core: add new AcceptFD= setting to .busname units

AcceptFD= defaults to true, thus making sure that by default fd passing
is enabled for all activatable names. Since for normal bus connections
fd passing is enabled too by default this makes sure fd passing works
correctly regardless whether a service is already activated or not.

Making this configurable on both busname units and in bus connections is
messy, but unavoidable since busnames are established and may queue
messages before the connection feature negotiation is done by the
service eventually activated. Conversely, feature negotiation on bus
connections takes place before the connection acquires its names.

Of course, this means developers really should make sure to keep the
settings in .busname units in sync with what they later intend to
negotiate.

src/core/busname.c

@@ -43,6 +43,7 @@ static void busname_init(Unit *u) {
         assert(u->load_state == UNIT_STUB);
 
         n->starter_fd = -1;
+        n->accept_fd = true;
 }
 
 static void busname_done(Unit *u) {
@@ -170,10 +171,12 @@ static void busname_dump(Unit *u, FILE *f, const char *prefix) {
         fprintf(f,
                 "%sBus Name State: %s\n"
                 "%sResult: %s\n"
-                "%sName: %s\n",
+                "%sName: %s\n"
+                "%sAccept FD: %s\n",
                 prefix, busname_state_to_string(n->state),
                 prefix, busname_result_to_string(n->result),
-                prefix, n->name);
+                prefix, n->name,
+                prefix, yes_no(n->accept_fd));
 }
 
 static void busname_unwatch_fd(BusName *n) {
@@ -226,8 +229,10 @@ static int busname_open_fd(BusName *n) {
         if (n->starter_fd >= 0)
                 return 0;
 
-        n->starter_fd = bus_kernel_create_starter(UNIT(n)->manager->running_as == SYSTEMD_SYSTEM ? "system" : "user",
-                                                  n->name, n->policy);
+        n->starter_fd = bus_kernel_create_starter(
+                        UNIT(n)->manager->running_as == SYSTEMD_SYSTEM ? "system" : "user",
+                        n->name, n->accept_fd, n->policy);
+
         if (n->starter_fd < 0) {
                 log_warning_unit(UNIT(n)->id, "Failed to create starter fd: %s", strerror(-n->starter_fd));
                 return n->starter_fd;

src/core/busname.h

@@ -56,6 +56,8 @@ struct BusName {
 
         sd_event_source *event_source;
 
+        bool accept_fd;
+
         LIST_HEAD(BusNamePolicy, policy);
 };
 

src/core/load-fragment-gperf.gperf.m4

@@ -256,6 +256,7 @@ BusName.AllowUser,               config_parse_bus_policy,            0,
 BusName.AllowGroup,              config_parse_bus_policy,            0,                             0
 BusName.AllowWorld,              config_parse_bus_policy,            0,                             0
 BusName.SELinuxContext,          config_parse_exec_selinux_context,  0,                             0
+BusName.AcceptFileDescriptors,   config_parse_bool,                  0,                             offsetof(BusName, accept_fd)
 m4_dnl
 Mount.What,                      config_parse_string,                0,                             offsetof(Mount, parameters_fragment.what)
 Mount.Where,                     config_parse_path,                  0,                             offsetof(Mount, where)

src/libsystemd/sd-bus/bus-kernel.c

@@ -1378,7 +1378,7 @@ static void bus_kernel_translate_policy(const BusNamePolicy *policy, struct kdbu
         }
 }
 
-int bus_kernel_create_starter(const char *bus, const char *name, BusNamePolicy *policy) {
+int bus_kernel_create_starter(const char *bus, const char *name, bool accept_fd, BusNamePolicy *policy) {
         struct kdbus_cmd_hello *hello;
         struct kdbus_item *n;
         size_t policy_cnt = 0;
@@ -1420,7 +1420,7 @@ int bus_kernel_create_starter(const char *bus, const char *name, BusNamePolicy *
         }
 
         hello->size = size;
-        hello->conn_flags = KDBUS_HELLO_ACTIVATOR|KDBUS_HELLO_ACCEPT_FD;
+        hello->conn_flags = KDBUS_HELLO_ACTIVATOR | (accept_fd ? KDBUS_HELLO_ACCEPT_FD : 0);
         hello->pool_size = KDBUS_POOL_SIZE;
 
         if (ioctl(fd, KDBUS_CMD_HELLO, hello) < 0) {

src/libsystemd/sd-bus/bus-kernel.h

@@ -67,7 +67,7 @@ int bus_kernel_read_message(sd_bus *bus, bool hint_priority, int64_t priority);
 
 int bus_kernel_create_bus(const char *name, bool world, char **s);
 int bus_kernel_create_domain(const char *name, char **s);
-int bus_kernel_create_starter(const char *bus, const char *name, BusNamePolicy *policy);
+int bus_kernel_create_starter(const char *bus, const char *name, bool accept_fd, BusNamePolicy *policy);
 int bus_kernel_create_monitor(const char *bus);
 
 int bus_kernel_pop_memfd(sd_bus *bus, void **address, size_t *mapped, size_t *allocated);