diff --git a/tools/testing/selftests/bpf/prog_tests/parse_tcp_hdr_opt.c b/tools/testing/selftests/bpf/prog_tests/parse_tcp_hdr_opt.c
index daa952711d8f..e9c07d561ded 100644
--- a/tools/testing/selftests/bpf/prog_tests/parse_tcp_hdr_opt.c
+++ b/tools/testing/selftests/bpf/prog_tests/parse_tcp_hdr_opt.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#define _GNU_SOURCE
 #include <test_progs.h>
 #include <network_helpers.h>
 #include "test_parse_tcp_hdr_opt.skel.h"