Comment 14 for bug 870297

Ok, some pointers in case that's useful, gdm has code in gdm-session-record.c

void
gdm_session_record_login (GPid session_pid,
...
        /* Handle wtmp */
        g_debug ("Writing wtmp session record to " GDM_NEW_SESSION_RECORDS_FILE);
#if defined(HAVE_UPDWTMPX)
        updwtmpx (GDM_NEW_SESSION_RECORDS_FILE, &session_record);
#elif defined(HAVE_UPDWTMP)
        updwtmp (GDM_NEW_SESSION_RECORDS_FILE, &session_record);
#elif defined(HAVE_LOGWTMP) && defined(HAVE_UT_UT_HOST)
#if defined(HAVE_UT_UT_USER)
        logwtmp (session_record.ut_line, session_record.ut_user, session_record.ut_host);
#elif defined(HAVE_UT_UT_NAME)
        logwtmp (session_record.ut_line, session_record.ut_name, session_record.ut_host);
#endif
#endif

        /*
         * Handle utmp
         * Update if entry already exists
         */
#if defined(HAVE_GETUTXENT)
        setutxent ();

        while ((u = getutxent ()) != NULL) {
                if (u->ut_type == USER_PROCESS &&
                    (session_record.ut_line != NULL &&
                     (strncmp (u->ut_line, session_record.ut_line,
                               sizeof (u->ut_line)) == 0 ||
                      u->ut_pid == session_record.ut_pid))) {
                        g_debug ("Updating existing utmp record");
                        pututxline (&session_record);
                        break;
                }
        }
        endutxent ();

        /* Add new entry if update did not work */
        if (u == NULL) {
                g_debug ("Adding new utmp record");
                pututxline (&session_record);
        }
#elif defined(HAVE_LOGIN)
 login (&session_record);
#endif
}
..."

So it seems it's somewhat that is not integrated with the system stack but that lightdm should be doing by itself on login