core: add LUOSession= unit setting (#42530)

Acquiring a LUO session from /dev/liveupdate requires privileges, and
also the device is a single-owner driver so only a single process can
open it at any given time.

Add a LUOSession= service settings that allows units running without
privileges to get a session assigned to them.

The kernel imposes a 64 chars limit on session names, which is too short
to avoid clashes, so derive a hash from joining the unit name with the
parameter name, that way two units using the same setting don't clash.

src/shared/bus-unit-util.c

@@ -2688,6 +2688,7 @@ static const BusProperty service_properties[] = {
         { "TimeoutStartFailureMode",               bus_append_string                             },
         { "TimeoutStopFailureMode",                bus_append_string                             },
         { "FileDescriptorStorePreserve",           bus_append_string                             },
+        { "LUOSession",                            bus_append_strv                               },
         { "PermissionsStartOnly",                  bus_append_parse_boolean                      },
         { "RootDirectoryStartOnly",                bus_append_parse_boolean                      },
         { "RemainAfterExit",                       bus_append_parse_boolean                      },

src/shared/luo-util.c

@@ -115,6 +115,11 @@ int luo_session_finish(int session_fd) {
         return RET_NERRNO(ioctl(session_fd, LIVEUPDATE_SESSION_FINISH, &args));
 }
 
+bool luo_session_name_is_valid(const char *name) {
+        /* Used for FDNAME: no whitespace, no ":", no control characters, etc. */
+        return fdname_is_valid(name) && string_is_safe(name, STRING_DISALLOW_WHITESPACE);
+}
+
 int luo_parse_serialization(sd_json_variant **ret, int **ret_fds, size_t *ret_n_fds) {
         _cleanup_(sd_json_variant_unrefp) sd_json_variant *root = NULL;
         _cleanup_free_ int *fd_list = NULL;

src/shared/luo-util.h

@@ -36,6 +36,8 @@ int luo_session_preserve_fd(int session_fd, int fd, uint64_t token);
 int luo_session_retrieve_fd(int session_fd, uint64_t token);
 int luo_session_finish(int session_fd);
 
+bool luo_session_name_is_valid(const char *name);
+
 int luo_parse_serialization(sd_json_variant **ret, int **ret_fds, size_t *ret_n_fds);
 int luo_preserve_fd_stores(sd_json_variant *serialization, int *ret_session_fd);