Port imported TTY fds to vfs2.

Refactor fs/host.TTYFileOperations so that the relevant functionality can be
shared with VFS2 (fsimpl/host.ttyFD).

Incorporate host.defaultFileFD into the default host.fileDescription. This way,
there is no need for a separate default_file.go. As in vfs1, the TTY file
implementation can be built on top of this default and override operations as
necessary (PRead/Read/PWrite/Write, Release, Ioctl).

Note that these changes still need to be plumbed into runsc, which refers to
imported TTYs in control/proc.go:ExecAsync.

Updates #1672.

PiperOrigin-RevId: 301718157

5 files changed, 679 insertions, 267 deletions

diff --git a/pkg/sentry/fsimpl/host/BUILD b/pkg/sentry/fsimpl/host/BUILD
index 5d67f88e3..0bb4a5c3e 100644
--- a/pkg/sentry/fsimpl/host/BUILD
+++ b/pkg/sentry/fsimpl/host/BUILD
@@ -5,8 +5,9 @@ licenses(["notice"])
 go_library(
     name = "host",
     srcs = [
-        "default_file.go",
         "host.go",
+        "ioctl_unsafe.go",
+        "tty.go",
         "util.go",
     ],
     visibility = ["//pkg/sentry:internal"],
@@ -17,9 +18,12 @@ go_library(
         "//pkg/log",
         "//pkg/refs",
         "//pkg/safemem",
+        "//pkg/sentry/arch",
         "//pkg/sentry/fsimpl/kernfs",
+        "//pkg/sentry/kernel",
         "//pkg/sentry/kernel/auth",
         "//pkg/sentry/memmap",
+        "//pkg/sentry/unimpl",
         "//pkg/sentry/vfs",
         "//pkg/sync",
         "//pkg/syserror",
diff --git a/pkg/sentry/fsimpl/host/default_file.go b/pkg/sentry/fsimpl/host/default_file.go
deleted file mode 100644
index 459238603..000000000
--- a/pkg/sentry/fsimpl/host/default_file.go
+++ /dev/null
@@ -1,247 +0,0 @@
-// Copyright 2020 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 host
-
-import (
-	"math"
-	"syscall"
-
-	"golang.org/x/sys/unix"
-	"gvisor.dev/gvisor/pkg/abi/linux"
-	"gvisor.dev/gvisor/pkg/context"
-	"gvisor.dev/gvisor/pkg/fd"
-	"gvisor.dev/gvisor/pkg/safemem"
-	"gvisor.dev/gvisor/pkg/sentry/memmap"
-	"gvisor.dev/gvisor/pkg/sentry/vfs"
-	"gvisor.dev/gvisor/pkg/sync"
-	"gvisor.dev/gvisor/pkg/syserror"
-	"gvisor.dev/gvisor/pkg/usermem"
-)
-
-// defaultFileFD implements FileDescriptionImpl for non-socket, non-TTY files.
-type defaultFileFD struct {
-	fileDescription
-
-	// canMap specifies whether we allow the file to be memory mapped.
-	canMap bool
-
-	// mu protects the fields below.
-	mu sync.Mutex
-
-	// offset specifies the current file offset.
-	offset int64
-}
-
-// TODO(gvisor.dev/issue/1672): Implement Waitable interface.
-
-// PRead implements FileDescriptionImpl.
-func (f *defaultFileFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
-	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
-	if f.inode.isStream {
-		return 0, syserror.ESPIPE
-	}
-
-	return readFromHostFD(ctx, f.inode.hostFD, dst, offset, int(opts.Flags))
-}
-
-// Read implements FileDescriptionImpl.
-func (f *defaultFileFD) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
-	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
-	if f.inode.isStream {
-		// These files can't be memory mapped, assert this.
-		if f.canMap {
-			panic("files that can return EWOULDBLOCK (sockets, pipes, etc.) cannot be memory mapped")
-		}
-
-		n, err := readFromHostFD(ctx, f.inode.hostFD, dst, -1, int(opts.Flags))
-		if isBlockError(err) {
-			// If we got any data at all, return it as a "completed" partial read
-			// rather than retrying until complete.
-			if n != 0 {
-				err = nil
-			} else {
-				err = syserror.ErrWouldBlock
-			}
-		}
-		return n, err
-	}
-	// TODO(gvisor.dev/issue/1672): Cache pages, when forced to do so.
-	f.mu.Lock()
-	n, err := readFromHostFD(ctx, f.inode.hostFD, dst, f.offset, int(opts.Flags))
-	f.offset += n
-	f.mu.Unlock()
-	return n, err
-}
-
-func readFromHostFD(ctx context.Context, hostFD int, dst usermem.IOSequence, offset int64, flags int) (int64, error) {
-	// TODO(gvisor.dev/issue/1672): Support select preadv2 flags.
-	if flags != 0 {
-		return 0, syserror.EOPNOTSUPP
-	}
-
-	var reader safemem.Reader
-	if offset == -1 {
-		reader = safemem.FromIOReader{fd.NewReadWriter(hostFD)}
-	} else {
-		reader = safemem.FromVecReaderFunc{
-			func(srcs [][]byte) (int64, error) {
-				n, err := unix.Preadv(hostFD, srcs, offset)
-				return int64(n), err
-			},
-		}
-	}
-	n, err := dst.CopyOutFrom(ctx, reader)
-	return int64(n), err
-}
-
-// PWrite implements FileDescriptionImpl.
-func (f *defaultFileFD) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
-	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
-	if f.inode.isStream {
-		return 0, syserror.ESPIPE
-	}
-	return writeToHostFD(ctx, f.inode.hostFD, src, offset, int(opts.Flags))
-}
-
-// Write implements FileDescriptionImpl.
-func (f *defaultFileFD) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
-	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
-	if f.inode.isStream {
-		// These files can't be memory mapped, assert this.
-		if f.canMap {
-			panic("files that can return EWOULDBLOCK (sockets, pipes, etc.) cannot be memory mapped")
-		}
-
-		n, err := writeToHostFD(ctx, f.inode.hostFD, src, -1, int(opts.Flags))
-		if isBlockError(err) {
-			err = syserror.ErrWouldBlock
-		}
-		return n, err
-	}
-	// TODO(gvisor.dev/issue/1672): Cache pages, when forced to do so.
-	// TODO(gvisor.dev/issue/1672): Write to end of file and update offset if O_APPEND is set on this file.
-	f.mu.Lock()
-	n, err := writeToHostFD(ctx, f.inode.hostFD, src, f.offset, int(opts.Flags))
-	f.offset += n
-	f.mu.Unlock()
-	return n, err
-}
-
-func writeToHostFD(ctx context.Context, hostFD int, src usermem.IOSequence, offset int64, flags int) (int64, error) {
-	// TODO(gvisor.dev/issue/1672): Support select pwritev2 flags.
-	if flags != 0 {
-		return 0, syserror.EOPNOTSUPP
-	}
-
-	limit, err := vfs.CheckLimit(ctx, offset, src.NumBytes())
-	if err != nil {
-		return 0, err
-	}
-	src = src.TakeFirst64(limit)
-
-	var writer safemem.Writer
-	if offset == -1 {
-		writer = safemem.FromIOWriter{fd.NewReadWriter(hostFD)}
-	} else {
-		writer = safemem.FromVecWriterFunc{
-			func(srcs [][]byte) (int64, error) {
-				n, err := unix.Pwritev(hostFD, srcs, offset)
-				return int64(n), err
-			},
-		}
-	}
-	n, err := src.CopyInTo(ctx, writer)
-	return int64(n), err
-}
-
-// Seek implements FileDescriptionImpl.
-//
-// Note that we do not support seeking on directories, since we do not even
-// allow directory fds to be imported at all.
-func (f *defaultFileFD) Seek(_ context.Context, offset int64, whence int32) (int64, error) {
-	// TODO(b/34716638): Some char devices do support seeking, e.g. /dev/null.
-	if f.inode.isStream {
-		return 0, syserror.ESPIPE
-	}
-
-	f.mu.Lock()
-	defer f.mu.Unlock()
-
-	switch whence {
-	case linux.SEEK_SET:
-		if offset < 0 {
-			return f.offset, syserror.EINVAL
-		}
-		f.offset = offset
-
-	case linux.SEEK_CUR:
-		// Check for overflow. Note that underflow cannot occur, since f.offset >= 0.
-		if offset > math.MaxInt64-f.offset {
-			return f.offset, syserror.EOVERFLOW
-		}
-		if f.offset+offset < 0 {
-			return f.offset, syserror.EINVAL
-		}
-		f.offset += offset
-
-	case linux.SEEK_END:
-		var s syscall.Stat_t
-		if err := syscall.Fstat(f.inode.hostFD, &s); err != nil {
-			return f.offset, err
-		}
-		size := s.Size
-
-		// Check for overflow. Note that underflow cannot occur, since size >= 0.
-		if offset > math.MaxInt64-size {
-			return f.offset, syserror.EOVERFLOW
-		}
-		if size+offset < 0 {
-			return f.offset, syserror.EINVAL
-		}
-		f.offset = size + offset
-
-	case linux.SEEK_DATA, linux.SEEK_HOLE:
-		// Modifying the offset in the host file table should not matter, since
-		// this is the only place where we use it.
-		//
-		// For reading and writing, we always rely on our internal offset.
-		n, err := unix.Seek(f.inode.hostFD, offset, int(whence))
-		if err != nil {
-			return f.offset, err
-		}
-		f.offset = n
-
-	default:
-		// Invalid whence.
-		return f.offset, syserror.EINVAL
-	}
-
-	return f.offset, nil
-}
-
-// Sync implements FileDescriptionImpl.
-func (f *defaultFileFD) Sync(context.Context) error {
-	// TODO(gvisor.dev/issue/1672): Currently we do not support the SyncData optimization, so we always sync everything.
-	return unix.Fsync(f.inode.hostFD)
-}
-
-// ConfigureMMap implements FileDescriptionImpl.
-func (f *defaultFileFD) ConfigureMMap(_ context.Context, opts *memmap.MMapOpts) error {
-	if !f.canMap {
-		return syserror.ENODEV
-	}
-	// TODO(gvisor.dev/issue/1672): Implement ConfigureMMap and Mappable interface.
-	return syserror.ENODEV
-}
diff --git a/pkg/sentry/fsimpl/host/host.go b/pkg/sentry/fsimpl/host/host.go
index 2eebcd60c..3afb41395 100644
--- a/pkg/sentry/fsimpl/host/host.go
+++ b/pkg/sentry/fsimpl/host/host.go
@@ -19,18 +19,23 @@ package host
 import (
 	"errors"
 	"fmt"
+	"math"
 	"syscall"
 
 	"golang.org/x/sys/unix"
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fd"
 	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/refs"
+	"gvisor.dev/gvisor/pkg/safemem"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
 )
 
 // filesystem implements vfs.FilesystemImpl.
@@ -70,10 +75,20 @@ func ImportFD(mnt *vfs.Mount, hostFD int, ownerUID auth.KUID, ownerGID auth.KGID
 		hostFD:   hostFD,
 		isStream: isStream,
 		isTTY:    isTTY,
+		canMap:   canMap(uint32(fileType)),
 		ino:      fs.NextIno(),
 		mode:     fileMode,
 		uid:      ownerUID,
 		gid:      ownerGID,
+		// For simplicity, set offset to 0. Technically, we should
+		// only set to 0 on files that are not seekable (sockets, pipes, etc.),
+		// and use the offset from the host fd otherwise.
+		offset: 0,
+	}
+
+	// These files can't be memory mapped, assert this.
+	if i.isStream && i.canMap {
+		panic("files that can return EWOULDBLOCK (sockets, pipes, etc.) cannot be memory mapped")
 	}
 
 	d := &kernfs.Dentry{}
@@ -110,12 +125,17 @@ type inode struct {
 	// This field is initialized at creation time and is immutable.
 	isTTY bool
 
+	// canMap specifies whether we allow the file to be memory mapped.
+	//
+	// This field is initialized at creation time and is immutable.
+	canMap bool
+
 	// ino is an inode number unique within this filesystem.
+	//
+	// This field is initialized at creation time and is immutable.
 	ino uint64
 
-	// mu protects the inode metadata below.
-	// TODO(gvisor.dev/issue/1672): actually protect fields below.
-	//mu sync.Mutex
+	// TODO(gvisor.dev/issue/1672): protect mode, uid, and gid with mutex.
 
 	// mode is the file mode of this inode. Note that this value may become out
 	// of date if the mode is changed on the host, e.g. with chmod.
@@ -125,6 +145,12 @@ type inode struct {
 	// file created on import, not the fd on the host.
 	uid auth.KUID
 	gid auth.KGID
+
+	// offsetMu protects offset.
+	offsetMu sync.Mutex
+
+	// offset specifies the current file offset.
+	offset int64
 }
 
 // Note that these flags may become out of date, since they can be modified
@@ -336,36 +362,40 @@ func (i *inode) open(d *vfs.Dentry, mnt *vfs.Mount) (*vfs.FileDescription, error
 
 	// TODO(gvisor.dev/issue/1672): Whitelist specific file types here, so that
 	// we don't allow importing arbitrary file types without proper support.
+	var (
+		vfsfd  *vfs.FileDescription
+		fdImpl vfs.FileDescriptionImpl
+	)
 	if i.isTTY {
-		// TODO(gvisor.dev/issue/1672): support importing host fd as TTY.
-		return nil, errors.New("importing host fd as TTY not supported")
-	}
-
-	// For simplicity, set offset to 0. Technically, we should
-	// only set to 0 on files that are not seekable (sockets, pipes, etc.),
-	// and use the offset from the host fd otherwise.
-	fd := &defaultFileFD{
-		fileDescription: fileDescription{
-			inode: i,
-		},
-		canMap: canMap(uint32(fileType)),
-		mu:     sync.Mutex{},
-		offset: 0,
+		fd := &ttyFD{
+			fileDescription: fileDescription{inode: i},
+			termios:         linux.DefaultSlaveTermios,
+		}
+		vfsfd = &fd.vfsfd
+		fdImpl = fd
+	} else {
+		// For simplicity, set offset to 0. Technically, we should
+		// only set to 0 on files that are not seekable (sockets, pipes, etc.),
+		// and use the offset from the host fd otherwise.
+		fd := &fileDescription{inode: i}
+		vfsfd = &fd.vfsfd
+		fdImpl = fd
 	}
 
-	vfsfd := &fd.vfsfd
 	flags, err := fileFlagsFromHostFD(i.hostFD)
 	if err != nil {
 		return nil, err
 	}
 
-	if err := vfsfd.Init(fd, uint32(flags), mnt, d, &vfs.FileDescriptionOptions{}); err != nil {
+	if err := vfsfd.Init(fdImpl, uint32(flags), mnt, d, &vfs.FileDescriptionOptions{}); err != nil {
 		return nil, err
 	}
 	return vfsfd, nil
 }
 
 // fileDescription is embedded by host fd implementations of FileDescriptionImpl.
+//
+// TODO(gvisor.dev/issue/1672): Implement Waitable interface.
 type fileDescription struct {
 	vfsfd vfs.FileDescription
 	vfs.FileDescriptionDefaultImpl
@@ -394,3 +424,193 @@ func (f *fileDescription) Stat(_ context.Context, opts vfs.StatOptions) (linux.S
 func (f *fileDescription) Release() {
 	// noop
 }
+
+// PRead implements FileDescriptionImpl.
+func (f *fileDescription) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	i := f.inode
+	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
+	if i.isStream {
+		return 0, syserror.ESPIPE
+	}
+
+	return readFromHostFD(ctx, i.hostFD, dst, offset, opts.Flags)
+}
+
+// Read implements FileDescriptionImpl.
+func (f *fileDescription) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	i := f.inode
+	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
+	if i.isStream {
+		n, err := readFromHostFD(ctx, i.hostFD, dst, -1, opts.Flags)
+		if isBlockError(err) {
+			// If we got any data at all, return it as a "completed" partial read
+			// rather than retrying until complete.
+			if n != 0 {
+				err = nil
+			} else {
+				err = syserror.ErrWouldBlock
+			}
+		}
+		return n, err
+	}
+	// TODO(gvisor.dev/issue/1672): Cache pages, when forced to do so.
+	i.offsetMu.Lock()
+	n, err := readFromHostFD(ctx, i.hostFD, dst, i.offset, opts.Flags)
+	i.offset += n
+	i.offsetMu.Unlock()
+	return n, err
+}
+
+func readFromHostFD(ctx context.Context, hostFD int, dst usermem.IOSequence, offset int64, flags uint32) (int64, error) {
+	// TODO(gvisor.dev/issue/1672): Support select preadv2 flags.
+	if flags != 0 {
+		return 0, syserror.EOPNOTSUPP
+	}
+
+	var reader safemem.Reader
+	if offset == -1 {
+		reader = safemem.FromIOReader{fd.NewReadWriter(hostFD)}
+	} else {
+		reader = safemem.FromVecReaderFunc{
+			func(srcs [][]byte) (int64, error) {
+				n, err := unix.Preadv(hostFD, srcs, offset)
+				return int64(n), err
+			},
+		}
+	}
+	n, err := dst.CopyOutFrom(ctx, reader)
+	return int64(n), err
+}
+
+// PWrite implements FileDescriptionImpl.
+func (f *fileDescription) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
+	i := f.inode
+	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
+	if i.isStream {
+		return 0, syserror.ESPIPE
+	}
+
+	return writeToHostFD(ctx, i.hostFD, src, offset, opts.Flags)
+}
+
+// Write implements FileDescriptionImpl.
+func (f *fileDescription) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
+	i := f.inode
+	// TODO(b/34716638): Some char devices do support offsets, e.g. /dev/null.
+	if i.isStream {
+		n, err := writeToHostFD(ctx, i.hostFD, src, -1, opts.Flags)
+		if isBlockError(err) {
+			err = syserror.ErrWouldBlock
+		}
+		return n, err
+	}
+	// TODO(gvisor.dev/issue/1672): Cache pages, when forced to do so.
+	// TODO(gvisor.dev/issue/1672): Write to end of file and update offset if O_APPEND is set on this file.
+	i.offsetMu.Lock()
+	n, err := writeToHostFD(ctx, i.hostFD, src, i.offset, opts.Flags)
+	i.offset += n
+	i.offsetMu.Unlock()
+	return n, err
+}
+
+func writeToHostFD(ctx context.Context, hostFD int, src usermem.IOSequence, offset int64, flags uint32) (int64, error) {
+	// TODO(gvisor.dev/issue/1672): Support select pwritev2 flags.
+	if flags != 0 {
+		return 0, syserror.EOPNOTSUPP
+	}
+
+	var writer safemem.Writer
+	if offset == -1 {
+		writer = safemem.FromIOWriter{fd.NewReadWriter(hostFD)}
+	} else {
+		writer = safemem.FromVecWriterFunc{
+			func(srcs [][]byte) (int64, error) {
+				n, err := unix.Pwritev(hostFD, srcs, offset)
+				return int64(n), err
+			},
+		}
+	}
+	n, err := src.CopyInTo(ctx, writer)
+	return int64(n), err
+}
+
+// Seek implements FileDescriptionImpl.
+//
+// Note that we do not support seeking on directories, since we do not even
+// allow directory fds to be imported at all.
+func (f *fileDescription) Seek(_ context.Context, offset int64, whence int32) (int64, error) {
+	i := f.inode
+	// TODO(b/34716638): Some char devices do support seeking, e.g. /dev/null.
+	if i.isStream {
+		return 0, syserror.ESPIPE
+	}
+
+	i.offsetMu.Lock()
+	defer i.offsetMu.Unlock()
+
+	switch whence {
+	case linux.SEEK_SET:
+		if offset < 0 {
+			return i.offset, syserror.EINVAL
+		}
+		i.offset = offset
+
+	case linux.SEEK_CUR:
+		// Check for overflow. Note that underflow cannot occur, since i.offset >= 0.
+		if offset > math.MaxInt64-i.offset {
+			return i.offset, syserror.EOVERFLOW
+		}
+		if i.offset+offset < 0 {
+			return i.offset, syserror.EINVAL
+		}
+		i.offset += offset
+
+	case linux.SEEK_END:
+		var s syscall.Stat_t
+		if err := syscall.Fstat(i.hostFD, &s); err != nil {
+			return i.offset, err
+		}
+		size := s.Size
+
+		// Check for overflow. Note that underflow cannot occur, since size >= 0.
+		if offset > math.MaxInt64-size {
+			return i.offset, syserror.EOVERFLOW
+		}
+		if size+offset < 0 {
+			return i.offset, syserror.EINVAL
+		}
+		i.offset = size + offset
+
+	case linux.SEEK_DATA, linux.SEEK_HOLE:
+		// Modifying the offset in the host file table should not matter, since
+		// this is the only place where we use it.
+		//
+		// For reading and writing, we always rely on our internal offset.
+		n, err := unix.Seek(i.hostFD, offset, int(whence))
+		if err != nil {
+			return i.offset, err
+		}
+		i.offset = n
+
+	default:
+		// Invalid whence.
+		return i.offset, syserror.EINVAL
+	}
+
+	return i.offset, nil
+}
+
+// Sync implements FileDescriptionImpl.
+func (f *fileDescription) Sync(context.Context) error {
+	// TODO(gvisor.dev/issue/1672): Currently we do not support the SyncData optimization, so we always sync everything.
+	return unix.Fsync(f.inode.hostFD)
+}
+
+// ConfigureMMap implements FileDescriptionImpl.
+func (f *fileDescription) ConfigureMMap(_ context.Context, opts *memmap.MMapOpts) error {
+	if !f.inode.canMap {
+		return syserror.ENODEV
+	}
+	// TODO(gvisor.dev/issue/1672): Implement ConfigureMMap and Mappable interface.
+	return syserror.ENODEV
+}
diff --git a/pkg/sentry/fsimpl/host/ioctl_unsafe.go b/pkg/sentry/fsimpl/host/ioctl_unsafe.go
new file mode 100644
index 000000000..0983bf7d8
--- /dev/null
+++ b/pkg/sentry/fsimpl/host/ioctl_unsafe.go
@@ -0,0 +1,56 @@
+// Copyright 2020 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 host
+
+import (
+	"syscall"
+	"unsafe"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+)
+
+func ioctlGetTermios(fd int) (*linux.Termios, error) {
+	var t linux.Termios
+	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), linux.TCGETS, uintptr(unsafe.Pointer(&t)))
+	if errno != 0 {
+		return nil, errno
+	}
+	return &t, nil
+}
+
+func ioctlSetTermios(fd int, req uint64, t *linux.Termios) error {
+	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), uintptr(req), uintptr(unsafe.Pointer(t)))
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+func ioctlGetWinsize(fd int) (*linux.Winsize, error) {
+	var w linux.Winsize
+	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), linux.TIOCGWINSZ, uintptr(unsafe.Pointer(&w)))
+	if errno != 0 {
+		return nil, errno
+	}
+	return &w, nil
+}
+
+func ioctlSetWinsize(fd int, w *linux.Winsize) error {
+	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), linux.TIOCSWINSZ, uintptr(unsafe.Pointer(w)))
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
diff --git a/pkg/sentry/fsimpl/host/tty.go b/pkg/sentry/fsimpl/host/tty.go
new file mode 100644
index 000000000..8936afb06
--- /dev/null
+++ b/pkg/sentry/fsimpl/host/tty.go
@@ -0,0 +1,379 @@
+// Copyright 2020 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 host
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/unimpl"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+// ttyFD implements vfs.FileDescriptionImpl for a host file descriptor
+// that wraps a TTY FD.
+type ttyFD struct {
+	fileDescription
+
+	// mu protects the fields below.
+	mu sync.Mutex `state:"nosave"`
+
+	// session is the session attached to this ttyFD.
+	session *kernel.Session
+
+	// fgProcessGroup is the foreground process group that is currently
+	// connected to this TTY.
+	fgProcessGroup *kernel.ProcessGroup
+
+	// termios contains the terminal attributes for this TTY.
+	termios linux.KernelTermios
+}
+
+// InitForegroundProcessGroup sets the foreground process group and session for
+// the TTY. This should only be called once, after the foreground process group
+// has been created, but before it has started running.
+func (t *ttyFD) InitForegroundProcessGroup(pg *kernel.ProcessGroup) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if t.fgProcessGroup != nil {
+		panic("foreground process group is already set")
+	}
+	t.fgProcessGroup = pg
+	t.session = pg.Session()
+}
+
+// ForegroundProcessGroup returns the foreground process for the TTY.
+func (t *ttyFD) ForegroundProcessGroup() *kernel.ProcessGroup {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return t.fgProcessGroup
+}
+
+// Release implements fs.FileOperations.Release.
+func (t *ttyFD) Release() {
+	t.mu.Lock()
+	t.fgProcessGroup = nil
+	t.mu.Unlock()
+
+	t.fileDescription.Release()
+}
+
+// PRead implements vfs.FileDescriptionImpl.
+//
+// Reading from a TTY is only allowed for foreground process groups. Background
+// process groups will either get EIO or a SIGTTIN.
+func (t *ttyFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	// Are we allowed to do the read?
+	// drivers/tty/n_tty.c:n_tty_read()=>job_control()=>tty_check_change().
+	if err := t.checkChange(ctx, linux.SIGTTIN); err != nil {
+		return 0, err
+	}
+
+	// Do the read.
+	return t.fileDescription.PRead(ctx, dst, offset, opts)
+}
+
+// Read implements vfs.FileDescriptionImpl.
+//
+// Reading from a TTY is only allowed for foreground process groups. Background
+// process groups will either get EIO or a SIGTTIN.
+func (t *ttyFD) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	// Are we allowed to do the read?
+	// drivers/tty/n_tty.c:n_tty_read()=>job_control()=>tty_check_change().
+	if err := t.checkChange(ctx, linux.SIGTTIN); err != nil {
+		return 0, err
+	}
+
+	// Do the read.
+	return t.fileDescription.Read(ctx, dst, opts)
+}
+
+// PWrite implements vfs.FileDescriptionImpl.
+func (t *ttyFD) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	// Check whether TOSTOP is enabled. This corresponds to the check in
+	// drivers/tty/n_tty.c:n_tty_write().
+	if t.termios.LEnabled(linux.TOSTOP) {
+		if err := t.checkChange(ctx, linux.SIGTTOU); err != nil {
+			return 0, err
+		}
+	}
+	return t.fileDescription.PWrite(ctx, src, offset, opts)
+}
+
+// Write implements vfs.FileDescriptionImpl.
+func (t *ttyFD) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	// Check whether TOSTOP is enabled. This corresponds to the check in
+	// drivers/tty/n_tty.c:n_tty_write().
+	if t.termios.LEnabled(linux.TOSTOP) {
+		if err := t.checkChange(ctx, linux.SIGTTOU); err != nil {
+			return 0, err
+		}
+	}
+	return t.fileDescription.Write(ctx, src, opts)
+}
+
+// Ioctl implements vfs.FileDescriptionImpl.
+func (t *ttyFD) Ioctl(ctx context.Context, io usermem.IO, args arch.SyscallArguments) (uintptr, error) {
+	// Ignore arg[0]. This is the real FD:
+	fd := t.inode.hostFD
+	ioctl := args[1].Uint64()
+	switch ioctl {
+	case linux.TCGETS:
+		termios, err := ioctlGetTermios(fd)
+		if err != nil {
+			return 0, err
+		}
+		_, err = usermem.CopyObjectOut(ctx, io, args[2].Pointer(), termios, usermem.IOOpts{
+			AddressSpaceActive: true,
+		})
+		return 0, err
+
+	case linux.TCSETS, linux.TCSETSW, linux.TCSETSF:
+		t.mu.Lock()
+		defer t.mu.Unlock()
+
+		if err := t.checkChange(ctx, linux.SIGTTOU); err != nil {
+			return 0, err
+		}
+
+		var termios linux.Termios
+		if _, err := usermem.CopyObjectIn(ctx, io, args[2].Pointer(), &termios, usermem.IOOpts{
+			AddressSpaceActive: true,
+		}); err != nil {
+			return 0, err
+		}
+		err := ioctlSetTermios(fd, ioctl, &termios)
+		if err == nil {
+			t.termios.FromTermios(termios)
+		}
+		return 0, err
+
+	case linux.TIOCGPGRP:
+		// Args: pid_t *argp
+		// When successful, equivalent to *argp = tcgetpgrp(fd).
+		// Get the process group ID of the foreground process group on this
+		// terminal.
+
+		pidns := kernel.PIDNamespaceFromContext(ctx)
+		if pidns == nil {
+			return 0, syserror.ENOTTY
+		}
+
+		t.mu.Lock()
+		defer t.mu.Unlock()
+
+		// Map the ProcessGroup into a ProcessGroupID in the task's PID namespace.
+		pgID := pidns.IDOfProcessGroup(t.fgProcessGroup)
+		_, err := usermem.CopyObjectOut(ctx, io, args[2].Pointer(), &pgID, usermem.IOOpts{
+			AddressSpaceActive: true,
+		})
+		return 0, err
+
+	case linux.TIOCSPGRP:
+		// Args: const pid_t *argp
+		// Equivalent to tcsetpgrp(fd, *argp).
+		// Set the foreground process group ID of this terminal.
+
+		task := kernel.TaskFromContext(ctx)
+		if task == nil {
+			return 0, syserror.ENOTTY
+		}
+
+		t.mu.Lock()
+		defer t.mu.Unlock()
+
+		// Check that we are allowed to set the process group.
+		if err := t.checkChange(ctx, linux.SIGTTOU); err != nil {
+			// drivers/tty/tty_io.c:tiocspgrp() converts -EIO from tty_check_change()
+			// to -ENOTTY.
+			if err == syserror.EIO {
+				return 0, syserror.ENOTTY
+			}
+			return 0, err
+		}
+
+		// Check that calling task's process group is in the TTY session.
+		if task.ThreadGroup().Session() != t.session {
+			return 0, syserror.ENOTTY
+		}
+
+		var pgID kernel.ProcessGroupID
+		if _, err := usermem.CopyObjectIn(ctx, io, args[2].Pointer(), &pgID, usermem.IOOpts{
+			AddressSpaceActive: true,
+		}); err != nil {
+			return 0, err
+		}
+
+		// pgID must be non-negative.
+		if pgID < 0 {
+			return 0, syserror.EINVAL
+		}
+
+		// Process group with pgID must exist in this PID namespace.
+		pidns := task.PIDNamespace()
+		pg := pidns.ProcessGroupWithID(pgID)
+		if pg == nil {
+			return 0, syserror.ESRCH
+		}
+
+		// Check that new process group is in the TTY session.
+		if pg.Session() != t.session {
+			return 0, syserror.EPERM
+		}
+
+		t.fgProcessGroup = pg
+		return 0, nil
+
+	case linux.TIOCGWINSZ:
+		// Args: struct winsize *argp
+		// Get window size.
+		winsize, err := ioctlGetWinsize(fd)
+		if err != nil {
+			return 0, err
+		}
+		_, err = usermem.CopyObjectOut(ctx, io, args[2].Pointer(), winsize, usermem.IOOpts{
+			AddressSpaceActive: true,
+		})
+		return 0, err
+
+	case linux.TIOCSWINSZ:
+		// Args: const struct winsize *argp
+		// Set window size.
+
+		// Unlike setting the termios, any process group (even background ones) can
+		// set the winsize.
+
+		var winsize linux.Winsize
+		if _, err := usermem.CopyObjectIn(ctx, io, args[2].Pointer(), &winsize, usermem.IOOpts{
+			AddressSpaceActive: true,
+		}); err != nil {
+			return 0, err
+		}
+		err := ioctlSetWinsize(fd, &winsize)
+		return 0, err
+
+	// Unimplemented commands.
+	case linux.TIOCSETD,
+		linux.TIOCSBRK,
+		linux.TIOCCBRK,
+		linux.TCSBRK,
+		linux.TCSBRKP,
+		linux.TIOCSTI,
+		linux.TIOCCONS,
+		linux.FIONBIO,
+		linux.TIOCEXCL,
+		linux.TIOCNXCL,
+		linux.TIOCGEXCL,
+		linux.TIOCNOTTY,
+		linux.TIOCSCTTY,
+		linux.TIOCGSID,
+		linux.TIOCGETD,
+		linux.TIOCVHANGUP,
+		linux.TIOCGDEV,
+		linux.TIOCMGET,
+		linux.TIOCMSET,
+		linux.TIOCMBIC,
+		linux.TIOCMBIS,
+		linux.TIOCGICOUNT,
+		linux.TCFLSH,
+		linux.TIOCSSERIAL,
+		linux.TIOCGPTPEER:
+
+		unimpl.EmitUnimplementedEvent(ctx)
+		fallthrough
+	default:
+		return 0, syserror.ENOTTY
+	}
+}
+
+// checkChange checks that the process group is allowed to read, write, or
+// change the state of the TTY.
+//
+// This corresponds to Linux drivers/tty/tty_io.c:tty_check_change(). The logic
+// is a bit convoluted, but documented inline.
+//
+// Preconditions: t.mu must be held.
+func (t *ttyFD) checkChange(ctx context.Context, sig linux.Signal) error {
+	task := kernel.TaskFromContext(ctx)
+	if task == nil {
+		// No task? Linux does not have an analog for this case, but
+		// tty_check_change is more of a blacklist of cases than a
+		// whitelist, and is surprisingly permissive. Allowing the
+		// change seems most appropriate.
+		return nil
+	}
+
+	tg := task.ThreadGroup()
+	pg := tg.ProcessGroup()
+
+	// If the session for the task is different than the session for the
+	// controlling TTY, then the change is allowed. Seems like a bad idea,
+	// but that's exactly what linux does.
+	if tg.Session() != t.fgProcessGroup.Session() {
+		return nil
+	}
+
+	// If we are the foreground process group, then the change is allowed.
+	if pg == t.fgProcessGroup {
+		return nil
+	}
+
+	// We are not the foreground process group.
+
+	// Is the provided signal blocked or ignored?
+	if (task.SignalMask()&linux.SignalSetOf(sig) != 0) || tg.SignalHandlers().IsIgnored(sig) {
+		// If the signal is SIGTTIN, then we are attempting to read
+		// from the TTY. Don't send the signal and return EIO.
+		if sig == linux.SIGTTIN {
+			return syserror.EIO
+		}
+
+		// Otherwise, we are writing or changing terminal state. This is allowed.
+		return nil
+	}
+
+	// If the process group is an orphan, return EIO.
+	if pg.IsOrphan() {
+		return syserror.EIO
+	}
+
+	// Otherwise, send the signal to the process group and return ERESTARTSYS.
+	//
+	// Note that Linux also unconditionally sets TIF_SIGPENDING on current,
+	// but this isn't necessary in gVisor because the rationale given in
+	// 040b6362d58f "tty: fix leakage of -ERESTARTSYS to userland" doesn't
+	// apply: the sentry will handle -ERESTARTSYS in
+	// kernel.runApp.execute() even if the kernel.Task isn't interrupted.
+	//
+	// Linux ignores the result of kill_pgrp().
+	_ = pg.SendSignal(kernel.SignalInfoPriv(sig))
+	return kernel.ERESTARTSYS
+}