Handle EBRs and tiny partitions when removing metadata

Extended boot records (EBRs) are used in the MSDOS partitioning
scheme to manage extended partitions. EBRs are small partitions
themselves and Ironic will therefore treat them like any other
partition during cleaning. Since EBRs are smaller than a GPT
(33 sectors), dd'ing 33 sectors will fail. This patch proposes
a fix to handle EBRs (and other tiny paritions) by adjusting
the number of blocks writtent to be the partition size at max.

Story: #2008539
Task: #41628
Change-Id: Ie9809b3e4fe2bea6b66ee95c442111942e85d618

ironic_lib/disk_utils.py

@@ -80,6 +80,8 @@ _PARTED_PRINT_RE = re.compile(r"^(\d+):([\d\.]+)MiB:"
 CONFIGDRIVE_LABEL = "config-2"
 MAX_CONFIG_DRIVE_SIZE_MB = 64
 
+GPT_SIZE_SECTORS = 33
+
 # Maximum disk size supported by MBR is 2TB (2 * 1024 * 1024 MB)
 MAX_DISK_SIZE_MB_SUPPORTED_BY_MBR = 2097152
 
@@ -497,13 +499,24 @@ def destroy_disk_metadata(dev, node_uuid):
     # erasing partition data.
     # This is the same bug as
     # https://bugs.launchpad.net/ironic-python-agent/+bug/1737556
+
+    # Overwrite the Primary GPT, catch very small partitions (like EBRs)
     dd_device = 'of=%s' % dev
-    gpt_backup = get_dev_block_size(dev) - 33
-    dd_seek = 'seek=%i' % gpt_backup
-    utils.execute('dd', 'bs=512', 'if=/dev/zero', dd_device, 'count=33',
+    dd_count = 'count=%s' % GPT_SIZE_SECTORS
+    dev_size = get_dev_block_size(dev)
+    if dev_size < GPT_SIZE_SECTORS:
+        dd_count = 'count=%s' % dev_size
+    utils.execute('dd', 'bs=512', 'if=/dev/zero', dd_device, dd_count,
                   run_as_root=True, use_standard_locale=True)
-    utils.execute('dd', 'bs=512', 'if=/dev/zero', dd_device, 'count=33',
-                  dd_seek, run_as_root=True, use_standard_locale=True)
+
+    # Overwrite the Secondary GPT, do this only if there could be one
+    if dev_size > GPT_SIZE_SECTORS:
+        gpt_backup = dev_size - GPT_SIZE_SECTORS
+        dd_seek = 'seek=%i' % gpt_backup
+        dd_count = 'count=%s' % GPT_SIZE_SECTORS
+        utils.execute('dd', 'bs=512', 'if=/dev/zero', dd_device, dd_count,
+                      dd_seek, run_as_root=True, use_standard_locale=True)
+
     # Go ahead and let sgdisk run as well.
     utils.execute('sgdisk', '-Z', dev, run_as_root=True,
                   use_standard_locale=True)

ironic_lib/tests/test_disk_utils.py

@@ -800,6 +800,58 @@ class DestroyMetaDataTestCase(base.IronicLibTestCase):
         disk_utils.destroy_disk_metadata(self.dev, self.node_uuid)
         mock_exec.assert_has_calls(expected_call)
 
+    def test_destroy_disk_metadata_ebr(self, mock_exec):
+        expected_calls = [mock.call('wipefs', '--force', '--all', 'fake-dev',
+                                    run_as_root=True,
+                                    use_standard_locale=True),
+                          mock.call('blockdev', '--getsz', 'fake-dev',
+                                    check_exit_code=[0],
+                                    run_as_root=True),
+                          mock.call('dd', 'bs=512', 'if=/dev/zero',
+                                    'of=fake-dev', 'count=2',
+                                    run_as_root=True,
+                                    use_standard_locale=True),
+                          mock.call('sgdisk', '-Z', 'fake-dev',
+                                    run_as_root=True,
+                                    use_standard_locale=True)]
+        mock_exec.side_effect = iter([
+            (None, None),
+            ('2\n', None),  # an EBR is 2 sectors
+            (None, None),
+            (None, None),
+            (None, None),
+            (None, None)])
+        disk_utils.destroy_disk_metadata(self.dev, self.node_uuid)
+        mock_exec.assert_has_calls(expected_calls)
+
+    def test_destroy_disk_metadata_tiny_partition(self, mock_exec):
+        expected_calls = [mock.call('wipefs', '--force', '--all', 'fake-dev',
+                                    run_as_root=True,
+                                    use_standard_locale=True),
+                          mock.call('blockdev', '--getsz', 'fake-dev',
+                                    check_exit_code=[0],
+                                    run_as_root=True),
+                          mock.call('dd', 'bs=512', 'if=/dev/zero',
+                                    'of=fake-dev', 'count=33',
+                                    run_as_root=True,
+                                    use_standard_locale=True),
+                          mock.call('dd', 'bs=512', 'if=/dev/zero',
+                                    'of=fake-dev', 'count=33', 'seek=9',
+                                    run_as_root=True,
+                                    use_standard_locale=True),
+                          mock.call('sgdisk', '-Z', 'fake-dev',
+                                    run_as_root=True,
+                                    use_standard_locale=True)]
+        mock_exec.side_effect = iter([
+            (None, None),
+            ('42\n', None),
+            (None, None),
+            (None, None),
+            (None, None),
+            (None, None)])
+        disk_utils.destroy_disk_metadata(self.dev, self.node_uuid)
+        mock_exec.assert_has_calls(expected_calls)
+
 
 @mock.patch.object(utils, 'execute', autospec=True)
 class GetDeviceBlockSizeTestCase(base.IronicLibTestCase):

releasenotes/notes/erase-tiny-partitions-c408a3a4afe60d44.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes cleaning errors when trying to erase a GPT from a partition
+    which is smaller than a GPT (33 sectors).
+