Merge release-20200323.0-177-g12bde95 (automated)

9 files changed, 448 insertions, 17 deletions

diff --git a/runsc/boot/config.go b/runsc/boot/config.go
index 7ea5bfade..715a19112 100644
--- a/runsc/boot/config.go
+++ b/runsc/boot/config.go
@@ -305,5 +305,10 @@ func (c *Config) ToFlags() []string {
 	if len(c.TestOnlyTestNameEnv) != 0 {
 		f = append(f, "--TESTONLY-test-name-env="+c.TestOnlyTestNameEnv)
 	}
+
+	if c.VFS2 {
+		f = append(f, "--vfs2=true")
+	}
+
 	return f
 }
diff --git a/runsc/boot/fds.go b/runsc/boot/fds.go
index 5314b0f2a..7e49f6f9f 100644
--- a/runsc/boot/fds.go
+++ b/runsc/boot/fds.go
@@ -20,6 +20,7 @@ import (
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/fs/host"
+	vfshost "gvisor.dev/gvisor/pkg/sentry/fsimpl/host"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
 )
 
@@ -31,6 +32,10 @@ func createFDTable(ctx context.Context, console bool, stdioFDs []int) (*kernel.F
 		return nil, fmt.Errorf("stdioFDs should contain exactly 3 FDs (stdin, stdout, and stderr), but %d FDs received", len(stdioFDs))
 	}
 
+	if kernel.VFS2Enabled {
+		return createFDTableVFS2(ctx, console, stdioFDs)
+	}
+
 	k := kernel.KernelFromContext(ctx)
 	fdTable := k.NewFDTable()
 	defer fdTable.DecRef()
@@ -78,3 +83,31 @@ func createFDTable(ctx context.Context, console bool, stdioFDs []int) (*kernel.F
 	fdTable.IncRef()
 	return fdTable, nil
 }
+
+func createFDTableVFS2(ctx context.Context, console bool, stdioFDs []int) (*kernel.FDTable, error) {
+	k := kernel.KernelFromContext(ctx)
+	fdTable := k.NewFDTable()
+	defer fdTable.DecRef()
+
+	hostMount, err := vfshost.NewMount(k.VFS())
+	if err != nil {
+		return nil, fmt.Errorf("creating host mount: %w", err)
+	}
+
+	for appFD, hostFD := range stdioFDs {
+		// TODO(gvisor.dev/issue/1482): Add TTY support.
+		appFile, err := vfshost.ImportFD(hostMount, hostFD, false)
+		if err != nil {
+			return nil, err
+		}
+
+		if err := fdTable.NewFDAtVFS2(ctx, int32(appFD), appFile, kernel.FDFlags{}); err != nil {
+			appFile.DecRef()
+			return nil, err
+		}
+		appFile.DecRef()
+	}
+
+	fdTable.IncRef()
+	return fdTable, nil
+}
diff --git a/runsc/boot/fs.go b/runsc/boot/fs.go
index 82cc612d2..98cce60af 100644
--- a/runsc/boot/fs.go
+++ b/runsc/boot/fs.go
@@ -278,6 +278,9 @@ func subtargets(root string, mnts []specs.Mount) []string {
 }
 
 func setupContainerFS(ctx context.Context, conf *Config, mntr *containerMounter, procArgs *kernel.CreateProcessArgs) error {
+	if conf.VFS2 {
+		return setupContainerVFS2(ctx, conf, mntr, procArgs)
+	}
 	mns, err := mntr.setupFS(conf, procArgs)
 	if err != nil {
 		return err
@@ -573,6 +576,9 @@ func newContainerMounter(spec *specs.Spec, goferFDs []int, k *kernel.Kernel, hin
 // should be mounted (e.g. a volume shared between containers). It must be
 // called for the root container only.
 func (c *containerMounter) processHints(conf *Config) error {
+	if conf.VFS2 {
+		return nil
+	}
 	ctx := c.k.SupervisorContext()
 	for _, hint := range c.hints.mounts {
 		// TODO(b/142076984): Only support tmpfs for now. Bind mounts require a
@@ -781,9 +787,6 @@ func (c *containerMounter) getMountNameAndOptions(conf *Config, m specs.Mount) (
 		useOverlay = conf.Overlay && !mountFlags(m.Options).ReadOnly
 
 	default:
-		// TODO(nlacasse): Support all the mount types and make this a fatal error.
-		// Most applications will "just work" without them, so this is a warning
-		// for now.
 		log.Warningf("ignoring unknown filesystem type %q", m.Type)
 	}
 	return fsName, opts, useOverlay, nil
diff --git a/runsc/boot/loader.go b/runsc/boot/loader.go
index 654441f65..cf1f47bc7 100644
--- a/runsc/boot/loader.go
+++ b/runsc/boot/loader.go
@@ -26,7 +26,6 @@ import (
 
 	specs "github.com/opencontainers/runtime-spec/specs-go"
 	"golang.org/x/sys/unix"
-	"gvisor.dev/gvisor/pkg/abi"
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/cpuid"
 	"gvisor.dev/gvisor/pkg/log"
@@ -73,6 +72,8 @@ import (
 	_ "gvisor.dev/gvisor/pkg/sentry/socket/unix"
 )
 
+var syscallTable *kernel.SyscallTable
+
 // Loader keeps state needed to start the kernel and run the container..
 type Loader struct {
 	// k is the kernel.
@@ -195,13 +196,14 @@ func New(args Args) (*Loader, error) {
 		return nil, fmt.Errorf("setting up memory usage: %v", err)
 	}
 
-	if args.Conf.VFS2 {
-		st, ok := kernel.LookupSyscallTable(abi.Linux, arch.Host)
-		if ok {
-			vfs2.Override(st.Table)
-		}
+	// Patch the syscall table.
+	kernel.VFS2Enabled = args.Conf.VFS2
+	if kernel.VFS2Enabled {
+		vfs2.Override(syscallTable.Table)
 	}
 
+	kernel.RegisterSyscallTable(syscallTable)
+
 	// Create kernel and platform.
 	p, err := createPlatform(args.Conf, args.Device)
 	if err != nil {
@@ -392,11 +394,16 @@ func newProcess(id string, spec *specs.Spec, creds *auth.Credentials, k *kernel.
 		return kernel.CreateProcessArgs{}, fmt.Errorf("creating limits: %v", err)
 	}
 
+	wd := spec.Process.Cwd
+	if wd == "" {
+		wd = "/"
+	}
+
 	// Create the process arguments.
 	procArgs := kernel.CreateProcessArgs{
 		Argv:                    spec.Process.Args,
 		Envv:                    spec.Process.Env,
-		WorkingDirectory:        spec.Process.Cwd, // Defaults to '/' if empty.
+		WorkingDirectory:        wd,
 		Credentials:             creds,
 		Umask:                   0022,
 		Limits:                  ls,
@@ -541,7 +548,15 @@ func (l *Loader) run() error {
 		}
 
 		// Add the HOME enviroment variable if it is not already set.
-		envv, err := maybeAddExecUserHome(ctx, l.rootProcArgs.MountNamespace, l.rootProcArgs.Credentials.RealKUID, l.rootProcArgs.Envv)
+		var envv []string
+		if kernel.VFS2Enabled {
+			envv, err = maybeAddExecUserHomeVFS2(ctx, l.rootProcArgs.MountNamespaceVFS2,
+				l.rootProcArgs.Credentials.RealKUID, l.rootProcArgs.Envv)
+
+		} else {
+			envv, err = maybeAddExecUserHome(ctx, l.rootProcArgs.MountNamespace,
+				l.rootProcArgs.Credentials.RealKUID, l.rootProcArgs.Envv)
+		}
 		if err != nil {
 			return err
 		}
diff --git a/runsc/boot/loader_amd64.go b/runsc/boot/loader_amd64.go
index b9669f2ac..78df86611 100755
--- a/runsc/boot/loader_amd64.go
+++ b/runsc/boot/loader_amd64.go
@@ -17,11 +17,10 @@
 package boot
 
 import (
-	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/sentry/syscalls/linux"
 )
 
 func init() {
-	// Register the global syscall table.
-	kernel.RegisterSyscallTable(linux.AMD64)
+	// Set the global syscall table.
+	syscallTable = linux.AMD64
 }
diff --git a/runsc/boot/loader_arm64.go b/runsc/boot/loader_arm64.go
index cf64d28c8..250785010 100755
--- a/runsc/boot/loader_arm64.go
+++ b/runsc/boot/loader_arm64.go
@@ -17,11 +17,10 @@
 package boot
 
 import (
-	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/sentry/syscalls/linux"
 )
 
 func init() {
-	// Register the global syscall table.
-	kernel.RegisterSyscallTable(linux.ARM64)
+	// Set the global syscall table.
+	syscallTable = linux.ARM64
 }
diff --git a/runsc/boot/user.go b/runsc/boot/user.go
index f0aa52135..332e4fce5 100644
--- a/runsc/boot/user.go
+++ b/runsc/boot/user.go
@@ -23,8 +23,10 @@ import (
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fspath"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/usermem"
 )
 
@@ -84,6 +86,48 @@ func getExecUserHome(ctx context.Context, rootMns *fs.MountNamespace, uid auth.K
 		File: f,
 	}
 
+	return findHomeInPasswd(uint32(uid), r, defaultHome)
+}
+
+type fileReaderVFS2 struct {
+	ctx context.Context
+	fd  *vfs.FileDescription
+}
+
+func (r *fileReaderVFS2) Read(buf []byte) (int, error) {
+	n, err := r.fd.Read(r.ctx, usermem.BytesIOSequence(buf), vfs.ReadOptions{})
+	return int(n), err
+}
+
+func getExecUserHomeVFS2(ctx context.Context, mns *vfs.MountNamespace, uid auth.KUID) (string, error) {
+	const defaultHome = "/"
+
+	root := mns.Root()
+	defer root.DecRef()
+
+	creds := auth.CredentialsFromContext(ctx)
+
+	target := &vfs.PathOperation{
+		Root:  root,
+		Start: root,
+		Path:  fspath.Parse("/etc/passwd"),
+	}
+
+	opts := &vfs.OpenOptions{
+		Flags: linux.O_RDONLY,
+	}
+
+	fd, err := root.Mount().Filesystem().VirtualFilesystem().OpenAt(ctx, creds, target, opts)
+	if err != nil {
+		return defaultHome, nil
+	}
+	defer fd.DecRef()
+
+	r := &fileReaderVFS2{
+		ctx: ctx,
+		fd:  fd,
+	}
+
 	homeDir, err := findHomeInPasswd(uint32(uid), r, defaultHome)
 	if err != nil {
 		return "", err
@@ -111,6 +155,26 @@ func maybeAddExecUserHome(ctx context.Context, mns *fs.MountNamespace, uid auth.
 	if err != nil {
 		return nil, fmt.Errorf("error reading exec user: %v", err)
 	}
+
+	return append(envv, "HOME="+homeDir), nil
+}
+
+func maybeAddExecUserHomeVFS2(ctx context.Context, vmns *vfs.MountNamespace, uid auth.KUID, envv []string) ([]string, error) {
+	// Check if the envv already contains HOME.
+	for _, env := range envv {
+		if strings.HasPrefix(env, "HOME=") {
+			// We have it. Return the original slice unmodified.
+			return envv, nil
+		}
+	}
+
+	// Read /etc/passwd for the user's HOME directory and set the HOME
+	// environment variable as required by POSIX if it is not overridden by
+	// the user.
+	homeDir, err := getExecUserHomeVFS2(ctx, vmns, uid)
+	if err != nil {
+		return nil, fmt.Errorf("error reading exec user: %v", err)
+	}
 	return append(envv, "HOME="+homeDir), nil
 }
 
diff --git a/runsc/boot/vfs.go b/runsc/boot/vfs.go
new file mode 100755
index 000000000..82083c57d
--- /dev/null
+++ b/runsc/boot/vfs.go
@@ -0,0 +1,310 @@
+// Copyright 2018 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package boot
+
+import (
+	"fmt"
+	"path"
+	"strconv"
+	"strings"
+
+	specs "github.com/opencontainers/runtime-spec/specs-go"
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/sentry/devices/memdev"
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+	devtmpfsimpl "gvisor.dev/gvisor/pkg/sentry/fsimpl/devtmpfs"
+	goferimpl "gvisor.dev/gvisor/pkg/sentry/fsimpl/gofer"
+	procimpl "gvisor.dev/gvisor/pkg/sentry/fsimpl/proc"
+	sysimpl "gvisor.dev/gvisor/pkg/sentry/fsimpl/sys"
+	tmpfsimpl "gvisor.dev/gvisor/pkg/sentry/fsimpl/tmpfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/log"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+)
+
+func registerFilesystems(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials) error {
+
+	vfsObj.MustRegisterFilesystemType(rootFsName, &goferimpl.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserList: true,
+	})
+
+	vfsObj.MustRegisterFilesystemType(bind, &goferimpl.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserList: true,
+	})
+
+	vfsObj.MustRegisterFilesystemType(devpts, &devtmpfsimpl.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserMount: true,
+		AllowUserList:  true,
+	})
+
+	vfsObj.MustRegisterFilesystemType(devtmpfs, &devtmpfsimpl.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserMount: true,
+		AllowUserList:  true,
+	})
+	vfsObj.MustRegisterFilesystemType(proc, &procimpl.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserMount: true,
+		AllowUserList:  true,
+	})
+	vfsObj.MustRegisterFilesystemType(sysfs, &sysimpl.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserMount: true,
+		AllowUserList:  true,
+	})
+	vfsObj.MustRegisterFilesystemType(tmpfs, &tmpfsimpl.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserMount: true,
+		AllowUserList:  true,
+	})
+	vfsObj.MustRegisterFilesystemType(nonefs, &sysimpl.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserMount: true,
+		AllowUserList:  true,
+	})
+
+	// Setup files in devtmpfs.
+	if err := memdev.Register(vfsObj); err != nil {
+		return fmt.Errorf("registering memdev: %w", err)
+	}
+	a, err := devtmpfsimpl.NewAccessor(ctx, vfsObj, creds, devtmpfsimpl.Name)
+	if err != nil {
+		return fmt.Errorf("creating devtmpfs accessor: %w", err)
+	}
+	defer a.Release()
+
+	if err := a.UserspaceInit(ctx); err != nil {
+		return fmt.Errorf("initializing userspace: %w", err)
+	}
+	if err := memdev.CreateDevtmpfsFiles(ctx, a); err != nil {
+		return fmt.Errorf("creating devtmpfs files: %w", err)
+	}
+	return nil
+}
+
+func setupContainerVFS2(ctx context.Context, conf *Config, mntr *containerMounter, procArgs *kernel.CreateProcessArgs) error {
+	if err := mntr.k.VFS().Init(); err != nil {
+		return fmt.Errorf("failed to initialize VFS: %w", err)
+	}
+	mns, err := mntr.setupVFS2(ctx, conf, procArgs)
+	if err != nil {
+		return fmt.Errorf("failed to setupFS: %w", err)
+	}
+	procArgs.MountNamespaceVFS2 = mns
+	return setExecutablePathVFS2(ctx, procArgs)
+}
+
+func setExecutablePathVFS2(ctx context.Context, procArgs *kernel.CreateProcessArgs) error {
+
+	exe := procArgs.Argv[0]
+
+	// Absolute paths can be used directly.
+	if path.IsAbs(exe) {
+		procArgs.Filename = exe
+		return nil
+	}
+
+	// Paths with '/' in them should be joined to the working directory, or
+	// to the root if working directory is not set.
+	if strings.IndexByte(exe, '/') > 0 {
+
+		if !path.IsAbs(procArgs.WorkingDirectory) {
+			return fmt.Errorf("working directory %q must be absolute", procArgs.WorkingDirectory)
+		}
+
+		procArgs.Filename = path.Join(procArgs.WorkingDirectory, exe)
+		return nil
+	}
+
+	// Paths with a '/' are relative to the CWD.
+	if strings.IndexByte(exe, '/') > 0 {
+		procArgs.Filename = path.Join(procArgs.WorkingDirectory, exe)
+		return nil
+	}
+
+	// Otherwise, We must lookup the name in the paths, starting from the
+	// root directory.
+	root := procArgs.MountNamespaceVFS2.Root()
+	defer root.DecRef()
+
+	paths := fs.GetPath(procArgs.Envv)
+	creds := procArgs.Credentials
+
+	for _, p := range paths {
+
+		binPath := path.Join(p, exe)
+
+		pop := &vfs.PathOperation{
+			Root:               root,
+			Start:              root,
+			Path:               fspath.Parse(binPath),
+			FollowFinalSymlink: true,
+		}
+
+		opts := &vfs.OpenOptions{
+			FileExec: true,
+			Flags:    linux.O_RDONLY,
+		}
+
+		dentry, err := root.Mount().Filesystem().VirtualFilesystem().OpenAt(ctx, creds, pop, opts)
+		if err == syserror.ENOENT || err == syserror.EACCES {
+			// Didn't find it here.
+			continue
+		}
+		if err != nil {
+			return err
+		}
+		dentry.DecRef()
+
+		procArgs.Filename = binPath
+		return nil
+	}
+
+	return fmt.Errorf("executable %q not found in $PATH=%q", exe, strings.Join(paths, ":"))
+}
+
+func (c *containerMounter) setupVFS2(ctx context.Context, conf *Config, procArgs *kernel.CreateProcessArgs) (*vfs.MountNamespace, error) {
+	log.Infof("Configuring container's file system with VFS2")
+
+	// Create context with root credentials to mount the filesystem (the current
+	// user may not be privileged enough).
+	rootProcArgs := *procArgs
+	rootProcArgs.WorkingDirectory = "/"
+	rootProcArgs.Credentials = auth.NewRootCredentials(procArgs.Credentials.UserNamespace)
+	rootProcArgs.Umask = 0022
+	rootProcArgs.MaxSymlinkTraversals = linux.MaxSymlinkTraversals
+	rootCtx := procArgs.NewContext(c.k)
+
+	creds := procArgs.Credentials
+	if err := registerFilesystems(rootCtx, c.k.VFS(), creds); err != nil {
+		return nil, fmt.Errorf("register filesystems: %w", err)
+	}
+
+	fd := c.fds.remove()
+
+	opts := strings.Join(p9MountOptionsVFS2(fd, conf.FileAccess), ",")
+
+	log.Infof("Mounting root over 9P, ioFD: %d", fd)
+	mns, err := c.k.VFS().NewMountNamespace(ctx, creds, "", rootFsName, &vfs.GetFilesystemOptions{Data: opts})
+	if err != nil {
+		return nil, fmt.Errorf("setting up mountnamespace: %w", err)
+	}
+
+	rootProcArgs.MountNamespaceVFS2 = mns
+
+	// Mount submounts.
+	if err := c.mountSubmountsVFS2(rootCtx, conf, mns, creds); err != nil {
+		return nil, fmt.Errorf("mounting submounts vfs2: %w", err)
+	}
+
+	return mns, nil
+}
+
+func (c *containerMounter) mountSubmountsVFS2(ctx context.Context, conf *Config, mns *vfs.MountNamespace, creds *auth.Credentials) error {
+
+	for _, submount := range c.mounts {
+		log.Debugf("Mounting %q to %q, type: %s, options: %s", submount.Source, submount.Destination, submount.Type, submount.Options)
+		if err := c.mountSubmountVFS2(ctx, conf, mns, creds, &submount); err != nil {
+			return err
+		}
+	}
+
+	// TODO(gvisor.dev/issue/1487): implement mountTmp from fs.go.
+
+	return c.checkDispenser()
+}
+
+// TODO(gvisor.dev/issue/1487): Implement submount options similar to the VFS1 version.
+func (c *containerMounter) mountSubmountVFS2(ctx context.Context, conf *Config, mns *vfs.MountNamespace, creds *auth.Credentials, submount *specs.Mount) error {
+	root := mns.Root()
+	defer root.DecRef()
+	target := &vfs.PathOperation{
+		Root:  root,
+		Start: root,
+		Path:  fspath.Parse(submount.Destination),
+	}
+
+	_, options, useOverlay, err := c.getMountNameAndOptionsVFS2(conf, *submount)
+	if err != nil {
+		return fmt.Errorf("mountOptions failed: %w", err)
+	}
+
+	opts := &vfs.MountOptions{
+		GetFilesystemOptions: vfs.GetFilesystemOptions{
+			Data: strings.Join(options, ","),
+		},
+		InternalMount: true,
+	}
+
+	// All writes go to upper, be paranoid and make lower readonly.
+	opts.ReadOnly = useOverlay
+
+	if err := c.k.VFS().MountAt(ctx, creds, "", target, submount.Type, opts); err != nil {
+		return fmt.Errorf("failed to mount %q (type: %s): %w, opts: %v", submount.Destination, submount.Type, err, opts)
+	}
+	log.Infof("Mounted %q to %q type: %s, internal-options: %q", submount.Source, submount.Destination, submount.Type, opts)
+	return nil
+}
+
+// getMountNameAndOptionsVFS2 retrieves the fsName, opts, and useOverlay values
+// used for mounts.
+func (c *containerMounter) getMountNameAndOptionsVFS2(conf *Config, m specs.Mount) (string, []string, bool, error) {
+	var (
+		fsName     string
+		opts       []string
+		useOverlay bool
+	)
+
+	switch m.Type {
+	case devpts, devtmpfs, proc, sysfs:
+		fsName = m.Type
+	case nonefs:
+		fsName = sysfs
+	case tmpfs:
+		fsName = m.Type
+
+		var err error
+		opts, err = parseAndFilterOptions(m.Options, tmpfsAllowedOptions...)
+		if err != nil {
+			return "", nil, false, err
+		}
+
+	case bind:
+		fd := c.fds.remove()
+		fsName = "9p"
+		opts = p9MountOptionsVFS2(fd, c.getMountAccessType(m))
+		// If configured, add overlay to all writable mounts.
+		useOverlay = conf.Overlay && !mountFlags(m.Options).ReadOnly
+
+	default:
+		log.Warningf("ignoring unknown filesystem type %q", m.Type)
+	}
+	return fsName, opts, useOverlay, nil
+}
+
+// p9MountOptions creates a slice of options for a p9 mount.
+// TODO(gvisor.dev/issue/1200): Remove this version in favor of the one in
+// fs.go when privateunixsocket lands.
+func p9MountOptionsVFS2(fd int, fa FileAccessType) []string {
+	opts := []string{
+		"trans=fd",
+		"rfdno=" + strconv.Itoa(fd),
+		"wfdno=" + strconv.Itoa(fd),
+	}
+	if fa == FileAccessShared {
+		opts = append(opts, "cache=remote_revalidating")
+	}
+	return opts
+}
diff --git a/runsc/main.go b/runsc/main.go
index c1c78529c..9d52f3006 100644
--- a/runsc/main.go
+++ b/runsc/main.go
@@ -84,6 +84,7 @@ var (
 	rootless           = flag.Bool("rootless", false, "it allows the sandbox to be started with a user that is not root. Sandbox and Gofer processes may run with same privileges as current user.")
 	referenceLeakMode  = flag.String("ref-leak-mode", "disabled", "sets reference leak check mode: disabled (default), log-names, log-traces.")
 	cpuNumFromQuota    = flag.Bool("cpu-num-from-quota", false, "set cpu number to cpu quota (least integer greater or equal to quota value, but not less than 2)")
+	vfs2Enabled        = flag.Bool("vfs2", false, "TEST ONLY; use while VFSv2 is landing. This uses the new experimental VFS layer.")
 
 	// Test flags, not to be used outside tests, ever.
 	testOnlyAllowRunAsCurrentUserWithoutChroot = flag.Bool("TESTONLY-unsafe-nonroot", false, "TEST ONLY; do not ever use! This skips many security measures that isolate the host from the sandbox.")
@@ -230,6 +231,7 @@ func main() {
 		ReferenceLeakMode:  refsLeakMode,
 		OverlayfsStaleRead: *overlayfsStaleRead,
 		CPUNumFromQuota:    *cpuNumFromQuota,
+		VFS2:               *vfs2Enabled,
 
 		TestOnlyAllowRunAsCurrentUserWithoutChroot: *testOnlyAllowRunAsCurrentUserWithoutChroot,
 		TestOnlyTestNameEnv:                        *testOnlyTestNameEnv,
@@ -313,6 +315,7 @@ func main() {
 	log.Infof("\t\tFileAccess: %v, overlay: %t", conf.FileAccess, conf.Overlay)
 	log.Infof("\t\tNetwork: %v, logging: %t", conf.Network, conf.LogPackets)
 	log.Infof("\t\tStrace: %t, max size: %d, syscalls: %s", conf.Strace, conf.StraceLogSize, conf.StraceSyscalls)
+	log.Infof("\t\tVFS2 enabled: %v", conf.VFS2)
 	log.Infof("***************************")
 
 	if *testOnlyAllowRunAsCurrentUserWithoutChroot {