containers: systemd exits with non-zero code

When a systemd service running in a container exits with a non-zero
code, it can be useful to terminate the container immediately and get
the exit code back to the host, when systemd-nspawn returns. This was
not possible to do. This patch adds the following to make it possible:

- Add a read-only "ExitCode" property on PID 1's "Manager" bus object.
  By default, it is 0 so the behaviour stays the same as previously.
- Add a method "SetExitCode" on the same object. The method fails when
  called on baremetal: it is only allowed in containers or in user
  session.
- Add support in systemctl to call "systemctl exit 42". It reuses the
  existing code for user session.
- Add exit.target to the system instance. It has the following
  condition: ConditionVirtualization=container.
- Change main() to actually allow exit() with the correct value.
- Update systemctl manpage.

I used the following to test it:

| $ sudo rkt --debug --insecure-skip-verify run \
|            --mds-register=false --local docker://busybox \
|            --exec=/bin/chroot -- /proc/1/root \
|            systemctl --force exit 42
| ...
| Container rkt-895a0cba-5c66-4fa5-831c-e3f8ddc5810d failed with error code 42.
| $ echo $?
| 42

I don't know why I have to use --force.

Fixes systemd#1290

Makefile.am

@@ -469,6 +469,7 @@ dist_systemunit_DATA = \
 	units/getty.target \
 	units/halt.target \
 	units/kexec.target \
+	units/exit.target \
 	units/local-fs.target \
 	units/local-fs-pre.target \
 	units/initrd.target \

man/systemctl.xml

@@ -1612,13 +1612,17 @@ kobject-uevent 1 systemd-udevd-kernel.socket systemd-udevd.service
         </varlistentry>
 
         <varlistentry>
-          <term><command>exit</command></term>
+          <term><command>exit <optional><replaceable>EXIT_CODE</replaceable></optional></command></term>
 
           <listitem>
             <para>Ask the systemd manager to quit. This is only
             supported for user service managers (i.e. in conjunction
-            with the <option>--user</option> option) and will fail
-            otherwise.</para>
+            with the <option>--user</option> option) or in containers
+            and will fail otherwise.</para>
+
+            <para>The systemd manager can exit with a non-zero exit
+            code if the optional argument
+            <replaceable>EXIT_CODE</replaceable> is given.</para>
           </listitem>
         </varlistentry>
 

src/core/dbus-manager.c

@@ -1206,8 +1206,8 @@ static int method_exit(sd_bus_message *message, void *userdata, sd_bus_error *er
         if (r < 0)
                 return r;
 
-        if (m->running_as == MANAGER_SYSTEM)
-                return sd_bus_error_setf(error, SD_BUS_ERROR_NOT_SUPPORTED, "Exit is only supported for user service managers.");
+        if (m->running_as == MANAGER_SYSTEM && detect_container(NULL) <= 0)
+                return sd_bus_error_setf(error, SD_BUS_ERROR_NOT_SUPPORTED, "Exit is only supported for user service managers or in containers.");
 
         m->exit_code = MANAGER_EXIT;
 
@@ -1455,6 +1455,30 @@ static int method_unset_and_set_environment(sd_bus_message *message, void *userd
         return sd_bus_reply_method_return(message, NULL);
 }
 
+static int method_set_exit_code(sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        unsigned code;
+        Manager *m = userdata;
+        int r;
+
+        assert(message);
+        assert(m);
+
+        r = mac_selinux_access_check(message, "exit", error);
+        if (r < 0)
+                return r;
+
+        r = sd_bus_message_read_basic(message, 'u', &code);
+        if (r < 0)
+                return r;
+
+        if (detect_container(NULL) <= 0)
+                return sd_bus_error_setf(error, BUS_ERROR_NOT_ALLOWED, "ExitCode can only be set in containers or user service manager.");
+
+        m->return_value = code;
+
+        return sd_bus_reply_method_return(message, NULL);
+}
+
 static int method_list_unit_files(sd_bus_message *message, void *userdata, sd_bus_error *error) {
         _cleanup_bus_message_unref_ sd_bus_message *reply = NULL;
         Manager *m = userdata;
@@ -1954,6 +1978,7 @@ const sd_bus_vtable bus_manager_vtable[] = {
         SD_BUS_WRITABLE_PROPERTY("ShutdownWatchdogUSec", "t", bus_property_get_usec, bus_property_set_usec, offsetof(Manager, shutdown_watchdog), 0),
         SD_BUS_PROPERTY("ControlGroup", "s", NULL, offsetof(Manager, cgroup_root), 0),
         SD_BUS_PROPERTY("SystemState", "s", property_get_system_state, 0, 0),
+        SD_BUS_PROPERTY("ExitCode", "u", bus_property_get_unsigned, offsetof(Manager, return_value), 0),
 
         SD_BUS_METHOD("GetUnit", "s", "o", method_get_unit, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("GetUnitByPID", "u", "o", method_get_unit_by_pid, SD_BUS_VTABLE_UNPRIVILEGED),
@@ -2007,6 +2032,7 @@ const sd_bus_vtable bus_manager_vtable[] = {
         SD_BUS_METHOD("GetDefaultTarget", NULL, "s", method_get_default_target, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("PresetAllUnitFiles", "sbb", "a(sss)", method_preset_all_unit_files, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("AddDependencyUnitFiles", "asssbb", "a(sss)", method_add_dependency_unit_files, SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD("SetExitCode", "u", NULL, method_set_exit_code, SD_BUS_VTABLE_UNPRIVILEGED),
 
         SD_BUS_SIGNAL("UnitNew", "so", 0),
         SD_BUS_SIGNAL("UnitRemoved", "so", 0),

src/core/main.c

@@ -1764,7 +1764,7 @@ int main(int argc, char *argv[]) {
                 switch (m->exit_code) {
 
                 case MANAGER_EXIT:
-                        retval = EXIT_SUCCESS;
+                        retval = m->return_value;
                         log_debug("Exit.");
                         goto finish;
 
@@ -1988,7 +1988,7 @@ int main(int argc, char *argv[]) {
 
         if (shutdown_verb) {
                 char log_level[DECIMAL_STR_MAX(int) + 1];
-                const char* command_line[9] = {
+                const char* command_line[10] = {
                         SYSTEMD_SHUTDOWN_BINARY_PATH,
                         shutdown_verb,
                         "--log-level", log_level,
@@ -2053,7 +2053,7 @@ int main(int argc, char *argv[]) {
                           getpid() == 1 ? "freezing" : "quitting");
         }
 
-        if (getpid() == 1) {
+        if (getpid() == 1 && detect_container(NULL) <= 0) {
                 if (error_message)
                         manager_status_printf(NULL, STATUS_TYPE_EMERGENCY,
                                               ANSI_HIGHLIGHT_RED_ON "!!!!!!" ANSI_HIGHLIGHT_OFF,

src/core/manager.h

@@ -295,6 +295,11 @@ struct Manager {
 
         const char *unit_log_field;
         const char *unit_log_format_string;
+
+        /* If non-zero, exit with the following value when the systemd
+         * process terminate. Useful for containers: systemd-nspawn could get
+         * the return value. */
+        unsigned return_value;
 };
 
 int manager_new(ManagerRunningAs running_as, bool test_run, Manager **m);

src/libsystemd/sd-bus/bus-common-errors.h

@@ -40,6 +40,7 @@
 #define BUS_ERROR_NO_ISOLATION "org.freedesktop.systemd1.NoIsolation"
 #define BUS_ERROR_SHUTTING_DOWN "org.freedesktop.systemd1.ShuttingDown"
 #define BUS_ERROR_SCOPE_NOT_RUNNING "org.freedesktop.systemd1.ScopeNotRunning"
+#define BUS_ERROR_NOT_ALLOWED "org.freedesktop.systemd1.NotAllowed"
 
 #define BUS_ERROR_NO_SUCH_MACHINE "org.freedesktop.machine1.NoSuchMachine"
 #define BUS_ERROR_NO_SUCH_IMAGE "org.freedesktop.machine1.NoSuchImage"

src/systemctl/systemctl.c

@@ -4838,6 +4838,31 @@ static int daemon_reload(sd_bus *bus, char **args) {
                                     /* "daemon-reload" */ "Reload";
         }
 
+        if (streq(args[0], "exit")) {
+                unsigned code = 0;
+                if (strv_length(args) > 1) {
+                        r = safe_atou(args[1], &code);
+                        if (r < 0) {
+                                log_error("Invalid exit code.");
+                                return -EINVAL;
+                        }
+                }
+
+                r = sd_bus_call_method(
+                                bus,
+                                "org.freedesktop.systemd1",
+                                "/org/freedesktop/systemd1",
+                                "org.freedesktop.systemd1.Manager",
+                                "SetExitCode",
+                                &error,
+                                NULL,
+                                "u", code);
+                if (r < 0) {
+                        log_error("Failed to execute operation: %s", bus_error_message(&error, r));
+                        return r;
+                }
+        }
+
         r = sd_bus_call_method(
                         bus,
                         "org.freedesktop.systemd1",
@@ -6155,7 +6180,7 @@ static void systemctl_help(void) {
                "  poweroff                        Shut down and power-off the system\n"
                "  reboot [ARG]                    Shut down and reboot the system\n"
                "  kexec                           Shut down and reboot the system with kexec\n"
-               "  exit                            Request user instance exit\n"
+               "  exit [EXIT_CODE]                Request user instance or container exit\n"
                "  switch-root ROOT [INIT]         Change to a different root file system\n"
                "  suspend                         Suspend the system\n"
                "  hibernate                       Hibernate the system\n"
@@ -7132,7 +7157,7 @@ static int systemctl_main(sd_bus *bus, int argc, char *argv[], int bus_error) {
                 { "default",               EQUAL, 1, start_special     },
                 { "rescue",                EQUAL, 1, start_special     },
                 { "emergency",             EQUAL, 1, start_special     },
-                { "exit",                  EQUAL, 1, start_special     },
+                { "exit",                  LESS,  2, start_special     },
                 { "reset-failed",          MORE,  1, reset_failed      },
                 { "enable",                MORE,  2, enable_unit,      NOBUS },
                 { "disable",               MORE,  2, enable_unit,      NOBUS },

units/exit.target

@@ -0,0 +1,18 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Exit the container
+Documentation=man:systemd.special(7)
+ConditionVirtualization=container
+DefaultDependencies=no
+Requires=systemd-exit.service
+After=systemd-exit.service
+AllowIsolate=yes
+
+[Install]
+Alias=ctrl-alt-del.target