diff --git a/libc/kernel/uapi/linux/ethtool.h b/libc/kernel/uapi/linux/ethtool.h
index baf417d..741ea2a 100644
--- a/libc/kernel/uapi/linux/ethtool.h
+++ b/libc/kernel/uapi/linux/ethtool.h
@@ -78,6 +78,7 @@
   ETHTOOL_RX_COPYBREAK,
   ETHTOOL_TX_COPYBREAK,
   ETHTOOL_PFC_PREVENTION_TOUT,
+  ETHTOOL_TX_COPYBREAK_BUF_SIZE,
   __ETHTOOL_TUNABLE_COUNT,
 };
 enum tunable_type_id {