[pty] Return enqueued data and then EIO if the replica is closed

PiperOrigin-RevId: 532496868

pkg/sentry/fsimpl/devpts/line_discipline.go

@@ -106,6 +106,9 @@ type lineDiscipline struct {
 	// handling certain special characters like backspace.
 	column int
 
+	// numReplicas is the number of replica file descriptors.
+	numReplicas int
+
 	// masterWaiter is used to wait on the master end of the TTY.
 	masterWaiter waiter.Queue
 
@@ -269,6 +272,20 @@ func (l *lineDiscipline) outputQueueWrite(ctx context.Context, src usermem.IOSeq
 	return 0, linuxerr.ErrWouldBlock
 }
 
+// replicaOpen is called when a replica file descriptor is opened.
+func (l *lineDiscipline) replicaOpen() {
+	l.termiosMu.Lock()
+	defer l.termiosMu.Unlock()
+	l.numReplicas++
+}
+
+// replicaClose is called when a replica file descriptor is closed.
+func (l *lineDiscipline) replicaClose() {
+	l.termiosMu.Lock()
+	defer l.termiosMu.Unlock()
+	l.numReplicas--
+}
+
 // transformer is a helper interface to make it easier to stateify queue.
 type transformer interface {
 	// transform functions require queue's mutex to be held.

pkg/sentry/fsimpl/devpts/queue.go

@@ -110,6 +110,9 @@ func (q *queue) read(ctx context.Context, dst usermem.IOSequence, l *lineDiscipl
 	defer q.mu.Unlock()
 
 	if !q.readable {
+		if l.numReplicas == 0 {
+			return 0, false, false, linuxerr.EIO
+		}
 		return 0, false, false, linuxerr.ErrWouldBlock
 	}
 

pkg/sentry/fsimpl/devpts/replica.go

@@ -69,6 +69,7 @@ func (ri *replicaInode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kern
 		// ignored silently.
 		_ = t.ThreadGroup().SetControllingTTY(fd.inode.t.replicaKTTY, false /* steal */, fd.vfsfd.IsReadable())
 	}
+	ri.t.ld.replicaOpen()
 	return &fd.vfsfd, nil
 
 }
@@ -114,7 +115,9 @@ type replicaFileDescription struct {
 var _ vfs.FileDescriptionImpl = (*replicaFileDescription)(nil)
 
 // Release implements fs.FileOperations.Release.
-func (rfd *replicaFileDescription) Release(ctx context.Context) {}
+func (rfd *replicaFileDescription) Release(ctx context.Context) {
+	rfd.inode.t.ld.replicaClose()
+}
 
 // EventRegister implements waiter.Waitable.EventRegister.
 func (rfd *replicaFileDescription) EventRegister(e *waiter.Entry) error {

test/syscalls/linux/pty.cc

@@ -650,6 +650,29 @@ TEST_F(PtyTest, WriteReplicaToMaster) {
   EXPECT_EQ(memcmp(buf, kExpected, sizeof(kExpected)), 0);
 }
 
+// Verifies that data enqueued into the replica is still readable by the master
+// after the replica is closed.
+TEST_F(PtyTest, WriteReplicaToMasterReadAfterReplicaClosed) {
+  // N.B. by default, the master reads nothing until the replica writes a
+  // newline, and the master gets a carriage return.
+  constexpr char kInput[] = "hello\n";
+  constexpr char kExpected[] = "hello\r\n";
+
+  EXPECT_THAT(WriteFd(replica_.get(), kInput, sizeof(kInput) - 1),
+              SyscallSucceedsWithValue(sizeof(kInput) - 1));
+
+  // Close the replica.
+  replica_.reset();
+
+  char buf[sizeof(kExpected)] = {};
+  ExpectReadable(master_, sizeof(buf) - 1, buf);
+  EXPECT_EQ(memcmp(buf, kExpected, sizeof(kExpected)), 0);
+
+  // After all data has been read, the master should return EIO.
+  char c;
+  EXPECT_THAT(ReadFd(master_.get(), &c, 1), SyscallFailsWithErrno(EIO));
+}
+
 TEST_F(PtyTest, WriteInvalidUTF8) {
   char c = 0xff;
   ASSERT_THAT(syscall(__NR_write, master_.get(), &c, sizeof(c)),