Fix operation not permitted with systemd-homed

Fix issue 2056

With this commit `nerdctl run hello-world` works, but `nerdctl run alpine` still fails with another error

```
$ ./nerdctl run -it --rm alpine:3.17.0
docker.io/library/alpine:3.17.0:                                                  resolved       |++++++++++++++++++++++++++++++++++++++|
index-sha256:8914eb54f968791faf6a8638949e480fef81e697984fba772b3976835194c6d4:    done           |++++++++++++++++++++++++++++++++++++++|
manifest-sha256:c0d488a800e4127c334ad20d61d7bc21b4097540327217dfab52262adc02380c: waiting        |--------------------------------------|
config-sha256:49176f190c7e9cdb51ac85ab6c6d5e4512352218190cd69b08e6fd803ffbf3da:   done           |++++++++++++++++++++++++++++++++++++++|
layer-sha256:c158987b05517b6f2c5913f3acef1f2182a32345a304fe357e3ace5fadcad715:    downloading    |+++++++++++++++++++++++---------------|  2.0 MiB/3.2 MiB
elapsed: 10.4s                                                                    total:  2.0 Mi (197.1 KiB/s)
FATA[0010] failed to extract layer sha256:ded7a220bb058e28ee3254fbba04ca90b679070424424761a53a043b93b612bf: mount callback failed on /var/lib/containerd/tmpmounts/containerd-mount762573051:
failed to Lchown "/var/lib/containerd/tmpmounts/containerd-mount762573051/etc/shadow" for UID 0, GID 42: lchown /var/lib/containerd/tmpmounts/containerd-mount762573051/etc/shadow: value too large for defined data type: unknown

$ containerd-rootless-setuptool.sh nsenter -- cat /proc/self/uid_map
         0      60444          1
         1     200000      65536

$ containerd-rootless-setuptool.sh nsenter -- cat /proc/self/gid_map
         0      60444          1
         1     200000      65536
```

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/nerdctl/container_run_mount.go

@@ -29,6 +29,7 @@ import (
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/oci"
+	"github.com/containerd/containerd/pkg/userns"
 	"github.com/containerd/continuity/fs"
 	"github.com/containerd/nerdctl/pkg/api/types"
 	"github.com/containerd/nerdctl/pkg/idgen"
@@ -189,6 +190,24 @@ func generateMountOpts(ctx context.Context, cmd *cobra.Command, client *containe
 					return nil, nil, nil, err
 				}
 			}
+		} else if runtime.GOOS == "linux" {
+			for _, m := range mounts {
+				defer unmounter(m.Source)
+				if m.Type == "bind" && userns.RunningInUserNS() {
+					// For https://github.com/containerd/nerdctl/issues/2056
+					unpriv, err := mountutil.UnprivilegedMountFlags(m.Source)
+					if err != nil {
+						return nil, nil, nil, err
+					}
+					m.Options = strutil.DedupeStrSlice(append(m.Options, unpriv...))
+				}
+				if err := m.Mount(tempDir); err != nil {
+					if err := s.Remove(ctx, tempDir); err != nil && !errdefs.IsNotFound(err) {
+						return nil, nil, nil, err
+					}
+					return nil, nil, nil, err
+				}
+			}
 		} else {
 			defer unmounter(tempDir)
 			if err := mount.All(mounts, tempDir); err != nil {

pkg/mountutil/mountutil.go

@@ -144,7 +144,7 @@ func ProcessFlagV(s string, volStore volumestore.VolumeStore) (*Processed, error
 		Options:     options,
 	}
 	if userns.RunningInUserNS() {
-		unpriv, err := getUnprivilegedMountFlags(src)
+		unpriv, err := UnprivilegedMountFlags(src)
 		if err != nil {
 			return nil, err
 		}

pkg/mountutil/mountutil_freebsd.go

@@ -26,7 +26,7 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
-func getUnprivilegedMountFlags(path string) ([]string, error) {
+func UnprivilegedMountFlags(path string) ([]string, error) {
 	m := []string{}
 	return m, nil
 }

pkg/mountutil/mountutil_linux.go

@@ -43,14 +43,14 @@ import (
    NOTICE: https://github.com/moby/moby/blob/v20.10.5/NOTICE
 */
 
-// getUnprivilegedMountFlags is from https://github.com/moby/moby/blob/v20.10.5/daemon/oci_linux.go#L420-L450
+// UnprivilegedMountFlags is from https://github.com/moby/moby/blob/v20.10.5/daemon/oci_linux.go#L420-L450
 //
 // Get the set of mount flags that are set on the mount that contains the given
 // path and are locked by CL_UNPRIVILEGED. This is necessary to ensure that
 // bind-mounting "with options" will not fail with user namespaces, due to
 // kernel restrictions that require user namespace mounts to preserve
 // CL_UNPRIVILEGED locked flags.
-func getUnprivilegedMountFlags(path string) ([]string, error) {
+func UnprivilegedMountFlags(path string) ([]string, error) {
 	var statfs unix.Statfs_t
 	if err := unix.Statfs(path, &statfs); err != nil {
 		return nil, err

pkg/mountutil/mountutil_windows.go

@@ -26,7 +26,7 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
-func getUnprivilegedMountFlags(path string) ([]string, error) {
+func UnprivilegedMountFlags(path string) ([]string, error) {
 	m := []string{}
 	return m, nil
 }