VM: Use virtiofsd chroot sandbox mode on pre pidfd_open kernels (#13794)

Otherwise virtiofsd doesn't start and this causes VMs to hang on start
up when used on Ubuntu 18.04 generic kernels (4.15) with QEMU >= 8.0
(which replaced the bundled virtiofsd with an external one written in
rust which defaults to namespace sandbox mode relying on pidfd_open
syscall).

Fixes #12856

Tested on Ubuntu 18.04 4.15 kernel and Ubuntu 22.04 6.5 HWE kernel.

lxd/device/device_utils_disk.go

@@ -21,6 +21,7 @@ import (
 	"github.com/canonical/lxd/shared"
 	"github.com/canonical/lxd/shared/osarch"
 	"github.com/canonical/lxd/shared/revert"
+	"github.com/canonical/lxd/shared/version"
 )
 
 // RBDFormatPrefix is the prefix used in disk paths to identify RBD.
@@ -424,7 +425,7 @@ func DiskVMVirtfsProxyStop(pidPath string) error {
 // Returns UnsupportedError error if the host system or instance does not support virtiosfd, returns normal error
 // type if process cannot be started for other reasons.
 // Returns revert function and listener file handle on success.
-func DiskVMVirtiofsdStart(execPath string, inst instance.Instance, socketPath string, pidPath string, logPath string, sharePath string, idmaps []idmap.IdmapEntry) (func(), net.Listener, error) {
+func DiskVMVirtiofsdStart(kernelVersion version.DottedVersion, inst instance.Instance, socketPath string, pidPath string, logPath string, sharePath string, idmaps []idmap.IdmapEntry) (func(), net.Listener, error) {
 	revert := revert.New()
 	defer revert.Fail()
 
@@ -502,7 +503,18 @@ func DiskVMVirtiofsdStart(execPath string, inst instance.Instance, socketPath st
 	defer func() { _ = unixFile.Close() }()
 
 	// Start the virtiofsd process in non-daemon mode.
-	args := []string{"--fd=3", "-o", fmt.Sprintf("source=%s", sharePath)}
+	args := []string{
+		"--fd=3",
+		"-o", fmt.Sprintf("source=%s", sharePath),
+	}
+
+	// Virtiofsd defaults to namespace sandbox mode which requires pidfd_open support.
+	// This was added in Linux 5.3, so if running an earlier kernel fallback to chroot sandbox mode.
+	minVer, _ := version.NewDottedVersion("5.3.0")
+	if kernelVersion.Compare(minVer) < 0 {
+		args = append(args, "--sandbox=chroot")
+	}
+
 	proc, err := subprocess.NewProcess(cmd, args, logPath, logPath)
 	if err != nil {
 		return nil, nil, err

lxd/device/disk.go

@@ -1157,7 +1157,7 @@ func (d *disk) startVM() (*deviceConfig.RunConfig, error) {
 					logPath := filepath.Join(d.inst.LogPath(), fmt.Sprintf("disk.%s.log", filesystem.PathNameEncode(d.name)))
 					_ = os.Remove(logPath) // Remove old log if needed.
 
-					revertFunc, unixListener, err := DiskVMVirtiofsdStart(d.state.OS.ExecPath, d.inst, sockPath, pidPath, logPath, mount.DevPath, rawIDMaps)
+					revertFunc, unixListener, err := DiskVMVirtiofsdStart(d.state.OS.KernelVersion, d.inst, sockPath, pidPath, logPath, mount.DevPath, rawIDMaps)
 					if err != nil {
 						var errUnsupported UnsupportedError
 						if errors.As(err, &errUnsupported) {

lxd/instance/drivers/driver_qemu.go

@@ -1388,7 +1388,7 @@ func (d *qemu) start(stateful bool, op *operationlock.InstanceOperation) error {
 	// This is used by the lxd-agent in preference to 9p (due to its improved performance) and in scenarios
 	// where 9p isn't available in the VM guest OS.
 	configSockPath, configPIDPath := d.configVirtiofsdPaths()
-	revertFunc, unixListener, err := device.DiskVMVirtiofsdStart(d.state.OS.ExecPath, d, configSockPath, configPIDPath, "", configMntPath, nil)
+	revertFunc, unixListener, err := device.DiskVMVirtiofsdStart(d.state.OS.KernelVersion, d, configSockPath, configPIDPath, "", configMntPath, nil)
 	if err != nil {
 		var errUnsupported device.UnsupportedError
 		if !errors.As(err, &errUnsupported) {