Fixes for procfs

- Return ENOENT for /proc/[pid]/task if task is zoombied or terminated
- Allow directory to be Seek() to the end
- Construct synthetic files for /proc/[pid]/ns/*
- Changed GenericDirectoryFD.Init to not register with FileDescription,
  otherwise other implementation cannot change behavior.

Updates #1195,1193

PiperOrigin-RevId: 308294649

15 files changed, 192 insertions, 36 deletions

diff --git a/pkg/sentry/fsimpl/devpts/devpts.go b/pkg/sentry/fsimpl/devpts/devpts.go
index 07a69b940..f36bf50fc 100644
--- a/pkg/sentry/fsimpl/devpts/devpts.go
+++ b/pkg/sentry/fsimpl/devpts/devpts.go
@@ -161,8 +161,10 @@ func (i *rootInode) masterClose(t *Terminal) {
 
 // Open implements kernfs.Inode.Open.
 func (i *rootInode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &kernfs.GenericDirectoryFD{}
-	fd.Init(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	if err != nil {
+		return nil, err
+	}
 	return fd.VFSFileDescription(), nil
 }
 
diff --git a/pkg/sentry/fsimpl/kernfs/fd_impl_util.go b/pkg/sentry/fsimpl/kernfs/fd_impl_util.go
index e8a4670b8..dd5806301 100644
--- a/pkg/sentry/fsimpl/kernfs/fd_impl_util.go
+++ b/pkg/sentry/fsimpl/kernfs/fd_impl_util.go
@@ -15,6 +15,8 @@
 package kernfs
 
 import (
+	"math"
+
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
@@ -43,15 +45,27 @@ type GenericDirectoryFD struct {
 	off      int64
 }
 
-// Init initializes a GenericDirectoryFD.
-func (fd *GenericDirectoryFD) Init(m *vfs.Mount, d *vfs.Dentry, children *OrderedChildren, opts *vfs.OpenOptions) error {
+// NewGenericDirectoryFD creates a new GenericDirectoryFD and returns its
+// dentry.
+func NewGenericDirectoryFD(m *vfs.Mount, d *vfs.Dentry, children *OrderedChildren, opts *vfs.OpenOptions) (*GenericDirectoryFD, error) {
+	fd := &GenericDirectoryFD{}
+	if err := fd.Init(children, opts); err != nil {
+		return nil, err
+	}
+	if err := fd.vfsfd.Init(fd, opts.Flags, m, d, &vfs.FileDescriptionOptions{}); err != nil {
+		return nil, err
+	}
+	return fd, nil
+}
+
+// Init initializes a GenericDirectoryFD. Use it when overriding
+// GenericDirectoryFD. Caller must call fd.VFSFileDescription.Init() with the
+// correct implementation.
+func (fd *GenericDirectoryFD) Init(children *OrderedChildren, opts *vfs.OpenOptions) error {
 	if vfs.AccessTypesForOpenFlags(opts)&vfs.MayWrite != 0 {
 		// Can't open directories for writing.
 		return syserror.EISDIR
 	}
-	if err := fd.vfsfd.Init(fd, opts.Flags, m, d, &vfs.FileDescriptionOptions{}); err != nil {
-		return err
-	}
 	fd.children = children
 	return nil
 }
@@ -187,6 +201,10 @@ func (fd *GenericDirectoryFD) Seek(ctx context.Context, offset int64, whence int
 		// Use offset as given.
 	case linux.SEEK_CUR:
 		offset += fd.off
+	case linux.SEEK_END:
+		// TODO(gvisor.dev/issue/1193): This can prevent new files from showing up
+		// if they are added after SEEK_END.
+		offset = math.MaxInt64
 	default:
 		return 0, syserror.EINVAL
 	}
diff --git a/pkg/sentry/fsimpl/kernfs/filesystem.go b/pkg/sentry/fsimpl/kernfs/filesystem.go
index 1d46dba25..3ccd92fc5 100644
--- a/pkg/sentry/fsimpl/kernfs/filesystem.go
+++ b/pkg/sentry/fsimpl/kernfs/filesystem.go
@@ -82,7 +82,7 @@ afterSymlink:
 	}
 	// Resolve any symlink at current path component.
 	if rp.ShouldFollowSymlink() && next.isSymlink() {
-		targetVD, targetPathname, err := next.inode.Getlink(ctx)
+		targetVD, targetPathname, err := next.inode.Getlink(ctx, rp.Mount())
 		if err != nil {
 			return nil, err
 		}
@@ -477,7 +477,7 @@ afterTrailingSymlink:
 	}
 	child := childVFSD.Impl().(*Dentry)
 	if rp.ShouldFollowSymlink() && child.isSymlink() {
-		targetVD, targetPathname, err := child.inode.Getlink(ctx)
+		targetVD, targetPathname, err := child.inode.Getlink(ctx, rp.Mount())
 		if err != nil {
 			return nil, err
 		}
diff --git a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
index a946645f6..02f35a675 100644
--- a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
+++ b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
@@ -182,7 +182,7 @@ func (InodeNotSymlink) Readlink(context.Context) (string, error) {
 }
 
 // Getlink implements Inode.Getlink.
-func (InodeNotSymlink) Getlink(context.Context) (vfs.VirtualDentry, string, error) {
+func (InodeNotSymlink) Getlink(context.Context, *vfs.Mount) (vfs.VirtualDentry, string, error) {
 	return vfs.VirtualDentry{}, "", syserror.EINVAL
 }
 
@@ -568,8 +568,10 @@ func (s *StaticDirectory) Init(creds *auth.Credentials, ino uint64, perm linux.F
 
 // Open implements kernfs.Inode.
 func (s *StaticDirectory) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &GenericDirectoryFD{}
-	fd.Init(rp.Mount(), vfsd, &s.OrderedChildren, &opts)
+	fd, err := NewGenericDirectoryFD(rp.Mount(), vfsd, &s.OrderedChildren, &opts)
+	if err != nil {
+		return nil, err
+	}
 	return fd.VFSFileDescription(), nil
 }
 
diff --git a/pkg/sentry/fsimpl/kernfs/kernfs.go b/pkg/sentry/fsimpl/kernfs/kernfs.go
index f5041824f..95cf6dc24 100644
--- a/pkg/sentry/fsimpl/kernfs/kernfs.go
+++ b/pkg/sentry/fsimpl/kernfs/kernfs.go
@@ -439,5 +439,5 @@ type inodeSymlink interface {
 	//
 	// - If the inode is not a symlink, Getlink returns (zero-value
 	// VirtualDentry, "", EINVAL).
-	Getlink(ctx context.Context) (vfs.VirtualDentry, string, error)
+	Getlink(ctx context.Context, mnt *vfs.Mount) (vfs.VirtualDentry, string, error)
 }
diff --git a/pkg/sentry/fsimpl/kernfs/kernfs_test.go b/pkg/sentry/fsimpl/kernfs/kernfs_test.go
index 465451f35..0964d5456 100644
--- a/pkg/sentry/fsimpl/kernfs/kernfs_test.go
+++ b/pkg/sentry/fsimpl/kernfs/kernfs_test.go
@@ -117,8 +117,8 @@ func (fs *filesystem) newReadonlyDir(creds *auth.Credentials, mode linux.FileMod
 }
 
 func (d *readonlyDir) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &kernfs.GenericDirectoryFD{}
-	if err := fd.Init(rp.Mount(), vfsd, &d.OrderedChildren, &opts); err != nil {
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &d.OrderedChildren, &opts)
+	if err != nil {
 		return nil, err
 	}
 	return fd.VFSFileDescription(), nil
@@ -147,8 +147,10 @@ func (fs *filesystem) newDir(creds *auth.Credentials, mode linux.FileMode, conte
 }
 
 func (d *dir) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &kernfs.GenericDirectoryFD{}
-	fd.Init(rp.Mount(), vfsd, &d.OrderedChildren, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &d.OrderedChildren, &opts)
+	if err != nil {
+		return nil, err
+	}
 	return fd.VFSFileDescription(), nil
 }
 
diff --git a/pkg/sentry/fsimpl/kernfs/symlink.go b/pkg/sentry/fsimpl/kernfs/symlink.go
index 018aa503c..0aa6dc979 100644
--- a/pkg/sentry/fsimpl/kernfs/symlink.go
+++ b/pkg/sentry/fsimpl/kernfs/symlink.go
@@ -56,7 +56,7 @@ func (s *StaticSymlink) Readlink(_ context.Context) (string, error) {
 }
 
 // Getlink implements Inode.Getlink.
-func (s *StaticSymlink) Getlink(_ context.Context) (vfs.VirtualDentry, string, error) {
+func (s *StaticSymlink) Getlink(context.Context, *vfs.Mount) (vfs.VirtualDentry, string, error) {
 	return vfs.VirtualDentry{}, s.target, nil
 }
 
diff --git a/pkg/sentry/fsimpl/proc/subtasks.go b/pkg/sentry/fsimpl/proc/subtasks.go
index a21313666..28ec2484a 100644
--- a/pkg/sentry/fsimpl/proc/subtasks.go
+++ b/pkg/sentry/fsimpl/proc/subtasks.go
@@ -88,6 +88,9 @@ func (i *subtasksInode) IterDirents(ctx context.Context, cb vfs.IterDirentsCallb
 	if len(tasks) == 0 {
 		return offset, syserror.ENOENT
 	}
+	if relOffset >= int64(len(tasks)) {
+		return offset, nil
+	}
 
 	tids := make([]int, 0, len(tasks))
 	for _, tid := range tasks {
@@ -110,10 +113,52 @@ func (i *subtasksInode) IterDirents(ctx context.Context, cb vfs.IterDirentsCallb
 	return offset, nil
 }
 
+type subtasksFD struct {
+	kernfs.GenericDirectoryFD
+
+	task *kernel.Task
+}
+
+func (fd *subtasksFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback) error {
+	if fd.task.ExitState() >= kernel.TaskExitZombie {
+		return syserror.ENOENT
+	}
+	return fd.GenericDirectoryFD.IterDirents(ctx, cb)
+}
+
+// Seek implements vfs.FileDecriptionImpl.Seek.
+func (fd *subtasksFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	if fd.task.ExitState() >= kernel.TaskExitZombie {
+		return 0, syserror.ENOENT
+	}
+	return fd.GenericDirectoryFD.Seek(ctx, offset, whence)
+}
+
+// Stat implements vfs.FileDescriptionImpl.Stat.
+func (fd *subtasksFD) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
+	if fd.task.ExitState() >= kernel.TaskExitZombie {
+		return linux.Statx{}, syserror.ENOENT
+	}
+	return fd.GenericDirectoryFD.Stat(ctx, opts)
+}
+
+// SetStat implements vfs.FileDescriptionImpl.SetStat.
+func (fd *subtasksFD) SetStat(ctx context.Context, opts vfs.SetStatOptions) error {
+	if fd.task.ExitState() >= kernel.TaskExitZombie {
+		return syserror.ENOENT
+	}
+	return fd.GenericDirectoryFD.SetStat(ctx, opts)
+}
+
 // Open implements kernfs.Inode.
 func (i *subtasksInode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &kernfs.GenericDirectoryFD{}
-	fd.Init(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	fd := &subtasksFD{task: i.task}
+	if err := fd.Init(&i.OrderedChildren, &opts); err != nil {
+		return nil, err
+	}
+	if err := fd.VFSFileDescription().Init(fd, opts.Flags, rp.Mount(), vfsd, &vfs.FileDescriptionOptions{}); err != nil {
+		return nil, err
+	}
 	return fd.VFSFileDescription(), nil
 }
 
diff --git a/pkg/sentry/fsimpl/proc/task.go b/pkg/sentry/fsimpl/proc/task.go
index 888afc0fd..e2790d35b 100644
--- a/pkg/sentry/fsimpl/proc/task.go
+++ b/pkg/sentry/fsimpl/proc/task.go
@@ -44,6 +44,7 @@ type taskInode struct {
 var _ kernfs.Inode = (*taskInode)(nil)
 
 func newTaskInode(inoGen InoGenerator, task *kernel.Task, pidns *kernel.PIDNamespace, isThreadGroup bool, cgroupControllers map[string]string) *kernfs.Dentry {
+	// TODO(gvisor.dev/issue/164): Fail with ESRCH if task exited.
 	contents := map[string]*kernfs.Dentry{
 		"auxv":      newTaskOwnedFile(task, inoGen.NextIno(), 0444, &auxvData{task: task}),
 		"cmdline":   newTaskOwnedFile(task, inoGen.NextIno(), 0444, &cmdlineData{task: task, arg: cmdlineDataArg}),
@@ -102,8 +103,10 @@ func (i *taskInode) Valid(ctx context.Context) bool {
 
 // Open implements kernfs.Inode.
 func (i *taskInode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &kernfs.GenericDirectoryFD{}
-	fd.Init(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	if err != nil {
+		return nil, err
+	}
 	return fd.VFSFileDescription(), nil
 }
 
diff --git a/pkg/sentry/fsimpl/proc/task_fds.go b/pkg/sentry/fsimpl/proc/task_fds.go
index 046265eca..a7622f1b6 100644
--- a/pkg/sentry/fsimpl/proc/task_fds.go
+++ b/pkg/sentry/fsimpl/proc/task_fds.go
@@ -143,8 +143,10 @@ func (i *fdDirInode) Lookup(ctx context.Context, name string) (*vfs.Dentry, erro
 
 // Open implements kernfs.Inode.
 func (i *fdDirInode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &kernfs.GenericDirectoryFD{}
-	fd.Init(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	if err != nil {
+		return nil, err
+	}
 	return fd.VFSFileDescription(), nil
 }
 
@@ -207,7 +209,7 @@ func (s *fdSymlink) Readlink(ctx context.Context) (string, error) {
 	return s.task.Kernel().VFS().PathnameWithDeleted(ctx, root, file.VirtualDentry())
 }
 
-func (s *fdSymlink) Getlink(ctx context.Context) (vfs.VirtualDentry, string, error) {
+func (s *fdSymlink) Getlink(ctx context.Context, mnt *vfs.Mount) (vfs.VirtualDentry, string, error) {
 	file, _ := getTaskFD(s.task, s.fd)
 	if file == nil {
 		return vfs.VirtualDentry{}, "", syserror.ENOENT
@@ -268,8 +270,10 @@ func (i *fdInfoDirInode) Lookup(ctx context.Context, name string) (*vfs.Dentry,
 
 // Open implements kernfs.Inode.
 func (i *fdInfoDirInode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &kernfs.GenericDirectoryFD{}
-	fd.Init(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	if err != nil {
+		return nil, err
+	}
 	return fd.VFSFileDescription(), nil
 }
 
diff --git a/pkg/sentry/fsimpl/proc/task_files.go b/pkg/sentry/fsimpl/proc/task_files.go
index f3173e197..410cc3552 100644
--- a/pkg/sentry/fsimpl/proc/task_files.go
+++ b/pkg/sentry/fsimpl/proc/task_files.go
@@ -622,7 +622,7 @@ func (s *exeSymlink) Readlink(ctx context.Context) (string, error) {
 }
 
 // Getlink implements kernfs.Inode.Getlink.
-func (s *exeSymlink) Getlink(ctx context.Context) (vfs.VirtualDentry, string, error) {
+func (s *exeSymlink) Getlink(ctx context.Context, _ *vfs.Mount) (vfs.VirtualDentry, string, error) {
 	if !kernel.ContextCanTrace(ctx, s.task, false) {
 		return vfs.VirtualDentry{}, "", syserror.EACCES
 	}
@@ -754,9 +754,79 @@ func (s *namespaceSymlink) Readlink(ctx context.Context) (string, error) {
 }
 
 // Getlink implements Inode.Getlink.
-func (s *namespaceSymlink) Getlink(ctx context.Context) (vfs.VirtualDentry, string, error) {
+func (s *namespaceSymlink) Getlink(ctx context.Context, mnt *vfs.Mount) (vfs.VirtualDentry, string, error) {
 	if err := checkTaskState(s.task); err != nil {
 		return vfs.VirtualDentry{}, "", err
 	}
-	return s.StaticSymlink.Getlink(ctx)
+
+	// Create a synthetic inode to represent the namespace.
+	dentry := &kernfs.Dentry{}
+	dentry.Init(&namespaceInode{})
+	vd := vfs.MakeVirtualDentry(mnt, dentry.VFSDentry())
+	vd.IncRef()
+	dentry.DecRef()
+	return vd, "", nil
+}
+
+// namespaceInode is a synthetic inode created to represent a namespace in
+// /proc/[pid]/ns/*.
+type namespaceInode struct {
+	kernfs.InodeAttrs
+	kernfs.InodeNoopRefCount
+	kernfs.InodeNotDirectory
+	kernfs.InodeNotSymlink
+}
+
+var _ kernfs.Inode = (*namespaceInode)(nil)
+
+// Init initializes a namespace inode.
+func (i *namespaceInode) Init(creds *auth.Credentials, ino uint64, perm linux.FileMode) {
+	if perm&^linux.PermissionsMask != 0 {
+		panic(fmt.Sprintf("Only permission mask must be set: %x", perm&linux.PermissionsMask))
+	}
+	i.InodeAttrs.Init(creds, ino, linux.ModeRegular|perm)
+}
+
+// Open implements Inode.Open.
+func (i *namespaceInode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
+	fd := &namespaceFD{inode: i}
+	i.IncRef()
+	if err := fd.vfsfd.Init(fd, opts.Flags, rp.Mount(), vfsd, &vfs.FileDescriptionOptions{}); err != nil {
+		return nil, err
+	}
+	return &fd.vfsfd, nil
+}
+
+// namespace FD is a synthetic file that represents a namespace in
+// /proc/[pid]/ns/*.
+type namespaceFD struct {
+	vfs.FileDescriptionDefaultImpl
+
+	vfsfd vfs.FileDescription
+	inode *namespaceInode
+}
+
+var _ vfs.FileDescriptionImpl = (*namespaceFD)(nil)
+
+// Stat implements FileDescriptionImpl.
+func (fd *namespaceFD) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
+	vfs := fd.vfsfd.VirtualDentry().Mount().Filesystem()
+	return fd.inode.Stat(vfs, opts)
+}
+
+// SetStat implements FileDescriptionImpl.
+func (fd *namespaceFD) SetStat(ctx context.Context, opts vfs.SetStatOptions) error {
+	vfs := fd.vfsfd.VirtualDentry().Mount().Filesystem()
+	creds := auth.CredentialsFromContext(ctx)
+	return fd.inode.SetStat(ctx, vfs, creds, opts)
+}
+
+// Release implements FileDescriptionImpl.
+func (fd *namespaceFD) Release() {
+	fd.inode.DecRef()
+}
+
+// OnClose implements FileDescriptionImpl.
+func (*namespaceFD) OnClose(context.Context) error {
+	return nil
 }
diff --git a/pkg/sentry/fsimpl/proc/tasks.go b/pkg/sentry/fsimpl/proc/tasks.go
index 9f2ef8200..26518ed03 100644
--- a/pkg/sentry/fsimpl/proc/tasks.go
+++ b/pkg/sentry/fsimpl/proc/tasks.go
@@ -202,8 +202,10 @@ func (i *tasksInode) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback
 
 // Open implements kernfs.Inode.
 func (i *tasksInode) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &kernfs.GenericDirectoryFD{}
-	fd.Init(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &i.OrderedChildren, &opts)
+	if err != nil {
+		return nil, err
+	}
 	return fd.VFSFileDescription(), nil
 }
 
diff --git a/pkg/sentry/fsimpl/proc/tasks_files.go b/pkg/sentry/fsimpl/proc/tasks_files.go
index 4621e2de0..92007df81 100644
--- a/pkg/sentry/fsimpl/proc/tasks_files.go
+++ b/pkg/sentry/fsimpl/proc/tasks_files.go
@@ -63,7 +63,7 @@ func (s *selfSymlink) Readlink(ctx context.Context) (string, error) {
 	return strconv.FormatUint(uint64(tgid), 10), nil
 }
 
-func (s *selfSymlink) Getlink(ctx context.Context) (vfs.VirtualDentry, string, error) {
+func (s *selfSymlink) Getlink(ctx context.Context, _ *vfs.Mount) (vfs.VirtualDentry, string, error) {
 	target, err := s.Readlink(ctx)
 	return vfs.VirtualDentry{}, target, err
 }
@@ -106,7 +106,7 @@ func (s *threadSelfSymlink) Readlink(ctx context.Context) (string, error) {
 	return fmt.Sprintf("%d/task/%d", tgid, tid), nil
 }
 
-func (s *threadSelfSymlink) Getlink(ctx context.Context) (vfs.VirtualDentry, string, error) {
+func (s *threadSelfSymlink) Getlink(ctx context.Context, _ *vfs.Mount) (vfs.VirtualDentry, string, error) {
 	target, err := s.Readlink(ctx)
 	return vfs.VirtualDentry{}, target, err
 }
diff --git a/pkg/sentry/fsimpl/sys/sys.go b/pkg/sentry/fsimpl/sys/sys.go
index 5c617270e..34e8e0cbe 100644
--- a/pkg/sentry/fsimpl/sys/sys.go
+++ b/pkg/sentry/fsimpl/sys/sys.go
@@ -106,8 +106,8 @@ func (*dir) SetStat(context.Context, *vfs.Filesystem, *auth.Credentials, vfs.Set
 
 // Open implements kernfs.Inode.Open.
 func (d *dir) Open(rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
-	fd := &kernfs.GenericDirectoryFD{}
-	if err := fd.Init(rp.Mount(), vfsd, &d.OrderedChildren, &opts); err != nil {
+	fd, err := kernfs.NewGenericDirectoryFD(rp.Mount(), vfsd, &d.OrderedChildren, &opts)
+	if err != nil {
 		return nil, err
 	}
 	return fd.VFSFileDescription(), nil
diff --git a/pkg/sentry/vfs/vfs.go b/pkg/sentry/vfs/vfs.go
index cb5bbd781..9015f2cc1 100644
--- a/pkg/sentry/vfs/vfs.go
+++ b/pkg/sentry/vfs/vfs.go
@@ -797,6 +797,14 @@ type VirtualDentry struct {
 	dentry *Dentry
 }
 
+// MakeVirtualDentry creates a VirtualDentry.
+func MakeVirtualDentry(mount *Mount, dentry *Dentry) VirtualDentry {
+	return VirtualDentry{
+		mount:  mount,
+		dentry: dentry,
+	}
+}
+
 // Ok returns true if vd is not empty. It does not require that a reference is
 // held.
 func (vd VirtualDentry) Ok() bool {