Add runc features command

Fix issue 3274

See `types/features/features.go`.

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

features.go

@@ -0,0 +1,74 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/opencontainers/runc/libcontainer/capabilities"
+	"github.com/opencontainers/runc/libcontainer/configs"
+	"github.com/opencontainers/runc/libcontainer/seccomp"
+	"github.com/opencontainers/runc/libcontainer/specconv"
+	"github.com/opencontainers/runc/types/features"
+	"github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/urfave/cli"
+)
+
+var featuresCommand = cli.Command{
+	Name:      "features",
+	Usage:     "show the enabled features",
+	ArgsUsage: "",
+	Description: `Show the enabled features.
+   The result is parsable as a JSON.
+   See https://pkg.go.dev/github.com/opencontainers/runc/types/features for the type definition.
+`,
+	Action: func(context *cli.Context) error {
+		if err := checkArgs(context, 0, exactArgs); err != nil {
+			return err
+		}
+
+		tru := true
+
+		feat := features.Features{
+			OCIVersionMin: "1.0.0",
+			OCIVersionMax: specs.Version,
+			Annotations: map[string]string{
+				features.AnnotationRuncVersion:           version,
+				features.AnnotationRuncCommit:            gitCommit,
+				features.AnnotationRuncCheckpointEnabled: "true",
+			},
+			Hooks:        configs.KnownHookNames(),
+			MountOptions: specconv.KnownMountOptions(),
+			Linux: &features.Linux{
+				Namespaces:   specconv.KnownNamespaces(),
+				Capabilities: capabilities.KnownCapabilities(),
+				Cgroup: &features.Cgroup{
+					V1:          &tru,
+					V2:          &tru,
+					Systemd:     &tru,
+					SystemdUser: &tru,
+				},
+				Apparmor: &features.Apparmor{
+					Enabled: &tru,
+				},
+				Selinux: &features.Selinux{
+					Enabled: &tru,
+				},
+			},
+		}
+
+		if seccomp.Enabled {
+			feat.Linux.Seccomp = &features.Seccomp{
+				Enabled:   &tru,
+				Actions:   seccomp.KnownActions(),
+				Operators: seccomp.KnownOperators(),
+				Archs:     seccomp.KnownArchs(),
+			}
+			major, minor, patch := seccomp.Version()
+			feat.Annotations[features.AnnotationLibseccompVersion] = fmt.Sprintf("%d.%d.%d", major, minor, patch)
+		}
+
+		enc := json.NewEncoder(context.App.Writer)
+		enc.SetIndent("", "    ")
+		return enc.Encode(feat)
+	},
+}