nspawn: Add support for sysroot pivoting (#5258)

Add a new --pivot-root argument to systemd-nspawn, which specifies a
directory to pivot to / inside the container; while the original / is
pivoted to another specified directory (if provided). This adds
support for booting container images which may contain several bootable
sysroots, as is common with OSTree disk images. When these disk images
are booted on real hardware, ostree-prepare-root is run in conjunction
with sysroot.mount in the initramfs to achieve the same results.

1 files changed, 29 insertions, 0 deletions

diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index 5594b87efa..a8d33ad907 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -132,6 +132,8 @@ typedef enum LinkJournal {
 static char *arg_directory = NULL;
 static char *arg_template = NULL;
 static char *arg_chdir = NULL;
+static char *arg_pivot_root_new = NULL;
+static char *arg_pivot_root_old = NULL;
 static char *arg_user = NULL;
 static sd_id128_t arg_uuid = {};
 static char *arg_machine = NULL;
@@ -221,6 +223,8 @@ static void help(void) {
                "  -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"
+               "     --pivot-root=PATH[:PATH]\n"
+               "                            Pivot root to given directory in the container\n"
                "  -u --user=USER            Run the command under specified user or uid\n"
                "  -M --machine=NAME         Set the machine name for the container\n"
                "     --uuid=UUID            Set a specific machine UUID for the container\n"
@@ -427,6 +431,7 @@ static int parse_argv(int argc, char *argv[]) {
                 ARG_KILL_SIGNAL,
                 ARG_SETTINGS,
                 ARG_CHDIR,
+                ARG_PIVOT_ROOT,
                 ARG_PRIVATE_USERS_CHOWN,
                 ARG_NOTIFY_READY,
                 ARG_ROOT_HASH,
@@ -478,6 +483,7 @@ static int parse_argv(int argc, char *argv[]) {
                 { "kill-signal",           required_argument, NULL, ARG_KILL_SIGNAL         },
                 { "settings",              required_argument, NULL, ARG_SETTINGS            },
                 { "chdir",                 required_argument, NULL, ARG_CHDIR               },
+                { "pivot-root",            required_argument, NULL, ARG_PIVOT_ROOT          },
                 { "notify-ready",          required_argument, NULL, ARG_NOTIFY_READY        },
                 { "root-hash",             required_argument, NULL, ARG_ROOT_HASH           },
                 {}
@@ -1012,6 +1018,14 @@ static int parse_argv(int argc, char *argv[]) {
                         arg_settings_mask |= SETTING_WORKING_DIRECTORY;
                         break;
 
+                case ARG_PIVOT_ROOT:
+                        r = pivot_root_parse(&arg_pivot_root_new, &arg_pivot_root_old, optarg);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse --pivot-root= argument %s: %m", optarg);
+
+                        arg_settings_mask |= SETTING_PIVOT_ROOT;
+                        break;
+
                 case ARG_NOTIFY_READY:
                         r = parse_boolean(optarg);
                         if (r < 0) {
@@ -2493,6 +2507,13 @@ static int outer_child(
         if (r < 0)
                 return r;
 
+        r = setup_pivot_root(
+                        directory,
+                        arg_pivot_root_new,
+                        arg_pivot_root_old);
+        if (r < 0)
+                return r;
+
         r = setup_volatile(
                         directory,
                         arg_volatile_mode,
@@ -2915,6 +2936,12 @@ static int load_settings(void) {
                 settings->parameters = NULL;
         }
 
+        if ((arg_settings_mask & SETTING_PIVOT_ROOT) == 0 &&
+            settings->pivot_root_new) {
+                free_and_replace(arg_pivot_root_new, settings->pivot_root_new);
+                free_and_replace(arg_pivot_root_old, settings->pivot_root_old);
+        }
+
         if ((arg_settings_mask & SETTING_WORKING_DIRECTORY) == 0 &&
             settings->working_directory) {
                 free(arg_chdir);
@@ -3915,6 +3942,8 @@ finish:
         free(arg_image);
         free(arg_machine);
         free(arg_user);
+        free(arg_pivot_root_new);
+        free(arg_pivot_root_old);
         free(arg_chdir);
         strv_free(arg_setenv);
         free(arg_network_bridge);