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.

1 files changed, 40 insertions, 12 deletions

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;