selftests: add devpts selftests

This adds tests to check:
- bind-mounts from /dev/pts/ptmx to /dev/ptmx work
- non-standard mounts of devpts work
- bind-mounts of /dev/pts/ptmx to locations that do not resolve to a valid
  slave pty path under the originating devpts mount fail

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 316 insertions, 1 deletions

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 7442dfb73b7f..dbda89c9d9b9 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -7,6 +7,7 @@ TARGETS += cpufreq
 TARGETS += cpu-hotplug
 TARGETS += efivarfs
 TARGETS += exec
+TARGETS += filesystems
 TARGETS += firmware
 TARGETS += ftrace
 TARGETS += futex
diff --git a/tools/testing/selftests/filesystems/.gitignore b/tools/testing/selftests/filesystems/.gitignore
index 31d6e426b6d4..8449cf6716ce 100644
--- a/tools/testing/selftests/filesystems/.gitignore
+++ b/tools/testing/selftests/filesystems/.gitignore
@@ -1 +1,2 @@
 dnotify_test
+devpts_pts
diff --git a/tools/testing/selftests/filesystems/Makefile b/tools/testing/selftests/filesystems/Makefile
index 13a73bf725b5..4e6d09fb166f 100644
--- a/tools/testing/selftests/filesystems/Makefile
+++ b/tools/testing/selftests/filesystems/Makefile
@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0
-TEST_PROGS := dnotify_test
+TEST_PROGS := dnotify_test devpts_pts
 all: $(TEST_PROGS)
 
 include ../lib.mk
diff --git a/tools/testing/selftests/filesystems/devpts_pts.c b/tools/testing/selftests/filesystems/devpts_pts.c
new file mode 100644
index 000000000000..b9055e974289
--- /dev/null
+++ b/tools/testing/selftests/filesystems/devpts_pts.c
@@ -0,0 +1,313 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <sched.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/ioctl.h>
+#include <sys/mount.h>
+#include <sys/wait.h>
+
+static bool terminal_dup2(int duplicate, int original)
+{
+        int ret;
+
+        ret = dup2(duplicate, original);
+        if (ret < 0)
+                return false;
+
+        return true;
+}
+
+static int terminal_set_stdfds(int fd)
+{
+        int i;
+
+        if (fd < 0)
+                return 0;
+
+        for (i = 0; i < 3; i++)
+                if (!terminal_dup2(fd, (int[]){STDIN_FILENO, STDOUT_FILENO,
+                                               STDERR_FILENO}[i]))
+                        return -1;
+
+        return 0;
+}
+
+static int login_pty(int fd)
+{
+        int ret;
+
+        setsid();
+
+        ret = ioctl(fd, TIOCSCTTY, NULL);
+        if (ret < 0)
+                return -1;
+
+        ret = terminal_set_stdfds(fd);
+        if (ret < 0)
+                return -1;
+
+        if (fd > STDERR_FILENO)
+                close(fd);
+
+        return 0;
+}
+
+static int wait_for_pid(pid_t pid)
+{
+        int status, ret;
+
+again:
+        ret = waitpid(pid, &status, 0);
+        if (ret == -1) {
+                if (errno == EINTR)
+                        goto again;
+                return -1;
+        }
+        if (ret != pid)
+                goto again;
+
+        if (!WIFEXITED(status) || WEXITSTATUS(status) != 0)
+                return -1;
+
+        return 0;
+}
+
+static int resolve_procfd_symlink(int fd, char *buf, size_t buflen)
+{
+        int ret;
+        char procfd[4096];
+
+        ret = snprintf(procfd, 4096, "/proc/self/fd/%d", fd);
+        if (ret < 0 || ret >= 4096)
+                return -1;
+
+        ret = readlink(procfd, buf, buflen);
+        if (ret < 0 || (size_t)ret >= buflen)
+                return -1;
+
+        buf[ret] = '\0';
+
+        return 0;
+}
+
+static int do_tiocgptpeer(char *ptmx, char *expected_procfd_contents)
+{
+        int ret;
+        int master = -1, slave = -1, fret = -1;
+
+        master = open(ptmx, O_RDWR | O_NOCTTY | O_CLOEXEC);
+        if (master < 0) {
+                fprintf(stderr, "Failed to open \"%s\": %s\n", ptmx,
+                        strerror(errno));
+                return -1;
+        }
+
+        /*
+         * grantpt() makes assumptions about /dev/pts/ so ignore it. It's also
+         * not really needed.
+         */
+        ret = unlockpt(master);
+        if (ret < 0) {
+                fprintf(stderr, "Failed to unlock terminal\n");
+                goto do_cleanup;
+        }
+
+#ifdef TIOCGPTPEER
+        slave = ioctl(master, TIOCGPTPEER, O_RDWR | O_NOCTTY | O_CLOEXEC);
+#endif
+        if (slave < 0) {
+                if (errno == EINVAL) {
+                        fprintf(stderr, "TIOCGPTPEER is not supported. "
+                                        "Skipping test.\n");
+                        fret = EXIT_SUCCESS;
+                }
+
+                fprintf(stderr, "Failed to perform TIOCGPTPEER ioctl\n");
+                goto do_cleanup;
+        }
+
+        pid_t pid = fork();
+        if (pid < 0)
+                goto do_cleanup;
+
+        if (pid == 0) {
+                char buf[4096];
+
+                ret = login_pty(slave);
+                if (ret < 0) {
+                        fprintf(stderr, "Failed to setup terminal\n");
+                        _exit(EXIT_FAILURE);
+                }
+
+                ret = resolve_procfd_symlink(STDIN_FILENO, buf, sizeof(buf));
+                if (ret < 0) {
+                        fprintf(stderr, "Failed to retrieve pathname of pts "
+                                        "slave file descriptor\n");
+                        _exit(EXIT_FAILURE);
+                }
+
+                if (strncmp(expected_procfd_contents, buf,
+                            strlen(expected_procfd_contents)) != 0) {
+                        fprintf(stderr, "Received invalid contents for "
+                                        "\"/proc/<pid>/fd/%d\" symlink: %s\n",
+                                        STDIN_FILENO, buf);
+                        _exit(-1);
+                }
+
+                fprintf(stderr, "Contents of \"/proc/<pid>/fd/%d\" "
+                                "symlink are valid: %s\n", STDIN_FILENO, buf);
+
+                _exit(EXIT_SUCCESS);
+        }
+
+        ret = wait_for_pid(pid);
+        if (ret < 0)
+                goto do_cleanup;
+
+        fret = EXIT_SUCCESS;
+
+do_cleanup:
+        if (master >= 0)
+                close(master);
+        if (slave >= 0)
+                close(slave);
+
+        return fret;
+}
+
+static int verify_non_standard_devpts_mount(void)
+{
+        char *mntpoint;
+        int ret = -1;
+        char devpts[] = P_tmpdir "/devpts_fs_XXXXXX";
+        char ptmx[] = P_tmpdir "/devpts_fs_XXXXXX/ptmx";
+
+        ret = umount("/dev/pts");
+        if (ret < 0) {
+                fprintf(stderr, "Failed to unmount \"/dev/pts\": %s\n",
+                                strerror(errno));
+                return -1;
+        }
+
+        (void)umount("/dev/ptmx");
+
+        mntpoint = mkdtemp(devpts);
+        if (!mntpoint) {
+                fprintf(stderr, "Failed to create temporary mountpoint: %s\n",
+                                 strerror(errno));
+                return -1;
+        }
+
+        ret = mount("devpts", mntpoint, "devpts", MS_NOSUID | MS_NOEXEC,
+                    "newinstance,ptmxmode=0666,mode=0620,gid=5");
+        if (ret < 0) {
+                fprintf(stderr, "Failed to mount devpts fs to \"%s\" in new "
+                                "mount namespace: %s\n", mntpoint,
+                                strerror(errno));
+                unlink(mntpoint);
+                return -1;
+        }
+
+        ret = snprintf(ptmx, sizeof(ptmx), "%s/ptmx", devpts);
+        if (ret < 0 || (size_t)ret >= sizeof(ptmx)) {
+                unlink(mntpoint);
+                return -1;
+        }
+
+        ret = do_tiocgptpeer(ptmx, mntpoint);
+        unlink(mntpoint);
+        if (ret < 0)
+                return -1;
+
+        return 0;
+}
+
+static int verify_ptmx_bind_mount(void)
+{
+        int ret;
+
+        ret = mount("/dev/pts/ptmx", "/dev/ptmx", NULL, MS_BIND, NULL);
+        if (ret < 0) {
+                fprintf(stderr, "Failed to bind mount \"/dev/pts/ptmx\" to "
+                                "\"/dev/ptmx\" mount namespace\n");
+                return -1;
+        }
+
+        ret = do_tiocgptpeer("/dev/ptmx", "/dev/pts/");
+        if (ret < 0)
+                return -1;
+
+        return 0;
+}
+
+static int verify_invalid_ptmx_bind_mount(void)
+{
+        int ret;
+        char mntpoint_fd;
+        char ptmx[] = P_tmpdir "/devpts_ptmx_XXXXXX";
+
+        mntpoint_fd = mkstemp(ptmx);
+        if (mntpoint_fd < 0) {
+                fprintf(stderr, "Failed to create temporary directory: %s\n",
+                                 strerror(errno));
+                return -1;
+        }
+
+        ret = mount("/dev/pts/ptmx", ptmx, NULL, MS_BIND, NULL);
+        close(mntpoint_fd);
+        if (ret < 0) {
+                fprintf(stderr, "Failed to bind mount \"/dev/pts/ptmx\" to "
+                                "\"%s\" mount namespace\n", ptmx);
+                return -1;
+        }
+
+        ret = do_tiocgptpeer(ptmx, "/dev/pts/");
+        if (ret == 0)
+                return -1;
+
+        return 0;
+}
+
+int main(int argc, char *argv[])
+{
+        int ret;
+
+        if (!isatty(STDIN_FILENO)) {
+                fprintf(stderr, "Standard input file desciptor is not attached "
+                                "to a terminal. Skipping test\n");
+                exit(EXIT_FAILURE);
+        }
+
+        ret = unshare(CLONE_NEWNS);
+        if (ret < 0) {
+                fprintf(stderr, "Failed to unshare mount namespace\n");
+                exit(EXIT_FAILURE);
+        }
+
+        ret = mount("", "/", NULL, MS_PRIVATE | MS_REC, 0);
+        if (ret < 0) {
+                fprintf(stderr, "Failed to make \"/\" MS_PRIVATE in new mount "
+                                "namespace\n");
+                exit(EXIT_FAILURE);
+        }
+
+        ret = verify_ptmx_bind_mount();
+        if (ret < 0)
+                exit(EXIT_FAILURE);
+
+        ret = verify_invalid_ptmx_bind_mount();
+        if (ret < 0)
+                exit(EXIT_FAILURE);
+
+        ret = verify_non_standard_devpts_mount();
+        if (ret < 0)
+                exit(EXIT_FAILURE);
+
+        exit(EXIT_SUCCESS);
+}