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.

1 files changed, 41 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;
+}