bus: don't switch to kdbus if not requested

Whenever systemd is re-executed, it tries to create a system bus via
kdbus. If the system did not have kdbus loaded during bootup, but the
module is loaded later on manually, this will cause two system buses
running (kdbus and dbus-daemon in parallel).

This patch makes sure we never try to create kdbus buses if it wasn't
explicitly requested on the command-line.

6 files changed, 37 insertions, 8 deletions

diff --git a/src/core/busname.c b/src/core/busname.c
index 1278c9663..3dc6e87e5 100644
--- a/src/core/busname.c
+++ b/src/core/busname.c
@@ -988,7 +988,7 @@ static bool busname_supported(void) {
         static int supported = -1;
 
         if (supported < 0)
-                supported = access("/sys/fs/kdbus", F_OK) >= 0;
+                supported = is_kdbus_available();
 
         return supported;
 }
diff --git a/src/core/kmod-setup.c b/src/core/kmod-setup.c
index c5117b47e..132c3e866 100644
--- a/src/core/kmod-setup.c
+++ b/src/core/kmod-setup.c
@@ -28,6 +28,7 @@
 
 #include "macro.h"
 #include "capability.h"
+#include "bus-util.h"
 #include "kmod-setup.h"
 
 #ifdef HAVE_KMOD
@@ -44,10 +45,6 @@ static void systemd_kmod_log(
         log_internalv(LOG_DEBUG, 0, file, line, fn, format, args);
         REENABLE_WARNING;
 }
-
-static bool cmdline_check_kdbus(void) {
-        return get_proc_cmdline_key("kdbus", NULL) > 0;
-}
 #endif
 
 int kmod_setup(void) {
@@ -69,7 +66,7 @@ int kmod_setup(void) {
                 { "unix",      "/proc/net/unix",            true,  NULL                },
 
                 /* IPC is needed before we bring up any other services */
-                { "kdbus",     "/sys/fs/kdbus",             false, cmdline_check_kdbus },
+                { "kdbus",     "/sys/fs/kdbus",             false, is_kdbus_wanted     },
 
                 /* netfilter is needed by networkd, nspawn among others, and cannot be autoloaded */
                 { "ip_tables", "/proc/net/ip_tables_names", false, NULL                },
diff --git a/src/core/manager.c b/src/core/manager.c
index 8c8645b68..c918f191c 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -721,6 +721,8 @@ static int manager_setup_kdbus(Manager *m) {
 
         if (m->test_run || m->kdbus_fd >= 0)
                 return 0;
+        if (!is_kdbus_available())
+                return -ESOCKTNOSUPPORT;
 
         if (m->running_as == SYSTEMD_SYSTEM && detect_container(NULL) <= 0)
                 bus_kernel_fix_attach_mask();
diff --git a/src/dbus1-generator/dbus1-generator.c b/src/dbus1-generator/dbus1-generator.c
index c909a4b1d..4980fccc3 100644
--- a/src/dbus1-generator/dbus1-generator.c
+++ b/src/dbus1-generator/dbus1-generator.c
@@ -302,7 +302,7 @@ int main(int argc, char *argv[]) {
 
         umask(0022);
 
-        if (access("/sys/fs/kdbus/control", F_OK) < 0)
+        if (!is_kdbus_available())
                 return 0;
 
         r = cg_pid_get_owner_uid(0, NULL);
@@ -310,7 +310,7 @@ int main(int argc, char *argv[]) {
                 path = "/usr/share/dbus-1/services";
                 type = "session";
                 units = USER_DATA_UNIT_PATH;
-        } else if (r == -ENOENT) {
+        } else if (r == -ENXIO) {
                 path = "/usr/share/dbus-1/system-services";
                 type = "system";
                 units = SYSTEM_DATA_UNIT_PATH;
diff --git a/src/libsystemd/sd-bus/bus-util.c b/src/libsystemd/sd-bus/bus-util.c
index f0695bfde..7536a96b8 100644
--- a/src/libsystemd/sd-bus/bus-util.c
+++ b/src/libsystemd/sd-bus/bus-util.c
@@ -2051,3 +2051,30 @@ int bus_path_decode_unique(const char *path, const char *prefix, char **ret_send
         *ret_external = external;
         return 1;
 }
+
+bool is_kdbus_wanted(void) {
+        _cleanup_free_ char *value = NULL;
+        int r;
+
+        if (get_proc_cmdline_key("kdbus", NULL) <= 0) {
+                r = get_proc_cmdline_key("kdbus=", &value);
+                if (r <= 0 || parse_boolean(value) != 1)
+                        return false;
+        }
+
+        return true;
+}
+
+bool is_kdbus_available(void) {
+        _cleanup_close_ int fd = -1;
+        struct kdbus_cmd cmd = { .size = sizeof(cmd), .flags = KDBUS_FLAG_NEGOTIATE };
+
+        if (!is_kdbus_wanted())
+                return false;
+
+        fd = open("/sys/fs/kdbus/control", O_RDWR | O_CLOEXEC | O_NONBLOCK | O_NOCTTY);
+        if (fd < 0)
+                return false;
+
+        return ioctl(fd, KDBUS_CMD_BUS_MAKE, &cmd) >= 0;
+}
diff --git a/src/libsystemd/sd-bus/bus-util.h b/src/libsystemd/sd-bus/bus-util.h
index d3a18e2d9..093b48b8f 100644
--- a/src/libsystemd/sd-bus/bus-util.h
+++ b/src/libsystemd/sd-bus/bus-util.h
@@ -205,3 +205,6 @@ int bus_deserialize_and_dump_unit_file_changes(sd_bus_message *m, bool quiet);
 
 int bus_path_encode_unique(sd_bus *b, const char *prefix, const char *sender_id, const char *external_id, char **ret_path);
 int bus_path_decode_unique(const char *path, const char *prefix, char **ret_sender, char **ret_external);
+
+bool is_kdbus_wanted(void);
+bool is_kdbus_available(void);