Detect overlay2 support on pre-4.0 kernels

daemon/graphdriver/overlay2/check.go

@@ -100,3 +100,35 @@ func doesSupportNativeDiff(d string) error {
 
 	return nil
 }
+
+// supportsMultipleLowerDir checks if the system supports multiple lowerdirs,
+// which is required for the overlay2 driver. On 4.x kernels, multiple lowerdirs
+// are always available (so this check isn't needed), and backported to RHEL and
+// CentOS 3.x kernels (3.10.0-693.el7.x86_64 and up). This function is to detect
+// support on those kernels, without doing a kernel version compare.
+func supportsMultipleLowerDir(d string) error {
+	td, err := ioutil.TempDir(d, "multiple-lowerdir-check")
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err := os.RemoveAll(td); err != nil {
+			logrus.Warnf("Failed to remove check directory %v: %v", td, err)
+		}
+	}()
+
+	for _, dir := range []string{"lower1", "lower2", "upper", "work", "merged"} {
+		if err := os.Mkdir(filepath.Join(td, dir), 0755); err != nil {
+			return err
+		}
+	}
+
+	opts := fmt.Sprintf("lowerdir=%s:%s,upperdir=%s,workdir=%s", path.Join(td, "lower2"), path.Join(td, "lower1"), path.Join(td, "upper"), path.Join(td, "work"))
+	if err := unix.Mount("overlay", filepath.Join(td, "merged"), "overlay", 0, opts); err != nil {
+		return errors.Wrap(err, "failed to mount overlay")
+	}
+	if err := unix.Unmount(filepath.Join(td, "merged"), 0); err != nil {
+		logrus.Warnf("Failed to unmount check directory %v: %v", filepath.Join(td, "merged"), err)
+	}
+	return nil
+}

daemon/graphdriver/overlay2/overlay.go

@@ -16,8 +16,6 @@ import (
 	"strings"
 	"sync"
 
-	"github.com/sirupsen/logrus"
-
 	"github.com/docker/docker/daemon/graphdriver"
 	"github.com/docker/docker/daemon/graphdriver/overlayutils"
 	"github.com/docker/docker/daemon/graphdriver/quota"
@@ -32,9 +30,9 @@ import (
 	"github.com/docker/docker/pkg/parsers"
 	"github.com/docker/docker/pkg/parsers/kernel"
 	"github.com/docker/docker/pkg/system"
-	units "github.com/docker/go-units"
-
+	"github.com/docker/go-units"
 	"github.com/opencontainers/selinux/go-selinux/label"
+	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )
 
@@ -134,12 +132,6 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 	if err != nil {
 		return nil, err
 	}
-	if kernel.CompareKernelVersion(*v, kernel.VersionInfo{Kernel: 4, Major: 0, Minor: 0}) < 0 {
-		if !opts.overrideKernelCheck {
-			return nil, graphdriver.ErrNotSupported
-		}
-		logrus.Warn("Using pre-4.0.0 kernel for overlay2, mount failures may require kernel update")
-	}
 
 	fsMagic, err := graphdriver.GetFSMagic(home)
 	if err != nil {
@@ -166,6 +158,17 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		}
 	}
 
+	if kernel.CompareKernelVersion(*v, kernel.VersionInfo{Kernel: 4, Major: 0, Minor: 0}) < 0 {
+		if opts.overrideKernelCheck {
+			logrus.Warn("Using pre-4.0.0 kernel for overlay2, mount failures may require kernel update")
+		} else {
+			if err := supportsMultipleLowerDir(filepath.Dir(home)); err != nil {
+				logrus.Debugf("Multiple lower dirs not supported: %v", err)
+				return nil, graphdriver.ErrNotSupported
+			}
+		}
+	}
+
 	rootUID, rootGID, err := idtools.GetRootUIDGID(uidMaps, gidMaps)
 	if err != nil {
 		return nil, err