Perform fsmagic detection on driver's home-dir if it exists

The fsmagic check was always performed on "data-root" (`/var/lib/docker`),
not on the storage-driver's home directory (e.g. `/var/lib/docker/<somedriver>`).

This caused detection to be done on the wrong filesystem in situations
where `/var/lib/docker/<somedriver>` was a mount, and a different
filesystem than `/var/lib/docker` itself.

This patch checks if the storage-driver's home directory exists, and only
falls back to `/var/lib/docker` if it doesn't exist.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/graphdriver/aufs/aufs.go

@@ -89,7 +89,16 @@ func Init(root string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, graphdriver.ErrNotSupported
 	}
 
-	fsMagic, err := graphdriver.GetFSMagic(root)
+	// Perform feature detection on /var/lib/docker/aufs if it's an existing directory.
+	// This covers situations where /var/lib/docker/aufs is a mount, and on a different
+	// filesystem than /var/lib/docker.
+	// If the path does not exist, fall back to using /var/lib/docker for feature detection.
+	testdir := root
+	if _, err := os.Stat(testdir); os.IsNotExist(err) {
+		testdir = filepath.Dir(testdir)
+	}
+
+	fsMagic, err := graphdriver.GetFSMagic(testdir)
 	if err != nil {
 		return nil, err
 	}

daemon/graphdriver/btrfs/btrfs.go

@@ -51,7 +51,16 @@ type btrfsOptions struct {
 // An error is returned if BTRFS is not supported.
 func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (graphdriver.Driver, error) {
 
-	fsMagic, err := graphdriver.GetFSMagic(home)
+	// Perform feature detection on /var/lib/docker/btrfs if it's an existing directory.
+	// This covers situations where /var/lib/docker/btrfs is a mount, and on a different
+	// filesystem than /var/lib/docker.
+	// If the path does not exist, fall back to using /var/lib/docker for feature detection.
+	testdir := home
+	if _, err := os.Stat(testdir); os.IsNotExist(err) {
+		testdir = filepath.Dir(testdir)
+	}
+
+	fsMagic, err := graphdriver.GetFSMagic(testdir)
 	if err != nil {
 		return nil, err
 	}

daemon/graphdriver/driver_linux.go

@@ -3,8 +3,6 @@
 package graphdriver
 
 import (
-	"path/filepath"
-
 	"github.com/docker/docker/pkg/mount"
 	"golang.org/x/sys/unix"
 )
@@ -82,7 +80,7 @@ var (
 // GetFSMagic returns the filesystem id given the path.
 func GetFSMagic(rootpath string) (FsMagic, error) {
 	var buf unix.Statfs_t
-	if err := unix.Statfs(filepath.Dir(rootpath), &buf); err != nil {
+	if err := unix.Statfs(rootpath, &buf); err != nil {
 		return 0, err
 	}
 	return FsMagic(buf.Type), nil

daemon/graphdriver/overlay/overlay.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"os/exec"
 	"path"
+	"path/filepath"
 	"strconv"
 
 	"github.com/docker/docker/daemon/graphdriver"
@@ -119,7 +120,16 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, graphdriver.ErrNotSupported
 	}
 
-	fsMagic, err := graphdriver.GetFSMagic(home)
+	// Perform feature detection on /var/lib/docker/overlay if it's an existing directory.
+	// This covers situations where /var/lib/docker/overlay is a mount, and on a different
+	// filesystem than /var/lib/docker.
+	// If the path does not exist, fall back to using /var/lib/docker for feature detection.
+	testdir := home
+	if _, err := os.Stat(testdir); os.IsNotExist(err) {
+		testdir = filepath.Dir(testdir)
+	}
+
+	fsMagic, err := graphdriver.GetFSMagic(testdir)
 	if err != nil {
 		return nil, err
 	}

daemon/graphdriver/overlay2/overlay.go

@@ -136,7 +136,16 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, err
 	}
 
-	fsMagic, err := graphdriver.GetFSMagic(home)
+	// Perform feature detection on /var/lib/docker/overlay2 if it's an existing directory.
+	// This covers situations where /var/lib/docker/overlay2 is a mount, and on a different
+	// filesystem than /var/lib/docker.
+	// If the path does not exist, fall back to using /var/lib/docker for feature detection.
+	testdir := home
+	if _, err := os.Stat(testdir); os.IsNotExist(err) {
+		testdir = filepath.Dir(testdir)
+	}
+
+	fsMagic, err := graphdriver.GetFSMagic(testdir)
 	if err != nil {
 		return nil, err
 	}