From 647909bcf34ba959bffcdfc6aeed5f23a8984018 Mon Sep 17 00:00:00 2001
From: Stephen Hemminger <stephen@networkplumber.org>
Date: Fri, 24 Apr 2020 16:36:57 -0700
Subject: [PATCH] net/tap: use netlink extended ack support

In recent Linux kernels, there is support for extended acknowledgment
to netlink messages. This is quite useful for diagnosing errors
in configuration in the kernel with TAP.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Keith Wiles <keith.wiles@intel.com>
---
 drivers/net/tap/tap_netlink.c | 80 +++++++++++++++++++++++++++++++++++
 1 file changed, 80 insertions(+)

diff --git a/drivers/net/tap/tap_netlink.c b/drivers/net/tap/tap_netlink.c
index 64220178a6..b73802b51e 100644
--- a/drivers/net/tap/tap_netlink.c
+++ b/drivers/net/tap/tap_netlink.c
@@ -9,10 +9,12 @@
 #include <string.h>
 #include <sys/socket.h>
 #include <unistd.h>
+#include <stdbool.h>
 
 #include <rte_malloc.h>
 #include <tap_netlink.h>
 #include <rte_random.h>
+
 #include "tap_log.h"
 
 /* Must be quite large to support dumping a huge list of QDISC or filters. */
@@ -43,6 +45,9 @@ tap_nl_init(uint32_t nl_groups)
 		.nl_family = AF_NETLINK,
 		.nl_groups = nl_groups,
 	};
+#ifdef NETLINK_EXT_ACK
+	int one = 1;
+#endif
 
 	fd = socket(AF_NETLINK, SOCK_RAW | SOCK_CLOEXEC, NETLINK_ROUTE);
 	if (fd < 0) {
@@ -59,6 +64,12 @@ tap_nl_init(uint32_t nl_groups)
 		close(fd);
 		return -1;
 	}
+
+#ifdef NETLINK_EXT_ACK
+	/* Ask for extended ACK response. on older kernel will ignore request. */
+	setsockopt(fd, SOL_NETLINK, NETLINK_EXT_ACK, &one, sizeof(one));
+#endif
+
 	if (bind(fd, (struct sockaddr *)&local, sizeof(local)) < 0) {
 		TAP_LOG(ERR, "Unable to bind to the netlink socket");
 		close(fd);
@@ -119,6 +130,74 @@ retry:
 	return send_bytes;
 }
 
+#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
+
 /**
  * Check that the kernel sends an appropriate ACK in response
  * to an tap_nl_send().
@@ -174,6 +253,7 @@ retry:
 			if (nh->nlmsg_type == NLMSG_ERROR) {
 				struct nlmsgerr *err_data = NLMSG_DATA(nh);
 
+				tap_nl_dump_ext_ack(nh, err_data);
 				if (err_data->error < 0) {
 					errno = -err_data->error;
 					return -1;
-- 
2.20.1