+#ifdef NETLINK_EXT_ACK
+static const struct nlattr *
+tap_nl_attr_first(const struct nlmsghdr *nh, size_t offset)
+{
+ return (const struct nlattr *)((const char *)nh + NLMSG_SPACE(offset));
+}
+
+static const struct nlattr *
+tap_nl_attr_next(const struct nlattr *attr)
+{
+ return (const struct nlattr *)((const char *)attr
+ + NLMSG_ALIGN(attr->nla_len));
+}
+
+static bool
+tap_nl_attr_ok(const struct nlattr *attr, int len)
+{
+ if (len < (int)sizeof(struct nlattr))
+ return false; /* missing header */
+ if (attr->nla_len < sizeof(struct nlattr))
+ return false; /* attribute length should include itself */
+ if ((int)attr->nla_len > len)
+ return false; /* attribute is truncated */
+ return true;
+}
+
+
+/* Decode extended errors from kernel */
+static void
+tap_nl_dump_ext_ack(const struct nlmsghdr *nh, const struct nlmsgerr *err)
+{
+ const struct nlattr *attr;
+ const char *tail = (const char *)nh + NLMSG_ALIGN(nh->nlmsg_len);
+ size_t hlen = sizeof(*err);
+
+ /* no TLVs, no extended response */
+ if (!(nh->nlmsg_flags & NLM_F_ACK_TLVS))
+ return;
+
+ if (!(nh->nlmsg_flags & NLM_F_CAPPED))
+ hlen += err->msg.nlmsg_len - NLMSG_HDRLEN;
+
+ for (attr = tap_nl_attr_first(nh, hlen);
+ tap_nl_attr_ok(attr, tail - (const char *)attr);
+ attr = tap_nl_attr_next(attr)) {
+ uint16_t type = attr->nla_type & NLA_TYPE_MASK;
+
+ if (type == NLMSGERR_ATTR_MSG) {
+ const char *msg = (const char *)attr
+ + NLMSG_ALIGN(sizeof(*attr));
+
+ if (err->error)
+ TAP_LOG(ERR, "%s", msg);
+ else
+
+ TAP_LOG(WARNING, "%s", msg);
+ break;
+ }
+ }
+}
+#else
+/*
+ * External ACK support was added in Linux kernel 4.17
+ * on older kernels, just ignore that part of message
+ */
+#define tap_nl_dump_ext_ack(nh, err) do { } while (0)
+#endif
+