Add --no-pivot option for containers on ramdisk

This adds a `--no-pivot` cli flag to runc so that a container's rootfs
can be located ontop of ramdisk/tmpfs and not fail because you cannot
pivot root.

This should be a cli flag and not part of the spec because this is a
detail of the host/runtime environment and not an attribute of a
container.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

libcontainer/specconv/spec_linux.go

@@ -158,9 +158,16 @@ var allowedDevices = []*configs.Device{
 	},
 }
 
+type CreateOpts struct {
+	CgroupName       string
+	UseSystemdCgroup bool
+	NoPivotRoot      bool
+	Spec             *specs.Spec
+}
+
 // CreateLibcontainerConfig creates a new libcontainer configuration from a
 // given specification and a cgroup name
-func CreateLibcontainerConfig(cgroupName string, useSystemdCgroup bool, spec *specs.Spec) (*configs.Config, error) {
+func CreateLibcontainerConfig(opts *CreateOpts) (*configs.Config, error) {
 	// runc's cwd will always be the bundle path
 	rcwd, err := os.Getwd()
 	if err != nil {
@@ -170,14 +177,16 @@ func CreateLibcontainerConfig(cgroupName string, useSystemdCgroup bool, spec *sp
 	if err != nil {
 		return nil, err
 	}
+	spec := opts.Spec
 	rootfsPath := spec.Root.Path
 	if !filepath.IsAbs(rootfsPath) {
 		rootfsPath = filepath.Join(cwd, rootfsPath)
 	}
 	config := &configs.Config{
-		Rootfs:     rootfsPath,
-		Readonlyfs: spec.Root.Readonly,
-		Hostname:   spec.Hostname,
+		Rootfs:      rootfsPath,
+		NoPivotRoot: opts.NoPivotRoot,
+		Readonlyfs:  spec.Root.Readonly,
+		Hostname:    spec.Hostname,
 		Labels: []string{
 			"bundle=" + cwd,
 		},
@@ -211,7 +220,7 @@ func CreateLibcontainerConfig(cgroupName string, useSystemdCgroup bool, spec *sp
 	if err := setupUserNamespace(spec, config); err != nil {
 		return nil, err
 	}
-	c, err := createCgroupConfig(cgroupName, useSystemdCgroup, spec)
+	c, err := createCgroupConfig(opts.CgroupName, opts.UseSystemdCgroup, spec)
 	if err != nil {
 		return nil, err
 	}

restore.go

@@ -73,6 +73,10 @@ using the runc checkpoint command.`,
 			Name:  "no-subreaper",
 			Usage: "disable the use of the subreaper used to reap reparented processes",
 		},
+		cli.BoolFlag{
+			Name:  "no-pivot",
+			Usage: "do not use pivot root to jail process inside rootfs.  This should be used whenever the rootfs is on top of a ramdisk",
+		},
 	},
 	Action: func(context *cli.Context) {
 		imagePath := context.String("image-path")
@@ -93,7 +97,12 @@ using the runc checkpoint command.`,
 		if err != nil {
 			fatal(err)
 		}
-		config, err := specconv.CreateLibcontainerConfig(id, context.GlobalBool("systemd-cgroup"), spec)
+		config, err := specconv.CreateLibcontainerConfig(&specconv.CreateOpts{
+			CgroupName:       id,
+			UseSystemdCgroup: context.GlobalBool("systemd-cgroup"),
+			NoPivotRoot:      context.Bool("no-pivot"),
+			Spec:             spec,
+		})
 		if err != nil {
 			fatal(err)
 		}

start.go

@@ -53,6 +53,10 @@ command(s) that get executed on start, edit the args parameter of the spec. See
 			Name:  "no-subreaper",
 			Usage: "disable the use of the subreaper used to reap reparented processes",
 		},
+		cli.BoolFlag{
+			Name:  "no-pivot",
+			Usage: "do not use pivot root to jail process inside rootfs.  This should be used whenever the rootfs is on top of a ramdisk",
+		},
 	},
 	Action: func(context *cli.Context) {
 		bundle := context.String("bundle")

utils.go

@@ -175,7 +175,12 @@ func createPidFile(path string, process *libcontainer.Process) error {
 }
 
 func createContainer(context *cli.Context, id string, spec *specs.Spec) (libcontainer.Container, error) {
-	config, err := specconv.CreateLibcontainerConfig(id, context.GlobalBool("systemd-cgroup"), spec)
+	config, err := specconv.CreateLibcontainerConfig(&specconv.CreateOpts{
+		CgroupName:       id,
+		UseSystemdCgroup: context.GlobalBool("systemd-cgroup"),
+		NoPivotRoot:      context.Bool("no-pivot"),
+		Spec:             spec,
+	})
 	if err != nil {
 		return nil, err
 	}