instances/%s/disk1_file' % instance.uuid)
 
     @mock.patch.object(compute_utils, 'disk_ops_semaphore')
     @mock.patch.object(host.Host, "has_min_version",
diff --git a/nova/virt/libvirt/driver.py b/nova/virt/libvirt/driver.py
index 9e852a4ccf7b..5171e9a7c31d 100644
--- a/nova/virt/libvirt/driver.py
+++ b/nova/virt/libvirt/driver.py
@@ -2953,7 +2953,16 @@ class LibvirtDriver(driver.ComputeDriver):
             # If the rebased image is going to have a backing file then
             # explicitly set the backing file format to avoid any security
             # concerns related to file format auto detection.
-            backing_file = rebase_base
+            if os.path.isabs(rebase_base):
+                backing_file = rebase_base
+            else:
+                # this is a probably a volume snapshot case where the
+                # rebase_base is relative. See bug
+                # https://bugs.launchpad.net/nova/+bug/1885528
+                backing_file_name = os.path.basename(rebase_base)
+                volume_path = os.path.dirname(source_path)
+                backing_file = os.path.join(volume_path, backing_file_name)
+
             b_file_fmt = images.qemu_img_info(backing_file).file_format
             qemu_img_extra_arg = ['-F', b_file_fmt]