diff --git a/criu/include/util.h b/criu/include/util.h
index 1894f6acc..d65a7d866 100644
--- a/criu/include/util.h
+++ b/criu/include/util.h
@@ -331,4 +331,6 @@ extern int epoll_del_rfd(int epfd, struct epoll_rfd *rfd);
 extern int epoll_run_rfds(int epfd, struct epoll_event *evs, int nr_fds, int tmo);
 extern int epoll_prepare(int nr_events, struct epoll_event **evs);
 
+extern int open_fd_of_real_pid(pid_t pid, int fd, int flags);
+
 #endif /* __CR_UTIL_H__ */
diff --git a/criu/namespaces.c b/criu/namespaces.c
index ee67bdad4..85444155c 100644
--- a/criu/namespaces.c
+++ b/criu/namespaces.c
@@ -1455,11 +1455,6 @@ static int usernsd(int sk)
 		unsc_msg_pid_fd(&um, &pid, &fd);
 		pr_debug("uns: daemon calls %p (%d, %d, %x)\n", call, pid, fd, flags);
 
-		if (fd < 0 && flags & UNS_FDOUT) {
-			pr_err("uns: bad flags/fd %p %d %x\n", call, fd, flags);
-			BUG();
-		}
-
 		/*
 		 * Caller has sent us bare address of the routine it
 		 * wants to call. Since the caller is fork()-ed from the
diff --git a/criu/util.c b/criu/util.c
index 6f740630c..6b3eda69a 100644
--- a/criu/util.c
+++ b/criu/util.c
@@ -1386,3 +1386,33 @@ free_events:
 	xfree(*events);
 	return -1;
 }
+
+static int fn_open_proc_r(void *path, int fd, pid_t pid)
+{
+	return openat(get_service_fd(CR_PROC_FD_OFF), path, O_RDONLY);
+}
+static int fn_open_proc_w(void *path, int fd, pid_t pid)
+{
+	return openat(get_service_fd(CR_PROC_FD_OFF), path, O_WRONLY);
+}
+static int fn_open_proc_rw(void *path, int fd, pid_t pid)
+{
+	return openat(get_service_fd(CR_PROC_FD_OFF), path, O_RDWR);
+}
+
+int open_fd_of_real_pid(pid_t pid, int fd, int flags)
+{
+	char path[64];
+	int ret;
+
+	ret = sprintf(path, "%d/fd/%d", pid, fd);
+	if (flags == O_RDONLY)
+		ret = userns_call(fn_open_proc_r, UNS_FDOUT, path, ret + 1, -1);
+	else if (flags == O_WRONLY)
+		ret = userns_call(fn_open_proc_w, UNS_FDOUT, path, ret + 1, -1);
+	else if (flags == O_RDWR)
+		ret = userns_call(fn_open_proc_rw, UNS_FDOUT, path, ret + 1, -1);
+	else
+		BUG();
+	return ret;
+}