DCO Linux: Fix setting DCO ifmode failing on big endian archs

The problem is that SITNL_ADDATTR is not forcing type safety and on
big endian architcutre passing a smaller size than the underlying
integer type of data causes only the more significant byte(s) to be passed
instead.

A proper fix would be to add specific methods for common integer types like
SITNL_ADDATTR_u8, SITNL_ADDATTR_u16, SITNL_ADDATTR_u32 like netlink library
does with NLA_PUT_U32, NLA_PUT_U16, NLA_PUT_U8.

Change-Id: I560f45fb0011180be8ca2b0e7fbc63030fa10f35
Github: closes OpenVPN/ovpn-dco#96
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <antonio@mandelbit.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1547
Message-Id: <20260219110954.21471-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg35752.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit b3e0e86)

Author: schwabe

src/openvpn/networking_sitnl.c

@@ -1356,7 +1356,15 @@ net_iface_new(openvpn_net_ctx_t *ctx, const char *iface, const char *type,
     {
         dco_context_t *dco = arg;
         struct rtattr *data = SITNL_NEST(&req.n, sizeof(req), IFLA_INFO_DATA);
-        SITNL_ADDATTR(&req.n, sizeof(req), IFLA_OVPN_MODE, &dco->ifmode,
+
+        /* the netlink format is uint8_t for this and using something
+         * other than uint8_t here (enum underlying type is undefined but
+         * commonly int) causes the values to be 0 when passed
+         * on big endian arch as we only take the (biggest endian) byte
+         * directly at the address
+         */
+        uint8_t ifmode = (uint8_t)dco->ifmode;
+        SITNL_ADDATTR(&req.n, sizeof(req), IFLA_OVPN_MODE, &ifmode,
                       sizeof(uint8_t));
         SITNL_NEST_END(&req.n, data);
     }