systemd: fix rootful-in-userns regression

shouldUseRootlessCgroupManager() was always returning true when we are inside a userns.

If the systemd OwnerUID is 0, we have rootful systemd inside the userns, and we can just use the rootful cgroup driver.

Fix #2724

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

rootless_linux.go

@@ -5,7 +5,9 @@ package main
 import (
 	"os"
 
+	"github.com/opencontainers/runc/libcontainer/cgroups/systemd"
 	"github.com/opencontainers/runc/libcontainer/system"
+	"github.com/sirupsen/logrus"
 	"github.com/urfave/cli"
 )
 
@@ -28,6 +30,23 @@ func shouldUseRootlessCgroupManager(context *cli.Context) (bool, error) {
 		return false, nil
 	}
 	// euid = 0, in a userns.
+	//
+	// [systemd driver]
+	// We can call DetectUID() to parse the OwnerUID value from `busctl --user --no-pager status` result.
+	// The value corresponds to sd_bus_creds_get_owner_uid(3).
+	// If the value is 0, we have rootful systemd inside userns, so we do not need the rootless cgroup manager.
+	//
+	// On error, we assume we are root. An error may happen during shelling out to `busctl` CLI,
+	// mostly when $DBUS_SESSION_BUS_ADDRESS is unset.
+	if context.GlobalBool("systemd-cgroup") {
+		ownerUID, err := systemd.DetectUID()
+		if err != nil {
+			logrus.WithError(err).Debug("failed to get the OwnerUID value, assuming the value to be 0")
+			ownerUID = 0
+		}
+		return ownerUID != 0, nil
+	}
+	// [cgroupfs driver]
 	// As we are unaware of cgroups path, we can't determine whether we have the full
 	// access to the cgroups path.
 	// Either way, we can safely decide to use the rootless cgroups manager.