core: warn if a generator is world-writable

... because that is obviously a security risk.

src/shared/exec-util.c

@@ -156,6 +156,18 @@ static int do_execute(
                         log_debug("About to execute %s%s%s", t, argv ? " " : "", argv ? strnull(args) : "");
                 }
 
+                if (FLAGS_SET(flags, EXEC_DIR_WARN_WORLD_WRITABLE)) {
+                        struct stat st;
+
+                        r = stat(t, &st);
+                        if (r < 0)
+                                log_warning_errno(errno, "Failed to stat '%s', ignoring: %m", t);
+                        else if (S_ISREG(st.st_mode) && (st.st_mode & 0002))
+                                log_warning("'%s' is marked world-writable, which is a security risk as it "
+                                            "is executed with privileges. Please remove world writability "
+                                            "permission bits. Proceeding anyway.", t);
+                }
+
                 r = do_spawn(t, argv, fd, FLAGS_SET(flags, EXEC_DIR_SET_SYSTEMD_EXEC_PID), &pid);
                 if (r <= 0)
                         continue;