add warning message when joining user namespace

join user namespace is not fully tested yet. So it is
not recommended to use this.

add a warning message when user tries to use join-ns with
user-namespace.
Signed-off-by: Deng Guangxing <dengguangxing@huawei.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/crtools.c

@@ -834,6 +834,8 @@ usage:
 "			    --join-ns net:12345 or --join-ns net:/foo/bar.\n"
 "			Extra_opts is optional, for now only user namespace support:\n"
 "			    --join-ns user:PID,UID,GID to specify uid and gid.\n"
+"			Please NOTE: join-ns with user-namespace is not fully tested.\n"
+"			It may be dangerous to use this feature\n"
 "Check options:\n"
 "  without any arguments, \"criu check\" checks availability of absolutely required\n"
 "  kernel features; if any of these features is missing dump and restore will fail\n"

criu/namespaces.c

@@ -48,6 +48,9 @@ int check_namespace_opts(void)
 		pr_perror("Conflict flags: -join-ns and -empty-ns");
 		return -1;
 	}
+	if (join_ns_flags & CLONE_NEWUSER)
+		pr_warn("join-ns with user-namespace is not fully tested and dangerous");
+
 	errno = 0;
 	return 0;
 }