Add a stub for /dev/tty.

Actual implementation to follow, but this will satisfy applications that
want it to just exist.

pkg/sentry/fs/dev/BUILD

@@ -11,6 +11,7 @@ go_library(
         "full.go",
         "null.go",
         "random.go",
+        "tty.go",
     ],
     importpath = "gvisor.dev/gvisor/pkg/sentry/fs/dev",
     visibility = ["//pkg/sentry:internal"],

pkg/sentry/fs/dev/dev.go

@@ -38,12 +38,20 @@ const (
 	urandomDevMinor uint32 = 9
 )
 
-func newCharacterDevice(ctx context.Context, iops fs.InodeOperations, msrc *fs.MountSource) *fs.Inode {
+// TTY major device number comes from include/uapi/linux/major.h.
+const (
+	ttyDevMinor = 0
+	ttyDevMajor = 5
+)
+
+func newCharacterDevice(ctx context.Context, iops fs.InodeOperations, msrc *fs.MountSource, major uint16, minor uint32) *fs.Inode {
 	return fs.NewInode(ctx, iops, msrc, fs.StableAttr{
-		DeviceID:  devDevice.DeviceID(),
-		InodeID:   devDevice.NextIno(),
-		BlockSize: usermem.PageSize,
-		Type:      fs.CharacterDevice,
+		DeviceID:        devDevice.DeviceID(),
+		InodeID:         devDevice.NextIno(),
+		BlockSize:       usermem.PageSize,
+		Type:            fs.CharacterDevice,
+		DeviceFileMajor: major,
+		DeviceFileMinor: minor,
 	})
 }
 
@@ -114,6 +122,8 @@ func New(ctx context.Context, msrc *fs.MountSource) *fs.Inode {
 		// If no devpts is mounted, this will simply be a dangling
 		// symlink, which is fine.
 		"ptmx": newSymlink(ctx, "pts/ptmx", msrc),
+
+		"tty": newCharacterDevice(ctx, newTTYDevice(ctx, fs.RootOwner, 0666), msrc, ttyDevMajor, ttyDevMinor),
 	}
 
 	iops := ramfs.NewDir(ctx, contents, fs.RootOwner, fs.FilePermsFromMode(0555))

pkg/sentry/fs/dev/tty.go

@@ -0,0 +1,79 @@
+// 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 dev
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/rand"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+	"gvisor.dev/gvisor/pkg/sentry/fs/fsutil"
+	"gvisor.dev/gvisor/pkg/sentry/safemem"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/waiter"
+)
+
+// +stateify savable
+type ttyInodeOperations struct {
+	fsutil.InodeGenericChecker       `state:"nosave"`
+	fsutil.InodeNoExtendedAttributes `state:"nosave"`
+	fsutil.InodeNoopAllocate         `state:"nosave"`
+	fsutil.InodeNoopRelease          `state:"nosave"`
+	fsutil.InodeNoopTruncate         `state:"nosave"`
+	fsutil.InodeNoopWriteOut         `state:"nosave"`
+	fsutil.InodeNotDirectory         `state:"nosave"`
+	fsutil.InodeNotMappable          `state:"nosave"`
+	fsutil.InodeNotSocket            `state:"nosave"`
+	fsutil.InodeNotSymlink           `state:"nosave"`
+	fsutil.InodeVirtual              `state:"nosave"`
+
+	fsutil.InodeSimpleAttributes
+}
+
+var _ fs.InodeOperations = (*ttyInodeOperations)(nil)
+
+func newTTYDevice(ctx context.Context, owner fs.FileOwner, mode linux.FileMode) *ttyInodeOperations {
+	return &ttyInodeOperations{
+		InodeSimpleAttributes: fsutil.NewInodeSimpleAttributes(ctx, owner, fs.FilePermsFromMode(mode), linux.TMPFS_MAGIC),
+	}
+}
+
+// GetFile implements fs.InodeOperations.GetFile.
+func (*ttyInodeOperations) GetFile(ctx context.Context, dirent *fs.Dirent, flags fs.FileFlags) (*fs.File, error) {
+	return fs.NewFile(ctx, dirent, flags, &ttyFileOperations{}), nil
+}
+
+// +stateify savable
+type ttyFileOperations struct {
+	fsutil.FileNoSeek               `state:"nosave"`
+	fsutil.FileNoIoctl              `state:"nosave"`
+	fsutil.FileNoMMap               `state:"nosave"`
+	fsutil.FileNoSplice             `state:"nosave"`
+	fsutil.FileNoopFlush            `state:"nosave"`
+	fsutil.FileNoopFsync            `state:"nosave"`
+	fsutil.FileNoopRelease          `state:"nosave"`
+	fsutil.FileNoopWrite            `state:"nosave"`
+	fsutil.FileNoopRead             `state:"nosave"`
+	fsutil.FileNotDirReaddir        `state:"nosave"`
+	fsutil.FileUseInodeUnstableAttr `state:"nosave"`
+	waiter.AlwaysReady              `state:"nosave"`
+}
+
+var _ fs.FileOperations = (*ttyFileOperations)(nil)
+
+// Read implements fs.FileOperations.Read.
+func (*ttyFileOperations) Read(ctx context.Context, _ *fs.File, dst usermem.IOSequence, _ int64) (int64, error) {
+	return dst.CopyOutFrom(ctx, safemem.FromIOReader{rand.Reader})
+}

test/syscalls/linux/dev.cc

@@ -13,6 +13,8 @@
 // limitations under the License.
 
 #include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 #include <unistd.h>
 #include <vector>
 
@@ -143,6 +145,11 @@ TEST(DevTest, WriteDevFull) {
   EXPECT_THAT(WriteFd(fd.get(), "a", 1), SyscallFailsWithErrno(ENOSPC));
 }
 
+TEST(DevTest, TTYExists) {
+  struct stat statbuf = {};
+  ASSERT_THAT(stat("/dev/tty", &statbuf), SyscallSucceeds());
+}
+
 }  // namespace
 }  // namespace testing