Job control: controlling TTYs and foreground process groups.

Adresses a deadlock with the rolled back change:
https://github.com/google/gvisor/commit/b6a5b950d28e0b474fdad160b88bc15314cf9259
Creating a session from an orphaned process group was causing a lock to be
acquired twice by a single goroutine. This behavior is addressed, and a test
(OrphanRegression) has been added to pty.cc.

Implemented the following ioctls:
- TIOCSCTTY - set controlling TTY
- TIOCNOTTY - remove controlling tty, maybe signal some other processes
- TIOCGPGRP - get foreground process group. Also enables tcgetpgrp().
- TIOCSPGRP - set foreground process group. Also enabled tcsetpgrp().

Next steps are to actually turn terminal-generated control characters (e.g. C^c)
into signals to the proper process groups, and to send SIGTTOU and SIGTTIN when
appropriate.

PiperOrigin-RevId: 270088599

7 files changed, 551 insertions, 19 deletions

diff --git a/test/syscalls/BUILD b/test/syscalls/BUILD
index 0135435ea..63e4c63dd 100644
--- a/test/syscalls/BUILD
+++ b/test/syscalls/BUILD
@@ -321,6 +321,10 @@ syscall_test(
 )
 
 syscall_test(
+    test = "//test/syscalls/linux:pty_root_test",
+)
+
+syscall_test(
     add_overlay = True,
     test = "//test/syscalls/linux:pwritev2_test",
 )
diff --git a/test/syscalls/linux/BUILD b/test/syscalls/linux/BUILD
index 56fe7be37..a4cebf46f 100644
--- a/test/syscalls/linux/BUILD
+++ b/test/syscalls/linux/BUILD
@@ -1287,8 +1287,10 @@ cc_binary(
     srcs = ["pty.cc"],
     linkstatic = 1,
     deps = [
+        "//test/util:capability_util",
         "//test/util:file_descriptor",
         "//test/util:posix_error",
+        "//test/util:pty_util",
         "//test/util:test_main",
         "//test/util:test_util",
         "//test/util:thread_util",
@@ -1301,6 +1303,23 @@ cc_binary(
 )
 
 cc_binary(
+    name = "pty_root_test",
+    testonly = 1,
+    srcs = ["pty_root.cc"],
+    linkstatic = 1,
+    deps = [
+        "//test/util:capability_util",
+        "//test/util:file_descriptor",
+        "//test/util:posix_error",
+        "//test/util:pty_util",
+        "//test/util:test_main",
+        "//test/util:thread_util",
+        "@com_google_absl//absl/base:core_headers",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_binary(
     name = "partial_bad_buffer_test",
     testonly = 1,
     srcs = ["partial_bad_buffer.cc"],
diff --git a/test/syscalls/linux/pty.cc b/test/syscalls/linux/pty.cc
index d1ab4703f..286388316 100644
--- a/test/syscalls/linux/pty.cc
+++ b/test/syscalls/linux/pty.cc
@@ -13,13 +13,17 @@
 // limitations under the License.
 
 #include <fcntl.h>
+#include <linux/capability.h>
 #include <linux/major.h>
 #include <poll.h>
+#include <sched.h>
+#include <signal.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
 #include <sys/sysmacros.h>
 #include <sys/types.h>
+#include <sys/wait.h>
 #include <termios.h>
 #include <unistd.h>
 
@@ -31,8 +35,10 @@
 #include "absl/synchronization/notification.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
+#include "test/util/capability_util.h"
 #include "test/util/file_descriptor.h"
 #include "test/util/posix_error.h"
+#include "test/util/pty_util.h"
 #include "test/util/test_util.h"
 #include "test/util/thread_util.h"
 
@@ -370,25 +376,6 @@ PosixErrorOr<size_t> PollAndReadFd(int fd, void* buf, size_t count,
   return PosixError(ETIMEDOUT, "Poll timed out");
 }
 
-// Opens the slave end of the passed master as R/W and nonblocking.
-PosixErrorOr<FileDescriptor> OpenSlave(const FileDescriptor& master) {
-  // Get pty index.
-  int n;
-  int ret = ioctl(master.get(), TIOCGPTN, &n);
-  if (ret < 0) {
-    return PosixError(errno, "ioctl(TIOCGPTN) failed");
-  }
-
-  // Unlock pts.
-  int unlock = 0;
-  ret = ioctl(master.get(), TIOCSPTLCK, &unlock);
-  if (ret < 0) {
-    return PosixError(errno, "ioctl(TIOSPTLCK) failed");
-  }
-
-  return Open(absl::StrCat("/dev/pts/", n), O_RDWR | O_NONBLOCK);
-}
-
 TEST(BasicPtyTest, StatUnopenedMaster) {
   struct stat s;
   ASSERT_THAT(stat("/dev/ptmx", &s), SyscallSucceeds());
@@ -1233,6 +1220,374 @@ TEST_F(PtyTest, SetMasterWindowSize) {
   EXPECT_EQ(retrieved_ws.ws_col, kCols);
 }
 
+class JobControlTest : public ::testing::Test {
+ protected:
+  void SetUp() override {
+    master_ = ASSERT_NO_ERRNO_AND_VALUE(Open("/dev/ptmx", O_RDWR | O_NONBLOCK));
+    slave_ = ASSERT_NO_ERRNO_AND_VALUE(OpenSlave(master_));
+
+    // Make this a session leader, which also drops the controlling terminal.
+    // In the gVisor test environment, this test will be run as the session
+    // leader already (as the sentry init process).
+    if (!IsRunningOnGvisor()) {
+      ASSERT_THAT(setsid(), SyscallSucceeds());
+    }
+  }
+
+  // Master and slave ends of the PTY. Non-blocking.
+  FileDescriptor master_;
+  FileDescriptor slave_;
+};
+
+TEST_F(JobControlTest, SetTTYMaster) {
+  ASSERT_THAT(ioctl(master_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+}
+
+TEST_F(JobControlTest, SetTTY) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+}
+
+TEST_F(JobControlTest, SetTTYNonLeader) {
+  // Fork a process that won't be the session leader.
+  pid_t child = fork();
+  if (!child) {
+    // We shouldn't be able to set the terminal.
+    TEST_PCHECK(ioctl(slave_.get(), TIOCSCTTY, 0));
+    _exit(0);
+  }
+
+  int wstatus;
+  ASSERT_THAT(waitpid(child, &wstatus, 0), SyscallSucceedsWithValue(child));
+  ASSERT_EQ(wstatus, 0);
+}
+
+TEST_F(JobControlTest, SetTTYBadArg) {
+  // Despite the man page saying arg should be 0 here, Linux doesn't actually
+  // check.
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 1), SyscallSucceeds());
+}
+
+TEST_F(JobControlTest, SetTTYDifferentSession) {
+  SKIP_IF(ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));
+
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  // Fork, join a new session, and try to steal the parent's controlling
+  // terminal, which should fail.
+  pid_t child = fork();
+  if (!child) {
+    TEST_PCHECK(setsid() >= 0);
+    // We shouldn't be able to steal the terminal.
+    TEST_PCHECK(ioctl(slave_.get(), TIOCSCTTY, 1));
+    _exit(0);
+  }
+
+  int wstatus;
+  ASSERT_THAT(waitpid(child, &wstatus, 0), SyscallSucceedsWithValue(child));
+  ASSERT_EQ(wstatus, 0);
+}
+
+TEST_F(JobControlTest, ReleaseTTY) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  // Make sure we're ignoring SIGHUP, which will be sent to this process once we
+  // disconnect they TTY.
+  struct sigaction sa = {
+      .sa_handler = SIG_IGN,
+      .sa_flags = 0,
+  };
+  sigemptyset(&sa.sa_mask);
+  struct sigaction old_sa;
+  EXPECT_THAT(sigaction(SIGHUP, &sa, &old_sa), SyscallSucceeds());
+  EXPECT_THAT(ioctl(slave_.get(), TIOCNOTTY), SyscallSucceeds());
+  EXPECT_THAT(sigaction(SIGHUP, &old_sa, NULL), SyscallSucceeds());
+}
+
+TEST_F(JobControlTest, ReleaseUnsetTTY) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCNOTTY), SyscallFailsWithErrno(ENOTTY));
+}
+
+TEST_F(JobControlTest, ReleaseWrongTTY) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  ASSERT_THAT(ioctl(master_.get(), TIOCNOTTY), SyscallFailsWithErrno(ENOTTY));
+}
+
+TEST_F(JobControlTest, ReleaseTTYNonLeader) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  pid_t child = fork();
+  if (!child) {
+    TEST_PCHECK(!ioctl(slave_.get(), TIOCNOTTY));
+    _exit(0);
+  }
+
+  int wstatus;
+  ASSERT_THAT(waitpid(child, &wstatus, 0), SyscallSucceedsWithValue(child));
+  ASSERT_EQ(wstatus, 0);
+}
+
+TEST_F(JobControlTest, ReleaseTTYDifferentSession) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  pid_t child = fork();
+  if (!child) {
+    // Join a new session, then try to disconnect.
+    TEST_PCHECK(setsid() >= 0);
+    TEST_PCHECK(ioctl(slave_.get(), TIOCNOTTY));
+    _exit(0);
+  }
+
+  int wstatus;
+  ASSERT_THAT(waitpid(child, &wstatus, 0), SyscallSucceedsWithValue(child));
+  ASSERT_EQ(wstatus, 0);
+}
+
+// Used by the child process spawned in ReleaseTTYSignals to track received
+// signals.
+static int received;
+
+void sig_handler(int signum) { received |= signum; }
+
+// When the session leader releases its controlling terminal, the foreground
+// process group gets SIGHUP, then SIGCONT. This test:
+// - Spawns 2 threads
+// - Has thread 1 return 0 if it gets both SIGHUP and SIGCONT
+// - Has thread 2 leave the foreground process group, and return non-zero if it
+//   receives any signals.
+// - Has the parent thread release its controlling terminal
+// - Checks that thread 1 got both signals
+// - Checks that thread 2 didn't get any signals.
+TEST_F(JobControlTest, ReleaseTTYSignals) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  received = 0;
+  struct sigaction sa = {
+      .sa_handler = sig_handler,
+      .sa_flags = 0,
+  };
+  sigemptyset(&sa.sa_mask);
+  sigaddset(&sa.sa_mask, SIGHUP);
+  sigaddset(&sa.sa_mask, SIGCONT);
+  sigprocmask(SIG_BLOCK, &sa.sa_mask, NULL);
+
+  pid_t same_pgrp_child = fork();
+  if (!same_pgrp_child) {
+    // The child will wait for SIGHUP and SIGCONT, then return 0. It begins with
+    // SIGHUP and SIGCONT blocked. We install signal handlers for those signals,
+    // then use sigsuspend to wait for those specific signals.
+    TEST_PCHECK(!sigaction(SIGHUP, &sa, NULL));
+    TEST_PCHECK(!sigaction(SIGCONT, &sa, NULL));
+    sigset_t mask;
+    sigfillset(&mask);
+    sigdelset(&mask, SIGHUP);
+    sigdelset(&mask, SIGCONT);
+    while (received != (SIGHUP | SIGCONT)) {
+      sigsuspend(&mask);
+    }
+    _exit(0);
+  }
+
+  // We don't want to block these anymore.
+  sigprocmask(SIG_UNBLOCK, &sa.sa_mask, NULL);
+
+  // This child will return non-zero if either SIGHUP or SIGCONT are received.
+  pid_t diff_pgrp_child = fork();
+  if (!diff_pgrp_child) {
+    TEST_PCHECK(!setpgid(0, 0));
+    TEST_PCHECK(pause());
+    _exit(1);
+  }
+
+  EXPECT_THAT(setpgid(diff_pgrp_child, diff_pgrp_child), SyscallSucceeds());
+
+  // Make sure we're ignoring SIGHUP, which will be sent to this process once we
+  // disconnect they TTY.
+  struct sigaction sighup_sa = {
+      .sa_handler = SIG_IGN,
+      .sa_flags = 0,
+  };
+  sigemptyset(&sighup_sa.sa_mask);
+  struct sigaction old_sa;
+  EXPECT_THAT(sigaction(SIGHUP, &sighup_sa, &old_sa), SyscallSucceeds());
+
+  // Release the controlling terminal, sending SIGHUP and SIGCONT to all other
+  // processes in this process group.
+  EXPECT_THAT(ioctl(slave_.get(), TIOCNOTTY), SyscallSucceeds());
+
+  EXPECT_THAT(sigaction(SIGHUP, &old_sa, NULL), SyscallSucceeds());
+
+  // The child in the same process group will get signaled.
+  int wstatus;
+  EXPECT_THAT(waitpid(same_pgrp_child, &wstatus, 0),
+              SyscallSucceedsWithValue(same_pgrp_child));
+  EXPECT_EQ(wstatus, 0);
+
+  // The other child will not get signaled.
+  EXPECT_THAT(waitpid(diff_pgrp_child, &wstatus, WNOHANG),
+              SyscallSucceedsWithValue(0));
+  EXPECT_THAT(kill(diff_pgrp_child, SIGKILL), SyscallSucceeds());
+}
+
+TEST_F(JobControlTest, GetForegroundProcessGroup) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+  pid_t foreground_pgid;
+  pid_t pid;
+  ASSERT_THAT(ioctl(slave_.get(), TIOCGPGRP, &foreground_pgid),
+              SyscallSucceeds());
+  ASSERT_THAT(pid = getpid(), SyscallSucceeds());
+
+  ASSERT_EQ(foreground_pgid, pid);
+}
+
+TEST_F(JobControlTest, GetForegroundProcessGroupNonControlling) {
+  // At this point there's no controlling terminal, so TIOCGPGRP should fail.
+  pid_t foreground_pgid;
+  ASSERT_THAT(ioctl(slave_.get(), TIOCGPGRP, &foreground_pgid),
+              SyscallFailsWithErrno(ENOTTY));
+}
+
+// This test:
+// - sets itself as the foreground process group
+// - creates a child process in a new process group
+// - sets that child as the foreground process group
+// - kills its child and sets itself as the foreground process group.
+TEST_F(JobControlTest, SetForegroundProcessGroup) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  // Ignore SIGTTOU so that we don't stop ourself when calling tcsetpgrp.
+  struct sigaction sa = {
+      .sa_handler = SIG_IGN,
+      .sa_flags = 0,
+  };
+  sigemptyset(&sa.sa_mask);
+  sigaction(SIGTTOU, &sa, NULL);
+
+  // Set ourself as the foreground process group.
+  ASSERT_THAT(tcsetpgrp(slave_.get(), getpgid(0)), SyscallSucceeds());
+
+  // Create a new process that just waits to be signaled.
+  pid_t child = fork();
+  if (!child) {
+    TEST_PCHECK(!pause());
+    // We should never reach this.
+    _exit(1);
+  }
+
+  // Make the child its own process group, then make it the controlling process
+  // group of the terminal.
+  ASSERT_THAT(setpgid(child, child), SyscallSucceeds());
+  ASSERT_THAT(tcsetpgrp(slave_.get(), child), SyscallSucceeds());
+
+  // Sanity check - we're still the controlling session.
+  ASSERT_EQ(getsid(0), getsid(child));
+
+  // Signal the child, wait for it to exit, then retake the terminal.
+  ASSERT_THAT(kill(child, SIGTERM), SyscallSucceeds());
+  int wstatus;
+  ASSERT_THAT(waitpid(child, &wstatus, 0), SyscallSucceedsWithValue(child));
+  ASSERT_TRUE(WIFSIGNALED(wstatus));
+  ASSERT_EQ(WTERMSIG(wstatus), SIGTERM);
+
+  // Set ourself as the foreground process.
+  pid_t pgid;
+  ASSERT_THAT(pgid = getpgid(0), SyscallSucceeds());
+  ASSERT_THAT(tcsetpgrp(slave_.get(), pgid), SyscallSucceeds());
+}
+
+TEST_F(JobControlTest, SetForegroundProcessGroupWrongTTY) {
+  pid_t pid = getpid();
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSPGRP, &pid),
+              SyscallFailsWithErrno(ENOTTY));
+}
+
+TEST_F(JobControlTest, SetForegroundProcessGroupNegPgid) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  pid_t pid = -1;
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSPGRP, &pid),
+              SyscallFailsWithErrno(EINVAL));
+}
+
+TEST_F(JobControlTest, SetForegroundProcessGroupEmptyProcessGroup) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  // Create a new process, put it in a new process group, make that group the
+  // foreground process group, then have the process wait.
+  pid_t child = fork();
+  if (!child) {
+    TEST_PCHECK(!setpgid(0, 0));
+    _exit(0);
+  }
+
+  // Wait for the child to exit.
+  int wstatus;
+  EXPECT_THAT(waitpid(child, &wstatus, 0), SyscallSucceedsWithValue(child));
+  // The child's process group doesn't exist anymore - this should fail.
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSPGRP, &child),
+              SyscallFailsWithErrno(ESRCH));
+}
+
+TEST_F(JobControlTest, SetForegroundProcessGroupDifferentSession) {
+  ASSERT_THAT(ioctl(slave_.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  // Create a new process and put it in a new session.
+  pid_t child = fork();
+  if (!child) {
+    TEST_PCHECK(setsid() >= 0);
+    // Tell the parent we're in a new session.
+    TEST_PCHECK(!raise(SIGSTOP));
+    TEST_PCHECK(!pause());
+    _exit(1);
+  }
+
+  // Wait for the child to tell us it's in a new session.
+  int wstatus;
+  EXPECT_THAT(waitpid(child, &wstatus, WUNTRACED),
+              SyscallSucceedsWithValue(child));
+  EXPECT_TRUE(WSTOPSIG(wstatus));
+
+  // Child is in a new session, so we can't make it the foregroup process group.
+  EXPECT_THAT(ioctl(slave_.get(), TIOCSPGRP, &child),
+              SyscallFailsWithErrno(EPERM));
+
+  EXPECT_THAT(kill(child, SIGKILL), SyscallSucceeds());
+}
+
+// Verify that we don't hang when creating a new session from an orphaned
+// process group (b/139968068). Calling setsid() creates an orphaned process
+// group, as process groups that contain the session's leading process are
+// orphans.
+//
+// We create 2 sessions in this test. The init process in gVisor is considered
+// not to be an orphan (see sessions.go), so we have to create a session from
+// which to create a session. The latter session is being created from an
+// orphaned process group.
+TEST_F(JobControlTest, OrphanRegression) {
+  pid_t session_2_leader = fork();
+  if (!session_2_leader) {
+    TEST_PCHECK(setsid() >= 0);
+
+    pid_t session_3_leader = fork();
+    if (!session_3_leader) {
+      TEST_PCHECK(setsid() >= 0);
+
+      _exit(0);
+    }
+
+    int wstatus;
+    TEST_PCHECK(waitpid(session_3_leader, &wstatus, 0) == session_3_leader);
+    TEST_PCHECK(wstatus == 0);
+
+    _exit(0);
+  }
+
+  int wstatus;
+  ASSERT_THAT(waitpid(session_2_leader, &wstatus, 0),
+              SyscallSucceedsWithValue(session_2_leader));
+  ASSERT_EQ(wstatus, 0);
+}
+
 }  // namespace
 }  // namespace testing
 }  // namespace gvisor
diff --git a/test/syscalls/linux/pty_root.cc b/test/syscalls/linux/pty_root.cc
new file mode 100644
index 000000000..14a4af980
--- /dev/null
+++ b/test/syscalls/linux/pty_root.cc
@@ -0,0 +1,68 @@
+// 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.
+
+#include <sys/ioctl.h>
+#include <termios.h>
+
+#include "gtest/gtest.h"
+#include "absl/base/macros.h"
+#include "test/util/capability_util.h"
+#include "test/util/file_descriptor.h"
+#include "test/util/posix_error.h"
+#include "test/util/pty_util.h"
+
+namespace gvisor {
+namespace testing {
+
+// These tests should be run as root.
+namespace {
+
+TEST(JobControlRootTest, StealTTY) {
+  SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));
+
+  // Make this a session leader, which also drops the controlling terminal.
+  // In the gVisor test environment, this test will be run as the session
+  // leader already (as the sentry init process).
+  if (!IsRunningOnGvisor()) {
+    ASSERT_THAT(setsid(), SyscallSucceeds());
+  }
+
+  FileDescriptor master =
+      ASSERT_NO_ERRNO_AND_VALUE(Open("/dev/ptmx", O_RDWR | O_NONBLOCK));
+  FileDescriptor slave = ASSERT_NO_ERRNO_AND_VALUE(OpenSlave(master));
+
+  // Make slave the controlling terminal.
+  ASSERT_THAT(ioctl(slave.get(), TIOCSCTTY, 0), SyscallSucceeds());
+
+  // Fork, join a new session, and try to steal the parent's controlling
+  // terminal, which should succeed when we have CAP_SYS_ADMIN and pass an arg
+  // of 1.
+  pid_t child = fork();
+  if (!child) {
+    ASSERT_THAT(setsid(), SyscallSucceeds());
+    // We shouldn't be able to steal the terminal with the wrong arg value.
+    TEST_PCHECK(ioctl(slave.get(), TIOCSCTTY, 0));
+    // We should be able to steal it here.
+    TEST_PCHECK(!ioctl(slave.get(), TIOCSCTTY, 1));
+    _exit(0);
+  }
+
+  int wstatus;
+  ASSERT_THAT(waitpid(child, &wstatus, 0), SyscallSucceedsWithValue(child));
+  ASSERT_EQ(wstatus, 0);
+}
+
+}  // namespace
+}  // namespace testing
+}  // namespace gvisor
diff --git a/test/util/BUILD b/test/util/BUILD
index 52f8b9e1f..25ed9c944 100644
--- a/test/util/BUILD
+++ b/test/util/BUILD
@@ -191,6 +191,17 @@ cc_test(
 )
 
 cc_library(
+    name = "pty_util",
+    testonly = 1,
+    srcs = ["pty_util.cc"],
+    hdrs = ["pty_util.h"],
+    deps = [
+        ":file_descriptor",
+        ":posix_error",
+    ],
+)
+
+cc_library(
     name = "signal_util",
     testonly = 1,
     srcs = ["signal_util.cc"],
diff --git a/test/util/pty_util.cc b/test/util/pty_util.cc
new file mode 100644
index 000000000..c0fd9a095
--- /dev/null
+++ b/test/util/pty_util.cc
@@ -0,0 +1,45 @@
+// Copyright 2019 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.
+
+#include "test/util/pty_util.h"
+
+#include <sys/ioctl.h>
+#include <termios.h>
+
+#include "test/util/file_descriptor.h"
+#include "test/util/posix_error.h"
+
+namespace gvisor {
+namespace testing {
+
+PosixErrorOr<FileDescriptor> OpenSlave(const FileDescriptor& master) {
+  // Get pty index.
+  int n;
+  int ret = ioctl(master.get(), TIOCGPTN, &n);
+  if (ret < 0) {
+    return PosixError(errno, "ioctl(TIOCGPTN) failed");
+  }
+
+  // Unlock pts.
+  int unlock = 0;
+  ret = ioctl(master.get(), TIOCSPTLCK, &unlock);
+  if (ret < 0) {
+    return PosixError(errno, "ioctl(TIOSPTLCK) failed");
+  }
+
+  return Open(absl::StrCat("/dev/pts/", n), O_RDWR | O_NONBLOCK);
+}
+
+}  // namespace testing
+}  // namespace gvisor
diff --git a/test/util/pty_util.h b/test/util/pty_util.h
new file mode 100644
index 000000000..367b14f15
--- /dev/null
+++ b/test/util/pty_util.h
@@ -0,0 +1,30 @@
+// Copyright 2019 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.
+
+#ifndef GVISOR_TEST_UTIL_PTY_UTIL_H_
+#define GVISOR_TEST_UTIL_PTY_UTIL_H_
+
+#include "test/util/file_descriptor.h"
+#include "test/util/posix_error.h"
+
+namespace gvisor {
+namespace testing {
+
+// Opens the slave end of the passed master as R/W and nonblocking.
+PosixErrorOr<FileDescriptor> OpenSlave(const FileDescriptor& master);
+
+}  // namespace testing
+}  // namespace gvisor
+
+#endif  // GVISOR_TEST_UTIL_PTY_UTIL_H_