Implement /dev/tty for donated host TTYs

pkg/sentry/control/proc.go

@@ -272,6 +272,13 @@ func (proc *Proc) execAsync(args *ExecArgs) (*kernel.ThreadGroup, kernel.ThreadI
 		return nil, 0, nil, err
 	}
 
+	if ttyFile != nil {
+		// Index does not matter here. This tty is not coming from a
+		// devpts mount, so it won't collide with any of the ptys
+		// created there.
+		initArgs.TTY = kernel.NewTTY(0, ttyFile)
+	}
+
 	// Set cgroups to the new exec task if cgroups are mounted.
 	cgroupRegistry := proc.Kernel.CgroupRegistry()
 	initialCgrps := map[kernel.Cgroup]struct{}{}

pkg/sentry/fsimpl/host/tty.go

@@ -31,6 +31,8 @@ import (
 // TTYFileDescription implements vfs.FileDescriptionImpl for a host file
 // descriptor that wraps a TTY FD.
 //
+// It implements kernel.TTYOperations.
+//
 // +stateify savable
 type TTYFileDescription struct {
 	fileDescription
@@ -49,6 +51,13 @@ type TTYFileDescription struct {
 	termios linux.KernelTermios
 }
 
+// Open re-opens the tty fd, for example via open(/dev/tty). See Linux's
+// tty_repoen().
+func (t *TTYFileDescription) Open(_ context.Context, _ *vfs.Mount, _ *vfs.Dentry, _ vfs.OpenOptions) (*vfs.FileDescription, error) {
+	t.vfsfd.IncRef()
+	return &t.vfsfd, nil
+}
+
 // 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.

pkg/sentry/kernel/kernel.go

@@ -991,6 +991,9 @@ type CreateProcessArgs struct {
 
 	// Origin indicates how the task was first created.
 	Origin TaskOrigin
+
+	// TTY is the optional TTY to associate with this process.
+	TTY *TTY
 }
 
 // NewContext returns a context.Context that represents the task that will be
@@ -1222,6 +1225,12 @@ func (k *Kernel) CreateProcess(args CreateProcessArgs) (*ThreadGroup, ThreadID,
 	}
 	t.traceExecEvent(image) // Simulate exec for tracing.
 
+	// Set TTY if configured.
+	if args.TTY != nil {
+		t.tg.tty = args.TTY
+		args.TTY.tg = t.tg
+	}
+
 	// Success.
 	cu.Release()
 	tgid := k.tasks.Root.IDOfThreadGroup(tg)

runsc/boot/loader.go

@@ -1110,6 +1110,13 @@ func (l *Loader) createContainerProcess(info *containerInfo) (*kernel.ThreadGrou
 	// ours either way.
 	info.procArgs.FDTable = fdTable
 
+	if ttyFile != nil {
+		// Index does not matter here. This tty is not coming from a
+		// devpts mount, so it won't collide with any of the ptys
+		// created there.
+		info.procArgs.TTY = kernel.NewTTY(0, ttyFile)
+	}
+
 	if info.execFD != nil {
 		if info.procArgs.Filename != "" {
 			return nil, nil, fmt.Errorf("process must either be started from a file or a filename, not both")

runsc/container/console_test.go

@@ -594,6 +594,16 @@ func TestMultiContainerTerminal(t *testing.T) {
 				if err := testutil.WaitUntilRead(ptyBuf, "foo-/-123", 5*time.Second); err != nil {
 					t.Fatalf("echo didn't execute: %v", err)
 				}
+
+				// Make sure we can open /dev/tty. We do this
+				// by asking `head` to to read 0 bytes, which
+				// causes it to simply open & close the file.
+				if _, err := tc.master.Write([]byte("head -n 0 /dev/tty; echo $?\n")); err != nil {
+					t.Fatalf("master.Write(): %v", err)
+				}
+				if err := testutil.WaitUntilRead(ptyBuf, "0", 5*time.Second); err != nil {
+					t.Fatalf("head didn't execute: %v", err)
+				}
 			}
 		})
 	}