dco linux: avoid redefining ovpn enums (2.6)

Starting with Linux kernel version 6.16, a couple of ovpn-related enum
definitions were introduced in the `include/uapi/linux/if_link.h`
header. Redefining them in openvpn when they are already present in the
system headers can lead to conflicts or build issues.

This commit ensures that enum redefinitions are avoided by conditionally
using the existing definitions from the system header when available.

This is the port to release/2.6 based on commit
1d3c2b6.

Change-Id: I41c5dfc7489352a9534ff6b1585a5a81e0623ab1
Signed-off-by: Frank Lichtenheld <[email protected]>
Acked-by: Antonio Quartulli <[email protected]>
Message-Id: <[email protected]>
URL: https://www.mail-archive.com/[email protected]/msg32470.html
Signed-off-by: Gert Doering <[email protected]>

Author: flichtenheld

src/openvpn/dco_linux.h

@@ -26,11 +26,13 @@
 
 #include "event.h"
 
-#include "ovpn_dco_linux.h"
-
 #include <netlink/socket.h>
 #include <netlink/netlink.h>
 
+/* include last since we need to behave differently if the kernel headers
+ * are from 6.16+ */
+#include "ovpn_dco_linux.h"
+
 typedef enum ovpn_key_slot dco_key_slot_t;
 typedef enum ovpn_cipher_alg dco_cipher_t;
 

src/openvpn/ovpn_dco_linux.h

@@ -237,14 +237,17 @@ enum ovpn_netlink_packet_attrs {
 	OVPN_PACKET_ATTR_MAX = __OVPN_PACKET_ATTR_AFTER_LAST - 1,
 };
 
+#ifndef IFLA_OVPN_MAX
+
 enum ovpn_ifla_attrs {
 	IFLA_OVPN_UNSPEC = 0,
 	IFLA_OVPN_MODE,
 
-	__IFLA_OVPN_AFTER_LAST,
-	IFLA_OVPN_MAX = __IFLA_OVPN_AFTER_LAST - 1,
+	__IFLA_OVPN_MAX,
 };
 
+#define IFLA_OVPN_MAX (__IFLA_OVPN_MAX - 1)
+
 enum ovpn_mode {
 	__OVPN_MODE_FIRST = 0,
 	OVPN_MODE_P2P = __OVPN_MODE_FIRST,
@@ -253,4 +256,6 @@ enum ovpn_mode {
 	__OVPN_MODE_AFTER_LAST,
 };
 
+#endif /* ifndef IFLA_OVPN_MAX */
+
 #endif /* _UAPI_LINUX_OVPN_DCO_H_ */