cri/sbserver: Copy userns helpers to podsandbox

Currently there is a big c&p of the helpers between these two folders
and a TODO in the platform agnostic file to organize them in the future,
when some other things settle.

So, let's just copy them for now.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

pkg/cri/sbserver/podsandbox/helpers_linux.go

@@ -33,7 +33,7 @@ import (
 	"github.com/containerd/containerd/pkg/seccomp"
 	"github.com/containerd/containerd/pkg/seutil"
 	"github.com/moby/sys/mountinfo"
-	"github.com/opencontainers/runtime-spec/specs-go"
+	runtimespec "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/opencontainers/selinux/go-selinux/label"
 	"golang.org/x/sys/unix"
 	runtime "k8s.io/cri-api/pkg/apis/runtime/v1"
@@ -242,7 +242,7 @@ func isVMBasedRuntime(runtimeType string) bool {
 	return false
 }
 
-func modifyProcessLabel(runtimeType string, spec *specs.Spec) error {
+func modifyProcessLabel(runtimeType string, spec *runtimespec.Spec) error {
 	if !isVMBasedRuntime(runtimeType) {
 		return nil
 	}
@@ -253,3 +253,71 @@ func modifyProcessLabel(runtimeType string, spec *specs.Spec) error {
 	spec.Process.SelinuxLabel = l
 	return nil
 }
+
+func parseUsernsIDMap(runtimeIDMap []*runtime.IDMapping) ([]runtimespec.LinuxIDMapping, error) {
+	var m []runtimespec.LinuxIDMapping
+
+	if len(runtimeIDMap) == 0 {
+		return m, nil
+	}
+
+	if len(runtimeIDMap) > 1 {
+		// We only accept 1 line, because containerd.WithRemappedSnapshot() only supports that.
+		return m, fmt.Errorf("only one mapping line supported, got %v mapping lines", len(runtimeIDMap))
+	}
+
+	// We know len is 1 now.
+	if runtimeIDMap[0] == nil {
+		return m, nil
+	}
+	uidMap := *runtimeIDMap[0]
+
+	if uidMap.Length < 1 {
+		return m, fmt.Errorf("invalid mapping length: %v", uidMap.Length)
+	}
+
+	m = []runtimespec.LinuxIDMapping{
+		{
+			ContainerID: uidMap.ContainerId,
+			HostID:      uidMap.HostId,
+			Size:        uidMap.Length,
+		},
+	}
+
+	return m, nil
+}
+
+func parseUsernsIDs(userns *runtime.UserNamespace) (uids, gids []runtimespec.LinuxIDMapping, retErr error) {
+	if userns == nil {
+		// If userns is not set, the kubelet doesn't support this option
+		// and we should just fallback to no userns. This is completely
+		// valid.
+		return nil, nil, nil
+	}
+
+	uids, err := parseUsernsIDMap(userns.GetUids())
+	if err != nil {
+		return nil, nil, fmt.Errorf("UID mapping: %w", err)
+	}
+
+	gids, err = parseUsernsIDMap(userns.GetGids())
+	if err != nil {
+		return nil, nil, fmt.Errorf("GID mapping: %w", err)
+	}
+
+	switch mode := userns.GetMode(); mode {
+	case runtime.NamespaceMode_NODE:
+		if len(uids) != 0 || len(gids) != 0 {
+			return nil, nil, fmt.Errorf("can't use user namespace mode %q with mappings. Got %v UID mappings and %v GID mappings", mode, len(uids), len(gids))
+		}
+	case runtime.NamespaceMode_POD:
+		// This is valid, we will handle it in WithPodNamespaces().
+		if len(uids) == 0 || len(gids) == 0 {
+			return nil, nil, fmt.Errorf("can't use user namespace mode %q without UID and GID mappings", mode)
+		}
+	default:
+		return nil, nil, fmt.Errorf("unsupported user namespace mode: %q", mode)
+	}
+
+	return uids, gids, nil
+}