core: introduce new Type=exec service type

Users are often surprised that "systemd-run" command lines like
"systemd-run -p User=idontexist /bin/true" will return successfully,
even though the logs show that the process couldn't be invoked, as the
user "idontexist" doesn't exist. This is because Type=simple will only
wait until fork() succeeded before returning start-up success.

This patch adds a new service type Type=exec, which is very similar to
Type=simple, but waits until the child process completed the execve()
before returning success. It uses a pipe that has O_CLOEXEC set for this
logic, so that the kernel automatically sends POLLHUP on it when the
execve() succeeded but leaves the pipe open if not. This means PID 1
waits exactly until the execve() succeeded in the child, and not longer
and not shorter, which is the desired functionality.

Making use of this new functionality, the command line
"systemd-run -p User=idontexist -p Type=exec /bin/true" will now fail,
as expected.

7 files changed, 254 insertions, 28 deletions

diff --git a/src/core/execute.c b/src/core/execute.c
index 8c5139dc02..0f36f6c825 100644
--- a/src/core/execute.c
+++ b/src/core/execute.c
@@ -2573,6 +2573,7 @@ static int close_remaining_fds(
                 const DynamicCreds *dcreds,
                 int user_lookup_fd,
                 int socket_fd,
+                int exec_fd,
                 int *fds, size_t n_fds) {
 
         size_t n_dont_close = 0;
@@ -2589,6 +2590,8 @@ static int close_remaining_fds(
 
         if (socket_fd >= 0)
                 dont_close[n_dont_close++] = socket_fd;
+        if (exec_fd >= 0)
+                dont_close[n_dont_close++] = exec_fd;
         if (n_fds > 0) {
                 memcpy(dont_close + n_dont_close, fds, sizeof(int) * n_fds);
                 n_dont_close += n_fds;
@@ -2732,9 +2735,10 @@ static int exec_child(
                 int *exit_status) {
 
         _cleanup_strv_free_ char **our_env = NULL, **pass_env = NULL, **accum_env = NULL, **final_argv = NULL;
-        _cleanup_free_ char *home_buffer = NULL;
+        int *fds_with_exec_fd, n_fds_with_exec_fd, r, ngids = 0, exec_fd = -1;
         _cleanup_free_ gid_t *supplementary_gids = NULL;
         const char *username = NULL, *groupname = NULL;
+        _cleanup_free_ char *home_buffer = NULL;
         const char *home = NULL, *shell = NULL;
         dev_t journal_stream_dev = 0;
         ino_t journal_stream_ino = 0;
@@ -2754,7 +2758,6 @@ static int exec_child(
 #endif
         uid_t uid = UID_INVALID;
         gid_t gid = GID_INVALID;
-        int r, ngids = 0;
         size_t n_fds;
         ExecDirectoryType dt;
         int secure_bits;
@@ -2798,8 +2801,8 @@ static int exec_child(
         /* In case anything used libc syslog(), close this here, too */
         closelog();
 
-        n_fds = n_storage_fds + n_socket_fds;
-        r = close_remaining_fds(params, runtime, dcreds, user_lookup_fd, socket_fd, fds, n_fds);
+        n_fds = n_socket_fds + n_storage_fds;
+        r = close_remaining_fds(params, runtime, dcreds, user_lookup_fd, socket_fd, params->exec_fd, fds, n_fds);
         if (r < 0) {
                 *exit_status = EXIT_FDS;
                 return log_unit_error_errno(unit, r, "Failed to close unwanted file descriptors: %m");
@@ -3172,9 +3175,45 @@ static int exec_child(
         }
 
         /* We repeat the fd closing here, to make sure that nothing is leaked from the PAM modules. Note that we are
-         * more aggressive this time since socket_fd and the netns fds we don't need anymore. The custom endpoint fd
-         * was needed to upload the policy and can now be closed as well. */
-        r = close_all_fds(fds, n_fds);
+         * more aggressive this time since socket_fd and the netns fds we don't need anymore. We do keep the exec_fd
+         * however if we have it as we want to keep it open until the final execve(). */
+
+        if (params->exec_fd >= 0) {
+                exec_fd = params->exec_fd;
+
+                if (exec_fd < 3 + (int) n_fds) {
+                        int moved_fd;
+
+                        /* Let's move the exec fd far up, so that it's outside of the fd range we want to pass to the
+                         * process we are about to execute. */
+
+                        moved_fd = fcntl(exec_fd, F_DUPFD_CLOEXEC, 3 + (int) n_fds);
+                        if (moved_fd < 0) {
+                                *exit_status = EXIT_FDS;
+                                return log_unit_error_errno(unit, errno, "Couldn't move exec fd up: %m");
+                        }
+
+                        safe_close(exec_fd);
+                        exec_fd = moved_fd;
+                } else {
+                        /* This fd should be FD_CLOEXEC already, but let's make sure. */
+                        r = fd_cloexec(exec_fd, true);
+                        if (r < 0) {
+                                *exit_status = EXIT_FDS;
+                                return log_unit_error_errno(unit, r, "Failed to make exec fd FD_CLOEXEC: %m");
+                        }
+                }
+
+                fds_with_exec_fd = newa(int, n_fds + 1);
+                memcpy(fds_with_exec_fd, fds, n_fds * sizeof(int));
+                fds_with_exec_fd[n_fds] = exec_fd;
+                n_fds_with_exec_fd = n_fds + 1;
+        } else {
+                fds_with_exec_fd = fds;
+                n_fds_with_exec_fd = n_fds;
+        }
+
+        r = close_all_fds(fds_with_exec_fd, n_fds_with_exec_fd);
         if (r >= 0)
                 r = shift_fds(fds, n_fds);
         if (r >= 0)
@@ -3184,6 +3223,11 @@ static int exec_child(
                 return log_unit_error_errno(unit, r, "Failed to adjust passed file descriptors: %m");
         }
 
+        /* At this point, the fds we want to pass to the program are all ready and set up, with O_CLOEXEC turned off
+         * and at the right fd numbers. The are no other fds open, with one exception: the exec_fd if it is defined,
+         * and it has O_CLOEXEC set, after all we want it to be closed by the execve(), so that our parent knows we
+         * came this far. */
+
         secure_bits = context->secure_bits;
 
         if (needs_sandboxing) {
@@ -3414,10 +3458,35 @@ static int exec_child(
                                    LOG_UNIT_INVOCATION_ID(unit));
         }
 
+        if (exec_fd >= 0) {
+                uint8_t hot = 1;
+
+                /* We have finished with all our initializations. Let's now let the manager know that. From this point
+                 * on, if the manager sees POLLHUP on the exec_fd, then execve() was successful. */
+
+                if (write(exec_fd, &hot, sizeof(hot)) < 0) {
+                        *exit_status = EXIT_EXEC;
+                        return log_unit_error_errno(unit, errno, "Failed to enable exec_fd: %m");
+                }
+        }
+
         execve(command->path, final_argv, accum_env);
+        r = -errno;
+
+        if (exec_fd >= 0) {
+                uint8_t hot = 0;
+
+                /* The execve() failed. This means the exec_fd is still open. Which means we need to tell the manager
+                 * that POLLHUP on it no longer means execve() succeeded. */
+
+                if (write(exec_fd, &hot, sizeof(hot)) < 0) {
+                        *exit_status = EXIT_EXEC;
+                        return log_unit_error_errno(unit, errno, "Failed to disable exec_fd: %m");
+                }
+        }
 
-        if (errno == ENOENT && (command->flags & EXEC_COMMAND_IGNORE_FAILURE)) {
-                log_struct_errno(LOG_INFO, errno,
+        if (r == -ENOENT && (command->flags & EXEC_COMMAND_IGNORE_FAILURE)) {
+                log_struct_errno(LOG_INFO, r,
                                  "MESSAGE_ID=" SD_MESSAGE_SPAWN_FAILED_STR,
                                  LOG_UNIT_ID(unit),
                                  LOG_UNIT_INVOCATION_ID(unit),
@@ -3428,7 +3497,7 @@ static int exec_child(
         }
 
         *exit_status = EXIT_EXEC;
-        return log_unit_error_errno(unit, errno, "Failed to execute command: %m");
+        return log_unit_error_errno(unit, r, "Failed to execute command: %m");
 }
 
 static int exec_context_load_environment(const Unit *unit, const ExecContext *c, char ***l);
diff --git a/src/core/execute.h b/src/core/execute.h
index aa6f1ccfda..cd5165c2d1 100644
--- a/src/core/execute.h
+++ b/src/core/execute.h
@@ -317,6 +317,9 @@ struct ExecParameters {
         int stdin_fd;
         int stdout_fd;
         int stderr_fd;
+
+        /* An fd that is closed by the execve(), and thus will result in EOF when the execve() is done */
+        int exec_fd;
 };
 
 #include "unit.h"
diff --git a/src/core/mount.c b/src/core/mount.c
index 21437dad08..16229d4af1 100644
--- a/src/core/mount.c
+++ b/src/core/mount.c
@@ -747,10 +747,11 @@ static void mount_dump(Unit *u, FILE *f, const char *prefix) {
 static int mount_spawn(Mount *m, ExecCommand *c, pid_t *_pid) {
 
         ExecParameters exec_params = {
-                .flags      = EXEC_APPLY_SANDBOXING|EXEC_APPLY_CHROOT|EXEC_APPLY_TTY_STDIN,
-                .stdin_fd   = -1,
-                .stdout_fd  = -1,
-                .stderr_fd  = -1,
+                .flags     = EXEC_APPLY_SANDBOXING|EXEC_APPLY_CHROOT|EXEC_APPLY_TTY_STDIN,
+                .stdin_fd  = -1,
+                .stdout_fd = -1,
+                .stderr_fd = -1,
+                .exec_fd   = -1,
         };
         pid_t pid;
         int r;
diff --git a/src/core/service.c b/src/core/service.c
index fd6450328e..6abf87cb39 100644
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -79,9 +79,10 @@ static const UnitActiveState state_translation_table_idle[_SERVICE_STATE_MAX] =
         [SERVICE_AUTO_RESTART] = UNIT_ACTIVATING
 };
 
-static int service_dispatch_io(sd_event_source *source, int fd, uint32_t events, void *userdata);
+static int service_dispatch_inotify_io(sd_event_source *source, int fd, uint32_t events, void *userdata);
 static int service_dispatch_timer(sd_event_source *source, usec_t usec, void *userdata);
 static int service_dispatch_watchdog(sd_event_source *source, usec_t usec, void *userdata);
+static int service_dispatch_exec_io(sd_event_source *source, int fd, uint32_t events, void *userdata);
 
 static void service_enter_signal(Service *s, ServiceState state, ServiceResult f);
 static void service_enter_reload_by_notify(Service *s);
@@ -389,6 +390,7 @@ static void service_done(Unit *u) {
         service_stop_watchdog(s);
 
         s->timer_event_source = sd_event_source_unref(s->timer_event_source);
+        s->exec_fd_event_source = sd_event_source_unref(s->exec_fd_event_source);
 
         service_release_resources(u);
 }
@@ -1066,6 +1068,9 @@ static void service_set_state(Service *s, ServiceState state) {
             !(state == SERVICE_DEAD && UNIT(s)->job))
                 service_close_socket_fd(s);
 
+        if (state != SERVICE_START)
+                s->exec_fd_event_source = sd_event_source_unref(s->exec_fd_event_source);
+
         if (!IN_SET(state, SERVICE_START_POST, SERVICE_RUNNING, SERVICE_RELOAD))
                 service_stop_watchdog(s);
 
@@ -1296,6 +1301,63 @@ static int service_collect_fds(
         return 0;
 }
 
+static int service_allocate_exec_fd_event_source(
+                Service *s,
+                int fd,
+                sd_event_source **ret_event_source) {
+
+        _cleanup_(sd_event_source_unrefp) sd_event_source *source = NULL;
+        int r;
+
+        assert(s);
+        assert(fd >= 0);
+        assert(ret_event_source);
+
+        r = sd_event_add_io(UNIT(s)->manager->event, &source, fd, 0, service_dispatch_exec_io, s);
+        if (r < 0)
+                return log_unit_error_errno(UNIT(s), r, "Failed to allocate exec_fd event source: %m");
+
+        /* This is a bit lower priority than SIGCHLD, as that carries a lot more interesting failure information */
+
+        r = sd_event_source_set_priority(source, SD_EVENT_PRIORITY_NORMAL-3);
+        if (r < 0)
+                return log_unit_error_errno(UNIT(s), r, "Failed to adjust priority of exec_fd event source: %m");
+
+        (void) sd_event_source_set_description(source, "service event_fd");
+
+        r = sd_event_source_set_io_fd_own(source, true);
+        if (r < 0)
+                return log_unit_error_errno(UNIT(s), r, "Failed to pass ownership of fd to event source: %m");
+
+        *ret_event_source = TAKE_PTR(source);
+        return 0;
+}
+
+static int service_allocate_exec_fd(
+                Service *s,
+                sd_event_source **ret_event_source,
+                int* ret_exec_fd) {
+
+        _cleanup_close_pair_ int p[2] = { -1, -1 };
+        int r;
+
+        assert(s);
+        assert(ret_event_source);
+        assert(ret_exec_fd);
+
+        if (pipe2(p, O_CLOEXEC|O_NONBLOCK) < 0)
+                return log_unit_error_errno(UNIT(s), errno, "Failed to allocate exec_fd pipe: %m");
+
+        r = service_allocate_exec_fd_event_source(s, p[0], ret_event_source);
+        if (r < 0)
+                return r;
+
+        p[0] = -1;
+        *ret_exec_fd = TAKE_FD(p[1]);
+
+        return 0;
+}
+
 static bool service_exec_needs_notify_socket(Service *s, ExecFlags flags) {
         assert(s);
 
@@ -1330,7 +1392,9 @@ static int service_spawn(
                 .exec_fd    = -1,
         };
         _cleanup_strv_free_ char **final_env = NULL, **our_env = NULL, **fd_names = NULL;
+        _cleanup_(sd_event_source_unrefp) sd_event_source *exec_fd_source = NULL;
         size_t n_socket_fds = 0, n_storage_fds = 0, n_env = 0;
+        _cleanup_close_ int exec_fd = -1;
         _cleanup_free_ int *fds = NULL;
         pid_t pid;
         int r;
@@ -1363,6 +1427,14 @@ static int service_spawn(
                 log_unit_debug(UNIT(s), "Passing %zu fds to service", n_socket_fds + n_storage_fds);
         }
 
+        if (!FLAGS_SET(flags, EXEC_IS_CONTROL) && s->type == SERVICE_EXEC) {
+                assert(!s->exec_fd_event_source);
+
+                r = service_allocate_exec_fd(s, &exec_fd_source, &exec_fd);
+                if (r < 0)
+                        return r;
+        }
+
         r = service_arm_timer(s, usec_add(now(CLOCK_MONOTONIC), timeout));
         if (r < 0)
                 return r;
@@ -1462,6 +1534,7 @@ static int service_spawn(
         exec_params.stdin_fd = s->stdin_fd;
         exec_params.stdout_fd = s->stdout_fd;
         exec_params.stderr_fd = s->stderr_fd;
+        exec_params.exec_fd = exec_fd;
 
         r = exec_spawn(UNIT(s),
                        c,
@@ -1473,6 +1546,9 @@ static int service_spawn(
         if (r < 0)
                 return r;
 
+        s->exec_fd_event_source = TAKE_PTR(exec_fd_source);
+        s->exec_fd_hot = false;
+
         r = unit_watch_pid(UNIT(s), pid);
         if (r < 0) /* FIXME: we need to do something here */
                 return r;
@@ -1984,14 +2060,12 @@ static void service_enter_start(Service *s) {
                 s->control_pid = pid;
                 service_set_state(s, SERVICE_START);
 
-        } else if (IN_SET(s->type, SERVICE_ONESHOT, SERVICE_DBUS, SERVICE_NOTIFY)) {
+        } else if (IN_SET(s->type, SERVICE_ONESHOT, SERVICE_DBUS, SERVICE_NOTIFY, SERVICE_EXEC)) {
 
-                /* For oneshot services we wait until the start
-                 * process exited, too, but it is our main process. */
+                /* For oneshot services we wait until the start process exited, too, but it is our main process. */
 
-                /* For D-Bus services we know the main pid right away,
-                 * but wait for the bus name to appear on the
-                 * bus. Notify services are similar. */
+                /* For D-Bus services we know the main pid right away, but wait for the bus name to appear on the
+                 * bus. 'notify' and 'exec' services are similar. */
 
                 service_set_main_pid(s, pid);
                 service_set_state(s, SERVICE_START);
@@ -2444,6 +2518,13 @@ static int service_serialize(Unit *u, FILE *f, FDSet *fds) {
         if (r < 0)
                 return r;
 
+        if (s->exec_fd_event_source) {
+                r = unit_serialize_item_fd(u, f, fds, "exec-fd", sd_event_source_get_io_fd(s->exec_fd_event_source));
+                if (r < 0)
+                        return r;
+                unit_serialize_item(u, f, "exec-fd-hot", yes_no(s->exec_fd_hot));
+        }
+
         if (UNIT_ISSET(s->accept_socket)) {
                 r = unit_serialize_item(u, f, "accept-socket", UNIT_DEREF(s->accept_socket)->id);
                 if (r < 0)
@@ -2777,6 +2858,18 @@ static int service_deserialize_item(Unit *u, const char *key, const char *value,
                         s->stderr_fd = fdset_remove(fds, fd);
                         s->exec_context.stdio_as_fds = true;
                 }
+        } else if (streq(key, "exec-fd")) {
+                int fd;
+
+                if (safe_atoi(value, &fd) < 0 || fd < 0 || !fdset_contains(fds, fd))
+                        log_unit_debug(u, "Failed to parse exec-fd value: %s", value);
+                else {
+                        s->exec_fd_event_source = sd_event_source_unref(s->exec_fd_event_source);
+
+                        fd = fdset_remove(fds, fd);
+                        if (service_allocate_exec_fd_event_source(s, fd, &s->exec_fd_event_source) < 0)
+                                safe_close(fd);
+                }
         } else if (streq(key, "watchdog-override-usec")) {
                 usec_t watchdog_override_usec;
                 if (timestamp_deserialize(value, &watchdog_override_usec) < 0)
@@ -2860,7 +2953,7 @@ static int service_watch_pid_file(Service *s) {
 
         log_unit_debug(UNIT(s), "Setting watch for PID file %s", s->pid_file_pathspec->path);
 
-        r = path_spec_watch(s->pid_file_pathspec, service_dispatch_io);
+        r = path_spec_watch(s->pid_file_pathspec, service_dispatch_inotify_io);
         if (r < 0)
                 goto fail;
 
@@ -2904,7 +2997,7 @@ static int service_demand_pid_file(Service *s) {
         return service_watch_pid_file(s);
 }
 
-static int service_dispatch_io(sd_event_source *source, int fd, uint32_t events, void *userdata) {
+static int service_dispatch_inotify_io(sd_event_source *source, int fd, uint32_t events, void *userdata) {
         PathSpec *p = userdata;
         Service *s;
 
@@ -2937,6 +3030,59 @@ fail:
         return 0;
 }
 
+static int service_dispatch_exec_io(sd_event_source *source, int fd, uint32_t events, void *userdata) {
+        Service *s = SERVICE(userdata);
+
+        assert(s);
+
+        log_unit_debug(UNIT(s), "got exec-fd event");
+
+        /* If Type=exec is set, we'll consider a service started successfully the instant we invoked execve()
+         * successfully for it. We implement this through a pipe() towards the child, which the kernel automatically
+         * closes for us due to O_CLOEXEC on execve() in the child, which then triggers EOF on the pipe in the
+         * parent. We need to be careful however, as there are other reasons that we might cause the child's side of
+         * the pipe to be closed (for example, a simple exit()). To deal with that we'll ignore EOFs on the pipe unless
+         * the child signalled us first that it is about to call the execve(). It does so by sending us a simple
+         * non-zero byte via the pipe. We also provide the child with a way to inform us in case execve() failed: if it
+         * sends a zero byte we'll ignore POLLHUP on the fd again. */
+
+        for (;;) {
+                uint8_t x;
+                ssize_t n;
+
+                n = read(fd, &x, sizeof(x));
+                if (n < 0) {
+                        if (errno == EAGAIN) /* O_NONBLOCK in effect → everything queued has now been processed. */
+                                return 0;
+
+                        return log_unit_error_errno(UNIT(s), errno, "Failed to read from exec_fd: %m");
+                }
+                if (n == 0) { /* EOF → the event we are waiting for */
+
+                        s->exec_fd_event_source = sd_event_source_unref(s->exec_fd_event_source);
+
+                        if (s->exec_fd_hot) { /* Did the child tell us to expect EOF now? */
+                                log_unit_debug(UNIT(s), "Got EOF on exec-fd");
+
+                                s->exec_fd_hot = false;
+
+                                /* Nice! This is what we have been waiting for. Transition to next state. */
+                                if (s->type == SERVICE_EXEC && s->state == SERVICE_START)
+                                        service_enter_start_post(s);
+                        } else
+                                log_unit_debug(UNIT(s), "Got EOF on exec-fd while it was disabled, ignoring.");
+
+                        return 0;
+                }
+
+                /* A byte was read → this turns on/off the exec fd logic */
+                assert(n == sizeof(x));
+                s->exec_fd_hot = x;
+        }
+
+        return 0;
+}
+
 static void service_notify_cgroup_empty_event(Unit *u) {
         Service *s = SERVICE(u);
 
@@ -3846,7 +3992,8 @@ static const char* const service_type_table[_SERVICE_TYPE_MAX] = {
         [SERVICE_ONESHOT] = "oneshot",
         [SERVICE_DBUS] = "dbus",
         [SERVICE_NOTIFY] = "notify",
-        [SERVICE_IDLE] = "idle"
+        [SERVICE_IDLE] = "idle",
+        [SERVICE_EXEC] = "exec",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(service_type, ServiceType);
diff --git a/src/core/service.h b/src/core/service.h
index 9c06e91883..b53e4204e7 100644
--- a/src/core/service.h
+++ b/src/core/service.h
@@ -30,6 +30,7 @@ typedef enum ServiceType {
         SERVICE_DBUS,     /* we fork and wait until a specific D-Bus name appears on the bus */
         SERVICE_NOTIFY,   /* we fork and wait until a daemon sends us a ready message with sd_notify() */
         SERVICE_IDLE,     /* much like simple, but delay exec() until all jobs are dispatched. */
+        SERVICE_EXEC,     /* we fork and wait until we execute exec() (this means our own setup is waited for) */
         _SERVICE_TYPE_MAX,
         _SERVICE_TYPE_INVALID = -1
 } ServiceType;
@@ -165,6 +166,8 @@ struct Service {
         NotifyAccess notify_access;
         NotifyState notify_state;
 
+        sd_event_source *exec_fd_event_source;
+
         ServiceFDStore *fd_store;
         size_t n_fd_store;
         unsigned n_fd_store_max;
@@ -179,6 +182,7 @@ struct Service {
 
         unsigned n_restarts;
         bool flush_n_restarts;
+        bool exec_fd_hot;
 };
 
 extern const UnitVTable service_vtable;
diff --git a/src/core/socket.c b/src/core/socket.c
index 56d32225c4..d488c64e91 100644
--- a/src/core/socket.c
+++ b/src/core/socket.c
@@ -1867,10 +1867,11 @@ static int socket_coldplug(Unit *u) {
 static int socket_spawn(Socket *s, ExecCommand *c, pid_t *_pid) {
 
         ExecParameters exec_params = {
-                .flags      = EXEC_APPLY_SANDBOXING|EXEC_APPLY_CHROOT|EXEC_APPLY_TTY_STDIN,
-                .stdin_fd   = -1,
-                .stdout_fd  = -1,
-                .stderr_fd  = -1,
+                .flags     = EXEC_APPLY_SANDBOXING|EXEC_APPLY_CHROOT|EXEC_APPLY_TTY_STDIN,
+                .stdin_fd  = -1,
+                .stdout_fd = -1,
+                .stderr_fd = -1,
+                .exec_fd   = -1,
         };
         pid_t pid;
         int r;
diff --git a/src/core/swap.c b/src/core/swap.c
index b78b1aa266..e01e61e56d 100644
--- a/src/core/swap.c
+++ b/src/core/swap.c
@@ -606,6 +606,7 @@ static int swap_spawn(Swap *s, ExecCommand *c, pid_t *_pid) {
                 .stdin_fd  = -1,
                 .stdout_fd = -1,
                 .stderr_fd = -1,
+                .exec_fd   = -1,
         };
         pid_t pid;
         int r;