Return nicer error message when cgroups v1 isn't available

Updates #3481
Closes #5430

PiperOrigin-RevId: 358923208

runsc/cgroup/BUILD

@@ -11,6 +11,7 @@ go_library(
         "//pkg/log",
         "@com_github_cenkalti_backoff//:go_default_library",
         "@com_github_opencontainers_runtime_spec//specs-go:go_default_library",
+        "@org_golang_x_sys//unix:go_default_library",
     ],
 )
 

runsc/cgroup/cgroup.go

@@ -32,6 +32,7 @@ import (
 
 	"github.com/cenkalti/backoff"
 	specs "github.com/opencontainers/runtime-spec/specs-go"
+	"golang.org/x/sys/unix"
 	"gvisor.dev/gvisor/pkg/cleanup"
 	"gvisor.dev/gvisor/pkg/log"
 )
@@ -59,6 +60,16 @@ var controllers = map[string]config{
 	"systemd":    {ctrlr: &noop{}},
 }
 
+// IsOnlyV2 checks whether cgroups V2 is enabled and V1 is not.
+func IsOnlyV2() bool {
+	var stat unix.Statfs_t
+	if err := unix.Statfs(cgroupRoot, &stat); err != nil {
+		// It's not used for anything important, assume not V2 on failure.
+		return false
+	}
+	return stat.Type == unix.CGROUP2_SUPER_MAGIC
+}
+
 func setOptionalValueInt(path, name string, val *int64) error {
 	if val == nil || *val == 0 {
 		return nil

runsc/container/container.go

@@ -230,7 +230,6 @@ func New(conf *config.Config, args Args) (*Container, error) {
 		if args.Spec.Linux.CgroupsPath == "" && !conf.TestOnlyAllowRunAsCurrentUserWithoutChroot {
 			args.Spec.Linux.CgroupsPath = "/" + args.ID
 		}
-
 		// Create and join cgroup before processes are created to ensure they are
 		// part of the cgroup from the start (and all their children processes).
 		cg, err := cgroup.New(args.Spec)
@@ -238,6 +237,10 @@ func New(conf *config.Config, args Args) (*Container, error) {
 			return nil, err
 		}
 		if cg != nil {
+			// TODO(gvisor.dev/issue/3481): Remove when cgroups v2 is supported.
+			if !conf.Rootless && cgroup.IsOnlyV2() {
+				return nil, fmt.Errorf("cgroups V2 is not yet supported. Enable cgroups V1 and retry")
+			}
 			// If there is cgroup config, install it before creating sandbox process.
 			if err := cg.Install(args.Spec.Linux.Resources); err != nil {
 				switch {