platform: print error message from netlink extended ack

From v4.12 the kernel appends some attributes to netlink acks
containing a textual description of the error and other fields (see
commit [1]). Parse those attributes and print the error message.

Examples:

platform-linux: netlink: recvmsg: error message from kernel: Network is unreachable (101) "Nexthop has invalid gateway" for request 12

platform-linux: netlink: recvmsg: error message from kernel: Invalid argument (22) "Local address cannot be multicast" for request 21

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2d4bc93368f5a0ddb57c8c885cdad9c9b7a10ed5

src/platform/nm-linux-platform.c

@@ -6596,11 +6596,30 @@ continue_reading:
 				abort_parsing = TRUE;
 			} else if (e->error) {
 				int errsv = e->error > 0 ? e->error : -e->error;
+				const char *extack_msg = NULL;
+
+				if (   NM_FLAGS_HAS (hdr->nlmsg_flags, NLM_F_ACK_TLVS)
+				    && hdr->nlmsg_len >= sizeof (*e) + e->msg.nlmsg_len) {
+					static const struct nla_policy policy[NLMSGERR_ATTR_MAX + 1] = {
+						[NLMSGERR_ATTR_MSG]     = { .type = NLA_STRING },
+						[NLMSGERR_ATTR_OFFS]    = { .type = NLA_U32 },
+					};
+					struct nlattr *tb[NLMSGERR_ATTR_MAX + 1];
+					struct nlattr *tlvs;
+
+					tlvs = (struct nlattr *) ((char *) e + sizeof (*e) + e->msg.nlmsg_len - NLMSG_HDRLEN);
+					if (!nla_parse (tb, NLMSGERR_ATTR_MAX, tlvs,
+					                hdr->nlmsg_len - sizeof (*e) - e->msg.nlmsg_len, policy)) {
+						if (tb[NLMSGERR_ATTR_MSG])
+							extack_msg = nla_get_string (tb[NLMSGERR_ATTR_MSG]);
+					}
+				}
 
 				/* Error message reported back from kernel. */
-				_LOGD ("netlink: recvmsg: error message from kernel: %s (%d) for request %d",
+				_LOGD ("netlink: recvmsg: error message from kernel: %s (%d)%s%s%s for request %d",
 				       strerror (errsv),
 				       errsv,
+				       NM_PRINT_FMT_QUOTED (extack_msg, " \"", extack_msg, "\"", ""),
 				       nlmsg_hdr (msg)->nlmsg_seq);
 				seq_result = -errsv;
 			} else
@@ -6948,6 +6967,10 @@ constructed (GObject *_object)
 	nle = nl_socket_set_buffer_size (priv->nlh, 8*1024*1024, 0);
 	g_assert (!nle);
 
+	nle = nl_socket_set_ext_ack (priv->nlh, TRUE);
+	if (nle)
+		_LOGD ("could not enable extended acks on netlink socket");
+
 	/* explicitly set the msg buffer size and disable MSG_PEEK.
 	 * If we later encounter NLE_MSG_TRUNC, we will adjust the buffer size. */
 	nl_socket_disable_msg_peek (priv->nlh);

src/platform/nm-netlink.c

@@ -38,6 +38,10 @@
 #define NL_MSG_PEEK_EXPLICIT    (1<<4)
 #define NL_NO_AUTO_ACK          (1<<5)
 
+#ifndef NETLINK_EXT_ACK
+#define NETLINK_EXT_ACK         11
+#endif
+
 #define NL_MSG_CRED_PRESENT 1
 
 struct nl_msg {
@@ -933,6 +937,22 @@ nl_socket_add_memberships (struct nl_sock *sk, int group, ...)
 	return 0;
 }
 
+int
+nl_socket_set_ext_ack (struct nl_sock *sk, gboolean enable)
+{
+	int err, val;
+
+	if (sk->s_fd == -1)
+		return -NLE_BAD_SOCK;
+
+	val = !!enable;
+	err = setsockopt (sk->s_fd, SOL_NETLINK, NETLINK_EXT_ACK, &val, sizeof (val));
+	if (err < 0)
+		return -nl_syserr2nlerr (errno);
+
+	return 0;
+}
+
 void nl_socket_disable_msg_peek (struct nl_sock *sk)
 {
 	sk->s_flags |= NL_MSG_PEEK_EXPLICIT;

src/platform/nm-netlink.h

@@ -26,7 +26,6 @@
 #include <linux/genetlink.h>
 
 /*****************************************************************************/
-
 #define _NLE_BASE               100000
 #define NLE_UNSPEC              (_NLE_BASE +  0)
 #define NLE_BUG                 (_NLE_BASE +  1)
@@ -42,6 +41,16 @@
 
 #define _NLE_BASE_END           (_NLE_BASE + 11)
 
+#define NLMSGERR_ATTR_UNUSED            0
+#define NLMSGERR_ATTR_MSG               1
+#define NLMSGERR_ATTR_OFFS              2
+#define NLMSGERR_ATTR_COOKIE            3
+#define NLMSGERR_ATTR_MAX               3
+
+#ifndef NLM_F_ACK_TLVS
+#define NLM_F_ACK_TLVS                  0x200
+#endif
+
 static inline int
 nl_errno (int err)
 {
@@ -484,6 +493,8 @@ int nl_recvmsgs (struct nl_sock *sk, const struct nl_cb *cb);
 int nl_wait_for_ack (struct nl_sock *sk,
                      const struct nl_cb *cb);
 
+int nl_socket_set_ext_ack (struct nl_sock *sk, gboolean enable);
+
 /*****************************************************************************/
 
 #endif /* __NM_NETLINK_H__ */