core: add DBUS method to bind mount new nodes without service restart

Allow to setup new bind mounts for a service at runtime (via either
DBUS or a new 'systemctl bind' verb) with a new helper that forks into
the unit's mount namespace.
Add a new integration test to cover this.

Useful for zero-downtime addition to services that are running inside
mount namespaces, especially when using RootImage/RootDirectory.

If a service runs with a read-only root, a tmpfs is added on /run
to ensure we can create the airlock directory for incoming mounts
under /run/host/incoming.

4 files changed, 67 insertions, 0 deletions

diff --git a/src/systemctl/systemctl-mount.c b/src/systemctl/systemctl-mount.c
new file mode 100644
index 0000000000..513a876f21
--- /dev/null
+++ b/src/systemctl/systemctl-mount.c
@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include "bus-error.h"
+#include "bus-locator.h"
+#include "systemctl-mount.h"
+#include "systemctl-util.h"
+#include "systemctl.h"
+
+int mount_bind(int argc, char *argv[], void *userdata) {
+        _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
+        _cleanup_free_ char *n = NULL;
+        sd_bus *bus;
+        int r;
+
+        r = acquire_bus(BUS_MANAGER, &bus);
+        if (r < 0)
+                return r;
+
+        polkit_agent_open_maybe();
+
+        r = unit_name_mangle(argv[1], arg_quiet ? 0 : UNIT_NAME_MANGLE_WARN, &n);
+        if (r < 0)
+                return log_error_errno(r, "Failed to mangle unit name: %m");
+
+        r = bus_call_method(
+                        bus,
+                        bus_systemd_mgr,
+                        "BindMountUnit",
+                        &error,
+                        NULL,
+                        "sssbb",
+                        n,
+                        argv[2],
+                        argv[3],
+                        arg_read_only,
+                        arg_mkdir);
+        if (r < 0)
+                return log_error_errno(r, "Failed to bind mount: %s", bus_error_message(&error, r));
+
+        return 0;
+}
diff --git a/src/systemctl/systemctl-mount.h b/src/systemctl/systemctl-mount.h
new file mode 100644
index 0000000000..1f9b3879fb
--- /dev/null
+++ b/src/systemctl/systemctl-mount.h
@@ -0,0 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+int mount_bind(int argc, char *argv[], void *userdata);
diff --git a/src/systemctl/systemctl.c b/src/systemctl/systemctl.c
index 7471fadd91..4726f65f97 100644
--- a/src/systemctl/systemctl.c
+++ b/src/systemctl/systemctl.c
@@ -38,6 +38,7 @@
 #include "systemctl-list-units.h"
 #include "systemctl-log-setting.h"
 #include "systemctl-logind.h"
+#include "systemctl-mount.h"
 #include "systemctl-preset-all.h"
 #include "systemctl-reset-failed.h"
 #include "systemctl-service-watchdogs.h"
@@ -105,6 +106,8 @@ bool arg_jobs_before = false;
 bool arg_jobs_after = false;
 char **arg_clean_what = NULL;
 TimestampStyle arg_timestamp_style = TIMESTAMP_PRETTY;
+bool arg_read_only = false;
+bool arg_mkdir = false;
 
 STATIC_DESTRUCTOR_REGISTER(arg_wall, strv_freep);
 STATIC_DESTRUCTOR_REGISTER(arg_root, freep);
@@ -157,6 +160,8 @@ static int systemctl_help(void) {
                "  freeze PATTERN...                   Freeze execution of unit processes\n"
                "  thaw PATTERN...                     Resume execution of a frozen unit\n"
                "  set-property UNIT PROPERTY=VALUE... Sets one or more properties of a unit\n"
+               "  bind UNIT PATH [PATH]               Bind-mount a path from the host into a\n"
+               "                                      unit's namespace\n"
                "  service-log-level SERVICE [LEVEL]   Get/set logging threshold for service\n"
                "  service-log-target SERVICE [TARGET] Get/set logging target for service\n"
                "  reset-failed [PATTERN...]           Reset failed state for all, one, or more\n"
@@ -286,6 +291,8 @@ static int systemctl_help(void) {
                "                         'us': 'Day YYYY-MM-DD HH:MM:SS.UUUUUU TZ\n"
                "                         'utc': 'Day YYYY-MM-DD HH:MM:SS UTC\n"
                "                         'us+utc': 'Day YYYY-MM-DD HH:MM:SS.UUUUUU UTC\n"
+               "     --read-only         Create read-only bind mount\n"
+               "     --mkdir             Create directory before bind-mounting, if missing\n"
                "\nSee the %2$s for details.\n"
                , program_invocation_short_name
                , link
@@ -401,6 +408,8 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
                 ARG_WHAT,
                 ARG_REBOOT_ARG,
                 ARG_TIMESTAMP_STYLE,
+                ARG_READ_ONLY,
+                ARG_MKDIR,
         };
 
         static const struct option options[] = {
@@ -457,6 +466,8 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
                 { "what",                required_argument, NULL, ARG_WHAT                },
                 { "reboot-argument",     required_argument, NULL, ARG_REBOOT_ARG          },
                 { "timestamp",           required_argument, NULL, ARG_TIMESTAMP_STYLE     },
+                { "read-only",           no_argument,       NULL, ARG_READ_ONLY           },
+                { "mkdir",               no_argument,       NULL, ARG_MKDIR               },
                 {}
         };
 
@@ -878,6 +889,14 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
 
                         break;
 
+                case ARG_READ_ONLY:
+                        arg_read_only = true;
+                        break;
+
+                case ARG_MKDIR:
+                        arg_mkdir = true;
+                        break;
+
                 case '.':
                         /* Output an error mimicking getopt, and print a hint afterwards */
                         log_error("%s: invalid option -- '.'", program_invocation_name);
@@ -1045,6 +1064,7 @@ static int systemctl_main(int argc, char *argv[]) {
                 { "add-wants",             3,        VERB_ANY, 0,                add_dependency          },
                 { "add-requires",          3,        VERB_ANY, 0,                add_dependency          },
                 { "edit",                  2,        VERB_ANY, VERB_ONLINE_ONLY, edit                    },
+                { "bind",                  3,        4,        VERB_ONLINE_ONLY, mount_bind              },
                 {}
         };
 
diff --git a/src/systemctl/systemctl.h b/src/systemctl/systemctl.h
index cdf56c7220..34650ebb44 100644
--- a/src/systemctl/systemctl.h
+++ b/src/systemctl/systemctl.h
@@ -90,3 +90,5 @@ extern bool arg_jobs_before;
 extern bool arg_jobs_after;
 extern char **arg_clean_what;
 extern TimestampStyle arg_timestamp_style;
+extern bool arg_read_only;
+extern bool arg_mkdir;