iotests: Test mirror-top filter permissions

Add a test accompanying commit 53431b9086
("block/mirror: Fix mirror_top's permissions").

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210331122815.51491-1-mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/tests/mirror-top-perms

@@ -0,0 +1,121 @@
+#!/usr/bin/env python3
+# group: rw
+#
+# Test permissions taken by the mirror-top filter
+#
+# Copyright (C) 2021 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+import os
+import iotests
+from iotests import qemu_img
+
+# Import qemu after iotests.py has amended sys.path
+# pylint: disable=wrong-import-order
+import qemu
+
+
+image_size = 1 * 1024 * 1024
+source = os.path.join(iotests.test_dir, 'source.img')
+
+
+class TestMirrorTopPerms(iotests.QMPTestCase):
+    def setUp(self):
+        assert qemu_img('create', '-f', iotests.imgfmt, source,
+                        str(image_size)) == 0
+        self.vm = iotests.VM()
+        self.vm.add_drive(source)
+        self.vm.add_blockdev(f'null-co,node-name=null,size={image_size}')
+        self.vm.launch()
+
+        # Will be created by the test function itself
+        self.vm_b = None
+
+    def tearDown(self):
+        try:
+            self.vm.shutdown()
+        except qemu.machine.AbnormalShutdown:
+            pass
+
+        if self.vm_b is not None:
+            self.vm_b.shutdown()
+
+        os.remove(source)
+
+    def test_cancel(self):
+        """
+        Before commit 53431b9086b28, mirror-top used to not take any
+        permissions but WRITE and share all permissions.  Because it
+        is inserted between the source's original parents and the
+        source, there generally was no parent that would have taken or
+        unshared any permissions on the source, which means that an
+        external process could access the image unhindered by locks.
+        (Unless there was a parent above the protocol node that would
+        take its own locks, e.g. a format driver.)
+        This is bad enough, but if the mirror job is then cancelled,
+        the mirroring VM tries to take back the image, restores the
+        original permissions taken and unshared, and assumes this must
+        just work.  But it will not, and so the VM aborts.
+
+        Commit 53431b9086b28 made mirror keep the original permissions
+        and so no other process can "steal" the image.
+
+        (Note that you cannot really do the same with the target image
+        and then completing the job, because the mirror job always
+        took/unshared the correct permissions on the target.  For
+        example, it does not share READ_CONSISTENT, which makes it
+        difficult to let some other qemu process open the image.)
+        """
+
+        result = self.vm.qmp('blockdev-mirror',
+                             job_id='mirror',
+                             device='drive0',
+                             target='null',
+                             sync='full')
+        self.assert_qmp(result, 'return', {})
+
+        self.vm.event_wait('BLOCK_JOB_READY')
+
+        # We want this to fail because the image cannot be locked.
+        # If it does not fail, continue still and see what happens.
+        self.vm_b = iotests.VM(path_suffix='b')
+        # Must use -blockdev -device so we can use share-rw.
+        # (And we need share-rw=on because mirror-top was always
+        # forced to take the WRITE permission so it can write to the
+        # source image.)
+        self.vm_b.add_blockdev(f'file,node-name=drive0,filename={source}')
+        self.vm_b.add_device('virtio-blk,drive=drive0,share-rw=on')
+        try:
+            self.vm_b.launch()
+            print('ERROR: VM B launched successfully, this should not have '
+                  'happened')
+        except qemu.qmp.QMPConnectError:
+            assert 'Is another process using the image' in self.vm_b.get_log()
+
+        result = self.vm.qmp('block-job-cancel',
+                             device='mirror')
+        self.assert_qmp(result, 'return', {})
+
+        self.vm.event_wait('BLOCK_JOB_COMPLETED')
+
+
+if __name__ == '__main__':
+    # No metadata format driver supported, because they would for
+    # example always unshare the WRITE permission.  The raw driver
+    # just passes through the permissions from the guest device, and
+    # those are the permissions that we want to test.
+    iotests.main(supported_fmts=['raw'],
+                 supported_protocols=['file'])

tests/qemu-iotests/tests/mirror-top-perms.out

@@ -0,0 +1,5 @@
+.
+----------------------------------------------------------------------
+Ran 1 tests
+
+OK