pty: Send SIGHUP to foreground process group when master is closed

When the PTY master file descriptor is closed, Linux sends SIGHUP to
the foreground process group of the session that has this PTY as its
controlling terminal.

gVisor was missing this behavior entirely: closing the master end did
not send any signal, causing child processes spawned via PTY to remain
running indefinitely instead of being notified of the hangup.

Fix by adding a TTY.Hangup() method that reuses the existing
ReleaseControllingTTY() logic to send SIGHUP/SIGCONT and clear the
controlling terminal. Call Hangup() for both masterKTTY and replicaKTTY
in masterClose().

Signed-off-by: Tan Yifeng <yiftan@tencent.com>

Author: tanyifeng

pkg/sentry/fsimpl/devpts/devpts.go

@@ -286,6 +286,12 @@ func (i *rootInode) allocateTerminal(ctx context.Context, creds *auth.Credential
 
 // masterClose is called when the master end of t is closed.
 func (i *rootInode) masterClose(ctx context.Context, t *Terminal) {
+	// When the master is closed, send SIGHUP to the foreground process group
+	// of the session that has this PTY as its controlling terminal.
+	// This corresponds to Linux's pty_close() calling tty_vhangup().
+	t.masterKTTY.Hangup(ctx)
+	t.replicaKTTY.Hangup(ctx)
+
 	i.mu.Lock()
 	defer i.mu.Unlock()
 

pkg/sentry/kernel/tty.go

@@ -158,3 +158,21 @@ func (tty *TTY) CheckChange(ctx context.Context, sig linux.Signal) error {
 	_ = pg.SendSignal(SignalInfoPriv(sig))
 	return linuxerr.ERESTARTSYS
 }
+
+// Hangup releases the controlling terminal and sends SIGHUP/SIGCONT to
+// the foreground process group. This is called when the PTY master is
+// closed and corresponds to Linux's tty_vhangup_session().
+func (tty *TTY) Hangup(ctx context.Context) {
+	tty.mu.Lock()
+	tg := tty.tg
+	tty.mu.Unlock()
+
+	if tg == nil {
+		// This TTY is not a controlling terminal.
+		return
+	}
+
+	// Reuse the existing ReleaseControllingTTY logic which handles
+	// sending SIGHUP/SIGCONT and clearing the controlling terminal.
+	_ = tg.ReleaseControllingTTY(tty)
+}

test/syscalls/linux/pty.cc

@@ -425,6 +425,12 @@ TEST(BasicPtyTest, OpenDevTTY) {
     // which will be opened by /dev/tty.
     setsid();
 
+    // Ignore SIGHUP: when the master fd is closed during cleanup, the
+    // kernel sends SIGHUP to the foreground process group of the
+    // controlling terminal session. Since this child *is* the session leader,
+    // it would be killed by the default SIGHUP disposition before _exit(0).
+    TEST_PCHECK(signal(SIGHUP, SIG_IGN) != SIG_ERR);
+
     FileDescriptor master =
         TEST_CHECK_NO_ERRNO_AND_VALUE(Open("/dev/ptmx", O_RDWR));
 
@@ -2213,6 +2219,47 @@ TEST_F(PtyTest, SignalCharConsumedWhenISIGEnabled) {
   EXPECT_EQ(buf, 'a');
 }
 
+// When the PTY master is closed, SIGHUP should be sent to the foreground
+// process group of the session that has this PTY as its controlling terminal.
+// This matches Linux pty_close() -> tty_vhangup() behavior.
+TEST_F(JobControlTest, SIGHUPOnMasterClose) {
+  pid_t child = fork();
+  if (child == 0) {
+    // Close the inherited master fd so that the parent holds the only
+    // reference. pty_close/tty_vhangup fires only when the last master
+    // fd is closed.
+    close(master_.release());
+
+    // Create new session and set the replica as controlling terminal.
+    TEST_PCHECK(setsid() >= 0);
+    TEST_PCHECK(ioctl(replica_.get(), TIOCSCTTY, 0) >= 0);
+
+    // Install a SIGHUP handler that exits with a known status.
+    struct sigaction sa = {};
+    sa.sa_handler = [](int) { _exit(42); };
+    sigemptyset(&sa.sa_mask);
+    TEST_PCHECK(sigaction(SIGHUP, &sa, nullptr) >= 0);
+
+    // Sleep waiting for the signal. Use a timeout to avoid hanging the test.
+    sleep(10);
+    // If we get here, SIGHUP was not received.
+    _exit(1);
+  }
+  ASSERT_GT(child, 0);
+
+  // Give the child time to set up its session and controlling terminal.
+  absl::SleepFor(absl::Milliseconds(500));
+
+  // Close the master end. This should trigger SIGHUP to the child.
+  master_.reset();
+
+  // Wait for the child and verify it received SIGHUP (exited with 42).
+  int wstatus;
+  ASSERT_THAT(waitpid(child, &wstatus, 0), SyscallSucceedsWithValue(child));
+  ASSERT_TRUE(WIFEXITED(wstatus));
+  EXPECT_EQ(WEXITSTATUS(wstatus), 42);
+}
+
 }  // namespace
 }  // namespace testing
 }  // namespace gvisor