diff --git a/libcomposefs/lcfs-fsverity.c b/libcomposefs/lcfs-fsverity.c
index 5e7a6cb..4d4e6cb 100644
--- a/libcomposefs/lcfs-fsverity.c
+++ b/libcomposefs/lcfs-fsverity.c
@@ -14,6 +14,8 @@
    You should have received a copy of the GNU Lesser General Public License
    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
 
+#define _GNU_SOURCE
+
 #include "config.h"
 
 #include <sys/stat.h>