diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index e1504833654d..ed012a7f0786 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -161,11 +161,11 @@ ifneq ($(KBUILD_OUTPUT),)
   # $(realpath ...) resolves symlinks
   abs_objtree := $(realpath $(abs_objtree))
   BUILD := $(abs_objtree)/kselftest
-  KHDR_INCLUDES := -isystem ${abs_objtree}/usr/include
+  KHDR_INCLUDES := -D_GNU_SOURCE -isystem ${abs_objtree}/usr/include
 else
   BUILD := $(CURDIR)
   abs_srctree := $(shell cd $(top_srcdir) && pwd)
-  KHDR_INCLUDES := -isystem ${abs_srctree}/usr/include
+  KHDR_INCLUDES := -D_GNU_SOURCE -isystem ${abs_srctree}/usr/include
   DEFAULT_INSTALL_HDR_PATH := 1
 endif
 
diff --git a/tools/testing/selftests/kselftest_harness.h b/tools/testing/selftests/kselftest_harness.h
index d98702b6955d..b2a1b6343896 100644
--- a/tools/testing/selftests/kselftest_harness.h
+++ b/tools/testing/selftests/kselftest_harness.h
@@ -51,7 +51,7 @@
 #define __KSELFTEST_HARNESS_H
 
 #ifndef _GNU_SOURCE
-#define _GNU_SOURCE
+static_assert(0, "kselftest harness requires _GNU_SOURCE to be defined");
 #endif
 #include <asm/types.h>
 #include <ctype.h>
diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
index 8ae203d8ed7f..7fa4a96e26ed 100644
--- a/tools/testing/selftests/lib.mk
+++ b/tools/testing/selftests/lib.mk
@@ -53,7 +53,7 @@ selfdir = $(realpath $(dir $(filter %/lib.mk,$(MAKEFILE_LIST))))
 top_srcdir = $(selfdir)/../../..
 
 ifeq ($(KHDR_INCLUDES),)
-KHDR_INCLUDES := -isystem $(top_srcdir)/usr/include
+KHDR_INCLUDES := -D_GNU_SOURCE -isystem $(top_srcdir)/usr/include
 endif
 
 # The following are built by lib.mk common compile rules.