nspawn: optionally run a stub init process as PID 1

This adds a new switch --as-pid2, which allows running commands as PID 2, while a stub init process is run as PID 1.
This is useful in order to run arbitrary commands in a container, as PID1's semantics are different from all other
processes regarding reaping of unknown children or signal handling.

9 files changed, 426 insertions, 43 deletions

diff --git a/Makefile.am b/Makefile.am
index a291c3c56f..f1f8315f30 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -2944,6 +2944,8 @@ systemd_nspawn_SOURCES = \
 	src/nspawn/nspawn-register.h \
 	src/nspawn/nspawn-setuid.c \
 	src/nspawn/nspawn-setuid.h \
+	src/nspawn/nspawn-stub-pid1.c \
+	src/nspawn/nspawn-stub-pid1.h \
 	src/core/mount-setup.c \
 	src/core/mount-setup.h \
 	src/core/loopback-setup.c \
diff --git a/man/systemd-nspawn.xml b/man/systemd-nspawn.xml
index 1cfef93df1..86cdb4e124 100644
--- a/man/systemd-nspawn.xml
+++ b/man/systemd-nspawn.xml
@@ -249,15 +249,68 @@
       </varlistentry>
 
       <varlistentry>
+        <term><option>-a</option></term>
+        <term><option>--as-pid2</option></term>
+
+        <listitem><para>Invoke the shell or specified program as process ID (PID) 2 instead of PID 1 (init). By
+        default, if neither this option nor <option>--boot</option> is used, the selected binary is run as process with
+        PID 1, a mode only suitable for programs that are aware of the special semantics that the process with PID 1
+        has on UNIX. For example, it needs to reap all processes reparented to it, and should implement
+        <command>sysvinit</command> compatible signal handling (specifically: it needs to reboot on SIGINT, reexecute
+        on SIGTERM, reload configuration on SIGHUP, and so on). With <option>--as-pid2</option> a minimal stub init
+        process is run as PID 1 and the selected binary is executed as PID 2 (and hence does not need to implement any
+        special semantics). The stub init process will reap processes as necessary and react appropriately to
+        signals. It is recommended to use this mode to invoke arbitrary commands in containers, unless they have been
+        modified to run correctly as PID 1. Or in other words: this switch should be used for pretty much all commands,
+        except when the command refers to an init or shell implementation, as these are generally capable of running
+        correctly as PID 1). This option may not be combined with <option>--boot</option> or
+        <option>--share-system</option>.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><option>-b</option></term>
         <term><option>--boot</option></term>
 
-        <listitem><para>Automatically search for an init binary and
-        invoke it instead of a shell or a user supplied program. If
-        this option is used, arguments specified on the command line
-        are used as arguments for the init binary. This option may not
-        be combined with <option>--share-system</option>.
-        </para></listitem>
+        <listitem><para>Automatically search for an init binary and invoke it as PID 1, instead of a shell or a user
+        supplied program. If this option is used, arguments specified on the command line are used as arguments for the
+        init binary. This option may not be combined with <option>--as-pid2</option> or
+        <option>--share-system</option>.</para>
+
+        <para>The following table explains the different modes of invocation and relationship to
+        <option>--as-pid2</option> (see above):</para>
+
+        <table>
+          <title>Invocation Mode</title>
+          <tgroup cols='2' align='left' colsep='1' rowsep='1'>
+            <colspec colname="switch" />
+            <colspec colname="explanation" />
+            <thead>
+              <row>
+                <entry>Switch</entry>
+                <entry>Explanation</entry>
+              </row>
+            </thead>
+            <tbody>
+              <row>
+                <entry>Neither <option>--as-pid2</option> nor <option>--boot</option> specified</entry>
+                <entry>The passed parameters are interpreted as command line, which is executed as PID 1 in the container.</entry>
+              </row>
+
+              <row>
+                <entry><option>--as-pid2</option> specified</entry>
+                <entry>The passed parameters are interpreted as command line, which are executed as PID 2 in the container. A stub init process is run as PID 1.</entry>
+              </row>
+
+              <row>
+                <entry><option>--boot</option> specified</entry>
+                <entry>An init binary as automatically searched and run as PID 1 in the container. The passed parameters are used as invocation parameters for this process.</entry>
+              </row>
+
+            </tbody>
+          </tgroup>
+        </table>
+        </listitem>
       </varlistentry>
 
       <varlistentry>
diff --git a/man/systemd.nspawn.xml b/man/systemd.nspawn.xml
index 7e604246f6..c07a4b0243 100644
--- a/man/systemd.nspawn.xml
+++ b/man/systemd.nspawn.xml
@@ -141,15 +141,21 @@
       <varlistentry>
         <term><varname>Boot=</varname></term>
 
-        <listitem><para>Takes a boolean argument, which defaults to off. If
-        enabled, <command>systemd-nspawn</command> will automatically
-        search for an <filename>init</filename> executable and invoke
-        it. In this case, the specified parameters using
-        <varname>Parameters=</varname> are passed as additional
-        arguments to the <filename>init</filename> process. This
-        setting corresponds to the <option>--boot</option> switch on
-        the <command>systemd-nspawn</command> command
-        line. </para></listitem>
+        <listitem><para>Takes a boolean argument, which defaults to off. If enabled, <command>systemd-nspawn</command>
+        will automatically search for an <filename>init</filename> executable and invoke it. In this case, the
+        specified parameters using <varname>Parameters=</varname> are passed as additional arguments to the
+        <filename>init</filename> process. This setting corresponds to the <option>--boot</option> switch on the
+        <command>systemd-nspawn</command> command line. This option may not be combined with
+        <varname>ProcessTwo=yes</varname>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>ProcessTwo=</varname></term>
+
+        <listitem><para>Takes a boolean argument, which defaults to off. If enabled, the specified program is run as
+        PID 2. A stub init process is run as PID 1. This setting corresponds to the <option>--as-pid2</option> switch
+        on the <command>systemd-nspawn</command> command line. This option may not be combined with
+        <varname>Boot=yes</varname>.</para></listitem>
       </varlistentry>
 
       <varlistentry>
diff --git a/src/nspawn/nspawn-gperf.gperf b/src/nspawn/nspawn-gperf.gperf
index 08020d510c..116655cdd2 100644
--- a/src/nspawn/nspawn-gperf.gperf
+++ b/src/nspawn/nspawn-gperf.gperf
@@ -15,7 +15,8 @@ struct ConfigPerfItem;
 %struct-type
 %includes
 %%
-Exec.Boot,                    config_parse_tristate,      0, offsetof(Settings, boot)
+Exec.Boot,                    config_parse_boot,          0, 0
+Exec.ProcessTwo,              config_parse_pid2,          0, 0,
 Exec.Parameters,              config_parse_strv,          0, offsetof(Settings, parameters)
 Exec.Environment,             config_parse_strv,          0, offsetof(Settings, environment)
 Exec.User,                    config_parse_string,        0, offsetof(Settings, user)
diff --git a/src/nspawn/nspawn-settings.c b/src/nspawn/nspawn-settings.c
index 3c552e0734..12524d3b89 100644
--- a/src/nspawn/nspawn-settings.c
+++ b/src/nspawn/nspawn-settings.c
@@ -24,6 +24,7 @@
 #include "conf-parser.h"
 #include "nspawn-network.h"
 #include "nspawn-settings.h"
+#include "parse-util.h"
 #include "process-util.h"
 #include "strv.h"
 #include "util.h"
@@ -39,7 +40,7 @@ int settings_load(FILE *f, const char *path, Settings **ret) {
         if (!s)
                 return -ENOMEM;
 
-        s->boot = -1;
+        s->start_mode = _START_MODE_INVALID;
         s->personality = PERSONALITY_INVALID;
 
         s->read_only = -1;
@@ -303,3 +304,93 @@ int config_parse_veth_extra(
 
         return 0;
 }
+
+int config_parse_boot(
+                const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        Settings *settings = data;
+        int r;
+
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+
+        r = parse_boolean(rvalue);
+        if (r < 0) {
+                log_syntax(unit, LOG_ERR, filename, line, r, "Failed to parse Boot= parameter %s, ignoring: %m", rvalue);
+                return 0;
+        }
+
+        if (r > 0) {
+                if (settings->start_mode == START_PID2)
+                        goto conflict;
+
+                settings->start_mode = START_BOOT;
+        } else {
+                if (settings->start_mode == START_BOOT)
+                        goto conflict;
+
+                if (settings->start_mode < 0)
+                        settings->start_mode = START_PID1;
+        }
+
+        return 0;
+
+conflict:
+        log_syntax(unit, LOG_ERR, filename, line, r, "Conflicting Boot= or ProcessTwo= setting found. Ignoring.");
+        return 0;
+}
+
+int config_parse_pid2(
+                const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        Settings *settings = data;
+        int r;
+
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+
+        r = parse_boolean(rvalue);
+        if (r < 0) {
+                log_syntax(unit, LOG_ERR, filename, line, r, "Failed to parse ProcessTwo= parameter %s, ignoring: %m", rvalue);
+                return 0;
+        }
+
+        if (r > 0) {
+                if (settings->start_mode == START_BOOT)
+                        goto conflict;
+
+                settings->start_mode = START_PID2;
+        } else {
+                if (settings->start_mode == START_PID2)
+                        goto conflict;
+
+                if (settings->start_mode < 0)
+                        settings->start_mode = START_PID1;
+        }
+
+        return 0;
+
+conflict:
+        log_syntax(unit, LOG_ERR, filename, line, r, "Conflicting Boot= or ProcessTwo= setting found. Ignoring.");
+        return 0;
+}
diff --git a/src/nspawn/nspawn-settings.h b/src/nspawn/nspawn-settings.h
index 236a1baa4f..fdb07486da 100644
--- a/src/nspawn/nspawn-settings.h
+++ b/src/nspawn/nspawn-settings.h
@@ -27,26 +27,34 @@
 #include "nspawn-expose-ports.h"
 #include "nspawn-mount.h"
 
+typedef enum StartMode {
+        START_PID1, /* Run parameters as command line as process 1 */
+        START_PID2, /* Use stub init process as PID 1, run parameters as command line as process 2 */
+        START_BOOT, /* Search for init system, pass arguments as parameters */
+        _START_MODE_MAX,
+        _START_MODE_INVALID = -1
+} StartMode;
+
 typedef enum SettingsMask {
-        SETTING_BOOT          = 1 << 0,
-        SETTING_ENVIRONMENT   = 1 << 1,
-        SETTING_USER          = 1 << 2,
-        SETTING_CAPABILITY    = 1 << 3,
-        SETTING_KILL_SIGNAL   = 1 << 4,
-        SETTING_PERSONALITY   = 1 << 5,
-        SETTING_MACHINE_ID    = 1 << 6,
-        SETTING_NETWORK       = 1 << 7,
-        SETTING_EXPOSE_PORTS  = 1 << 8,
-        SETTING_READ_ONLY     = 1 << 9,
-        SETTING_VOLATILE_MODE = 1 << 10,
-        SETTING_CUSTOM_MOUNTS = 1 << 11,
+        SETTING_START_MODE        = 1 << 0,
+        SETTING_ENVIRONMENT       = 1 << 1,
+        SETTING_USER              = 1 << 2,
+        SETTING_CAPABILITY        = 1 << 3,
+        SETTING_KILL_SIGNAL       = 1 << 4,
+        SETTING_PERSONALITY       = 1 << 5,
+        SETTING_MACHINE_ID        = 1 << 6,
+        SETTING_NETWORK           = 1 << 7,
+        SETTING_EXPOSE_PORTS      = 1 << 8,
+        SETTING_READ_ONLY         = 1 << 9,
+        SETTING_VOLATILE_MODE     = 1 << 10,
+        SETTING_CUSTOM_MOUNTS     = 1 << 11,
         SETTING_WORKING_DIRECTORY = 1 << 12,
-        _SETTINGS_MASK_ALL    = (1 << 13) -1
+        _SETTINGS_MASK_ALL        = (1 << 13) -1
 } SettingsMask;
 
 typedef struct Settings {
         /* [Run] */
-        int boot;
+        StartMode start_mode;
         char **parameters;
         char **environment;
         char *user;
@@ -91,3 +99,5 @@ int config_parse_volatile_mode(const char *unit, const char *filename, unsigned
 int config_parse_bind(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 int config_parse_tmpfs(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 int config_parse_veth_extra(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
+int config_parse_boot(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
+int config_parse_pid2(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
diff --git a/src/nspawn/nspawn-stub-pid1.c b/src/nspawn/nspawn-stub-pid1.c
new file mode 100644
index 0000000000..2de87e3c63
--- /dev/null
+++ b/src/nspawn/nspawn-stub-pid1.c
@@ -0,0 +1,170 @@
+/***
+  This file is part of systemd.
+
+  Copyright 2016 Lennart Poettering
+
+  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.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <sys/reboot.h>
+#include <sys/unistd.h>
+#include <sys/wait.h>
+
+#include "fd-util.h"
+#include "log.h"
+#include "nspawn-stub-pid1.h"
+#include "process-util.h"
+#include "signal-util.h"
+#include "time-util.h"
+#include "def.h"
+
+int stub_pid1(void) {
+        enum {
+                STATE_RUNNING,
+                STATE_REBOOT,
+                STATE_POWEROFF,
+        } state = STATE_RUNNING;
+
+        sigset_t fullmask, oldmask, waitmask;
+        usec_t quit_usec = USEC_INFINITY;
+        pid_t pid;
+        int r;
+
+        /* Implements a stub PID 1, that reaps all processes and processes a couple of standard signals. This is useful
+         * for allowing arbitrary processes run in a container, and still have all zombies reaped. */
+
+        assert_se(sigfillset(&fullmask) >= 0);
+        assert_se(sigprocmask(SIG_BLOCK, &fullmask, &oldmask) >= 0);
+
+        pid = fork();
+        if (pid < 0)
+                return log_error_errno(errno, "Failed to fork child pid: %m");
+
+        if (pid == 0) {
+                /* Return in the child */
+                assert_se(sigprocmask(SIG_SETMASK, &oldmask, NULL) >= 0);
+                setsid();
+                return 0;
+        }
+
+        reset_all_signal_handlers();
+
+        log_close();
+        close_all_fds(NULL, 0);
+        log_open();
+
+        rename_process("STUBINIT");
+
+        assert_se(sigemptyset(&waitmask) >= 0);
+        assert_se(sigset_add_many(&waitmask,
+                                  SIGCHLD,          /* posix: process died */
+                                  SIGINT,           /* sysv: ctrl-alt-del */
+                                  SIGRTMIN+3,       /* systemd: halt */
+                                  SIGRTMIN+4,       /* systemd: poweroff */
+                                  SIGRTMIN+5,       /* systemd: reboot */
+                                  SIGRTMIN+6,       /* systemd: kexec */
+                                  SIGRTMIN+13,      /* systemd: halt */
+                                  SIGRTMIN+14,      /* systemd: poweroff */
+                                  SIGRTMIN+15,      /* systemd: reboot */
+                                  SIGRTMIN+16,      /* systemd: kexec */
+                                  -1) >= 0);
+
+        /* Note that we ignore SIGTERM (sysv's reexec), SIGHUP (reload), and all other signals here, since we don't
+         * support reexec/reloading in this stub process. */
+
+        for (;;) {
+                siginfo_t si;
+                usec_t current_usec;
+
+                si.si_pid = 0;
+                r = waitid(P_ALL, 0, &si, WEXITED|WNOHANG);
+                if (r < 0) {
+                        r = log_error_errno(errno, "Failed to reap children: %m");
+                        goto finish;
+                }
+
+                current_usec = now(CLOCK_MONOTONIC);
+
+                if (si.si_pid == pid || current_usec >= quit_usec) {
+
+                        /* The child we started ourselves died or we reached a timeout. */
+
+                        if (state == STATE_REBOOT) { /* dispatch a queued reboot */
+                                (void) reboot(RB_AUTOBOOT);
+                                r = log_error_errno(errno, "Failed to reboot: %m");
+                                goto finish;
+
+                        } else if (state == STATE_POWEROFF)
+                                (void) reboot(RB_POWER_OFF); /* if this fails, fall back to normal exit. */
+
+                        if (si.si_pid == pid && si.si_code == CLD_EXITED)
+                                r = si.si_status; /* pass on exit code */
+                        else
+                                r = 255; /* signal, coredump, timeout, … */
+
+                        goto finish;
+                }
+                if (si.si_pid != 0)
+                        /* We reaped something. Retry until there's nothing more to reap. */
+                        continue;
+
+                if (quit_usec == USEC_INFINITY)
+                        r = sigwaitinfo(&waitmask, &si);
+                else {
+                        struct timespec ts;
+                        r = sigtimedwait(&waitmask, &si, timespec_store(&ts, quit_usec - current_usec));
+                }
+                if (r < 0) {
+                        if (errno == EINTR) /* strace -p attach can result in EINTR, let's handle this nicely. */
+                                continue;
+                        if (errno == EAGAIN) /* timeout reached */
+                                continue;
+
+                        r = log_error_errno(errno, "Failed to wait for signal: %m");
+                        goto finish;
+                }
+
+                if (si.si_signo == SIGCHLD)
+                        continue; /* Let's reap this */
+
+                if (state != STATE_RUNNING)
+                        continue;
+
+                /* Would love to use a switch() statement here, but SIGRTMIN is actually a function call, not a
+                 * constant… */
+
+                if (si.si_signo == SIGRTMIN+3 ||
+                    si.si_signo == SIGRTMIN+4 ||
+                    si.si_signo == SIGRTMIN+13 ||
+                    si.si_signo == SIGRTMIN+14)
+
+                        state = STATE_POWEROFF;
+
+                else if (si.si_signo == SIGINT ||
+                         si.si_signo == SIGRTMIN+5 ||
+                         si.si_signo == SIGRTMIN+6 ||
+                         si.si_signo == SIGRTMIN+15 ||
+                         si.si_signo == SIGRTMIN+16)
+
+                        state = STATE_REBOOT;
+                else
+                        assert_not_reached("Got unexpected signal");
+
+                /* (void) kill_and_sigcont(pid, SIGTERM); */
+                quit_usec = now(CLOCK_MONOTONIC) + DEFAULT_TIMEOUT_USEC;
+        }
+
+finish:
+        _exit(r < 0 ? EXIT_FAILURE : r);
+}
diff --git a/src/nspawn/nspawn-stub-pid1.h b/src/nspawn/nspawn-stub-pid1.h
new file mode 100644
index 0000000000..36c1aaf5dd
--- /dev/null
+++ b/src/nspawn/nspawn-stub-pid1.h
@@ -0,0 +1,22 @@
+#pragma once
+
+/***
+  This file is part of systemd.
+
+  Copyright 2016 Lennart Poettering
+
+  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.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+int stub_pid1(void);
diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index 0dae9984c9..e7314ac29c 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -79,6 +79,7 @@
 #include "nspawn-register.h"
 #include "nspawn-settings.h"
 #include "nspawn-setuid.h"
+#include "nspawn-stub-pid1.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "process-util.h"
@@ -123,7 +124,7 @@ static const char *arg_selinux_apifs_context = NULL;
 static const char *arg_slice = NULL;
 static bool arg_private_network = false;
 static bool arg_read_only = false;
-static bool arg_boot = false;
+static StartMode arg_start_mode = START_PID1;
 static bool arg_ephemeral = false;
 static LinkJournal arg_link_journal = LINK_AUTO;
 static bool arg_link_journal_try = false;
@@ -193,6 +194,7 @@ static void help(void) {
                "  -x --ephemeral            Run container with snapshot of root directory, and\n"
                "                            remove it after exit\n"
                "  -i --image=PATH           File system device or disk image for the container\n"
+               "  -a --as-pid2              Maintain a stub init as PID1, invoke binary as PID2\n"
                "  -b --boot                 Boot up full system (i.e. invoke init)\n"
                "     --chdir=PATH           Set working directory in the container\n"
                "  -u --user=USER            Run the command under specified user or uid\n"
@@ -358,6 +360,7 @@ static int parse_argv(int argc, char *argv[]) {
                 { "ephemeral",             no_argument,       NULL, 'x'                   },
                 { "user",                  required_argument, NULL, 'u'                   },
                 { "private-network",       no_argument,       NULL, ARG_PRIVATE_NETWORK   },
+                { "as-pid2",               no_argument,       NULL, 'a'                   },
                 { "boot",                  no_argument,       NULL, 'b'                   },
                 { "uuid",                  required_argument, NULL, ARG_UUID              },
                 { "read-only",             no_argument,       NULL, ARG_READ_ONLY         },
@@ -404,7 +407,7 @@ static int parse_argv(int argc, char *argv[]) {
         assert(argc >= 0);
         assert(argv);
 
-        while ((c = getopt_long(argc, argv, "+hD:u:bL:M:jS:Z:qi:xp:n", options, NULL)) >= 0)
+        while ((c = getopt_long(argc, argv, "+hD:u:abL:M:jS:Z:qi:xp:n", options, NULL)) >= 0)
 
                 switch (c) {
 
@@ -495,8 +498,23 @@ static int parse_argv(int argc, char *argv[]) {
                         break;
 
                 case 'b':
-                        arg_boot = true;
-                        arg_settings_mask |= SETTING_BOOT;
+                        if (arg_start_mode == START_PID2) {
+                                log_error("--boot and --as-pid2 may not be combined.");
+                                return -EINVAL;
+                        }
+
+                        arg_start_mode = START_BOOT;
+                        arg_settings_mask |= SETTING_START_MODE;
+                        break;
+
+                case 'a':
+                        if (arg_start_mode == START_BOOT) {
+                                log_error("--boot and --as-pid2 may not be combined.");
+                                return -EINVAL;
+                        }
+
+                        arg_start_mode = START_PID2;
+                        arg_settings_mask |= SETTING_START_MODE;
                         break;
 
                 case ARG_UUID:
@@ -876,7 +894,7 @@ static int parse_argv(int argc, char *argv[]) {
         if (arg_share_system)
                 arg_register = false;
 
-        if (arg_boot && arg_share_system) {
+        if (arg_start_mode != START_PID1 && arg_share_system) {
                 log_error("--boot and --share-system may not be combined.");
                 return -EINVAL;
         }
@@ -924,7 +942,7 @@ static int parse_argv(int argc, char *argv[]) {
                 if (!arg_parameters)
                         return log_oom();
 
-                arg_settings_mask |= SETTING_BOOT;
+                arg_settings_mask |= SETTING_START_MODE;
         }
 
         /* Load all settings from .nspawn files */
@@ -960,7 +978,7 @@ static int verify_arguments(void) {
                 return -EINVAL;
         }
 
-        if (arg_boot && arg_kill_signal <= 0)
+        if (arg_start_mode == START_BOOT && arg_kill_signal <= 0)
                 arg_kill_signal = SIGRTMIN+3;
 
         return 0;
@@ -2584,6 +2602,12 @@ static int inner_child(
                 if (chdir(arg_chdir) < 0)
                         return log_error_errno(errno, "Failed to change to specified working directory %s: %m", arg_chdir);
 
+        if (arg_start_mode == START_PID2) {
+                r = stub_pid1();
+                if (r < 0)
+                        return r;
+        }
+
         /* Now, explicitly close the log, so that we
          * then can close all remaining fds. Closing
          * the log explicitly first has the benefit
@@ -2595,7 +2619,7 @@ static int inner_child(
         log_close();
         (void) fdset_close_others(fds);
 
-        if (arg_boot) {
+        if (arg_start_mode == START_BOOT) {
                 char **a;
                 size_t m;
 
@@ -2917,9 +2941,9 @@ static int load_settings(void) {
         /* Copy over bits from the settings, unless they have been
          * explicitly masked by command line switches. */
 
-        if ((arg_settings_mask & SETTING_BOOT) == 0 &&
-            settings->boot >= 0) {
-                arg_boot = settings->boot;
+        if ((arg_settings_mask & SETTING_START_MODE) == 0 &&
+            settings->start_mode >= 0) {
+                arg_start_mode = settings->start_mode;
 
                 strv_free(arg_parameters);
                 arg_parameters = settings->parameters;
@@ -3074,6 +3098,10 @@ int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
 
+        /* Make sure rename_process() in the stub init process can work */
+        saved_argv = argv;
+        saved_argc = argc;
+
         r = parse_argv(argc, argv);
         if (r <= 0)
                 goto finish;
@@ -3180,7 +3208,7 @@ int main(int argc, char *argv[]) {
                         }
                 }
 
-                if (arg_boot) {
+                if (arg_start_mode == START_BOOT) {
                         if (path_is_os_tree(arg_directory) <= 0) {
                                 log_error("Directory %s doesn't look like an OS root directory (os-release file is missing). Refusing.", arg_directory);
                                 r = -EINVAL;