osutils/disks: work-around old UC 20 bug where udev did not run on de…

…vice

There is a bug in old UC20 kernels where udev does not retrieve data
from cold plugged disks initialized in initrd.

It was fixed in:

* canonical/core-initrd#58
* canonical/core-initrd#203

osutil/disks/disks_linux.go

@@ -976,7 +976,22 @@ func DMCryptUUIDFromMountPoint(mountpoint string) (string, error) {
 
 	dmUUID, hasDmUUID := props["DM_UUID"]
 	if !hasDmUUID {
-		return "", ErrNoDmUUID
+		// There is a bug in old UC20 kernels where udev does
+		// not retrieve data from cold plugged disks
+		// initialized in initrd.
+		// https://github.com/canonical/core-initrd/pull/58
+		// https://github.com/canonical/core-initrd/pull/203
+		devPath, hasDevPath := props["DEVPATH"]
+		if !hasDevPath {
+			return "", ErrNoDmUUID
+		}
+		devUUIDPath := filepath.Join(dirs.SysfsDir,  devPath, "dm", "uuid")
+		data, err := os.ReadFile(devUUIDPath)
+		if err != nil {
+			return "", ErrNoDmUUID
+		}
+		dmUUID = strings.TrimSuffix(string(data), "\n")
+
 	}
 
 	match := dmUUIDRe.FindStringSubmatchIndex(dmUUID)

osutil/disks/disks_linux_test.go

@@ -1913,6 +1913,36 @@ func (s *diskSuite) TestDMCryptUUIDFromMountPoint(c *C) {
 	c.Assert(uuid, Equals, "5a522809-c87e-4dfa-81a8-8dc5667d1304")
 }
 
+func (s *diskSuite) TestDMCryptUUIDFromMountPointFallback(c *C) {
+	restore := osutil.MockMountInfo(`130 30 42:1 / /run/mnt/point rw,relatime shared:54 - ext4 /dev/mapper/something rw
+`)
+	defer restore()
+	restore = disks.MockUdevPropertiesForDevice(func(typeOpt, dev string) (map[string]string, error) {
+		c.Assert(typeOpt, Equals, "--name")
+		switch dev {
+		case "/dev/mapper/something":
+			return map[string]string{
+				"DEVPATH": "/devices/virtual/mydevice",
+
+			}, nil
+		default:
+			c.Errorf("unexpected udev device properties requested: %s", dev)
+			return nil, fmt.Errorf("unexpected udev device: %s", dev)
+		}
+	})
+	defer restore()
+
+	err := os.MkdirAll(filepath.Join(dirs.SysfsDir, "devices/virtual/mydevice", "dm"), 0755)
+	c.Assert(err, IsNil)
+	b := []byte("CRYPT-LUKS2-5a522809c87e4dfa81a88dc5667d1304-something\n")
+	err = os.WriteFile(filepath.Join(dirs.SysfsDir, "devices/virtual/mydevice", "dm/uuid"), b, 0644)
+	c.Assert(err, IsNil)
+
+	uuid, err := disks.DMCryptUUIDFromMountPoint("/run/mnt/point")
+	c.Assert(err, IsNil)
+	c.Assert(uuid, Equals, "5a522809-c87e-4dfa-81a8-8dc5667d1304")
+}
+
 func (s *diskSuite) TestFilesystemTypeForPartition(c *C) {
 	restore := disks.MockUdevPropertiesForDevice(func(typeOpt, dev string) (map[string]string, error) {
 		c.Assert(typeOpt, Equals, "--name")