netlink: Split up functionality of nl_link()

nl_link() performs a number of functions: it can bring links up, set MAC
address and MTU and also retrieve the existing MAC.  This makes for a small
number of lines of code, but high conceptual complexity: it's quite hard
to follow what's going on both in nl_link() itself and it's also not very
obvious which function its callers are intending to use.

Clarify this, by splitting nl_link() into nl_link_up(), nl_link_set_mac(),
and nl_link_get_mac().  The first brings up a link, optionally setting the
MTU, the others get or set the MAC address.

This fixes an arguable bug in pasta_ns_conf(): it looks as though that was
intended to retrieve the guest MAC whether or not c->pasta_conf_ns is set.
However, it only actually does so in the !c->pasta_conf_ns case: the fact
that we set up==1 means we would only ever set, never get, the MAC in the
nl_link() call in the other path.  We get away with this because the MAC
will quickly be discovered once we receive packets on the tap interface.
Still, it's neater to always get the MAC address here.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -671,7 +671,7 @@ static unsigned int conf_ip4(unsigned int ifi,
 	memcpy(&ip4->addr_seen, &ip4->addr, sizeof(ip4->addr_seen));
 
 	if (MAC_IS_ZERO(mac))
-		nl_link(0, ifi, mac, 0, 0);
+		nl_link_get_mac(0, ifi, mac);
 
 	if (IN4_IS_ADDR_UNSPECIFIED(&ip4->addr) ||
 	    MAC_IS_ZERO(mac))
@@ -712,7 +712,7 @@ static unsigned int conf_ip6(unsigned int ifi,
 	memcpy(&ip6->addr_ll_seen, &ip6->addr_ll, sizeof(ip6->addr_ll));
 
 	if (MAC_IS_ZERO(mac))
-		nl_link(0, ifi, mac, 0, 0);
+		nl_link_get_mac(0, ifi, mac);
 
 	if (IN6_IS_ADDR_UNSPECIFIED(&ip6->addr) ||
 	    IN6_IS_ADDR_UNSPECIFIED(&ip6->addr_ll) ||

netlink.c

@@ -486,83 +486,44 @@ next:
 }
 
 /**
- * nl_link() - Get/set link attributes
+ * nl_link_get_mac() - Get link MAC address
  * @ns:		Use netlink socket in namespace
  * @ifi:	Interface index
- * @mac:	MAC address to fill, if passed as zero, to set otherwise
+ * @mac:	Fill with current MAC address
- * @up:		If set, bring up the link
- * @mtu:	If non-zero, set interface MTU
  */
-void nl_link(int ns, unsigned int ifi, void *mac, int up, int mtu)
+void nl_link_get_mac(int ns, unsigned int ifi, void *mac)
 {
-	int change = !MAC_IS_ZERO(mac) || up || mtu;
 	struct req_t {
 		struct nlmsghdr nlh;
 		struct ifinfomsg ifm;
-		struct rtattr rta;
-		union {
-			unsigned char mac[ETH_ALEN];
-			struct {
-				unsigned int mtu;
-			} mtu;
-		} set;
 	} req = {
-		.nlh.nlmsg_type   = change ? RTM_NEWLINK : RTM_GETLINK,
+		.nlh.nlmsg_type	  = RTM_GETLINK,
-		.nlh.nlmsg_len    = NLMSG_LENGTH(sizeof(struct ifinfomsg)),
+		.nlh.nlmsg_len	  = sizeof(req),
-		.nlh.nlmsg_flags  = NLM_F_REQUEST | (change ? NLM_F_ACK : 0),
+		.nlh.nlmsg_flags  = NLM_F_REQUEST | NLM_F_ACK,
 		.nlh.nlmsg_seq	  = nl_seq++,
 		.ifm.ifi_family	  = AF_UNSPEC,
 		.ifm.ifi_index	  = ifi,
-		.ifm.ifi_flags	  = up ? IFF_UP : 0,
-		.ifm.ifi_change	  = up ? IFF_UP : 0,
 	};
-	struct ifinfomsg *ifm;
 	struct nlmsghdr *nh;
-	struct rtattr *rta;
 	char buf[NLBUFSIZ];
 	ssize_t n;
+
+	n = nl_req(ns, buf, &req, sizeof(req));
+	if (n < 0)
+		return;
+
+	for (nh = (struct nlmsghdr *)buf;
+	     NLMSG_OK(nh, n) && nh->nlmsg_type != NLMSG_DONE;
+	     nh = NLMSG_NEXT(nh, n)) {
+		struct ifinfomsg *ifm = (struct ifinfomsg *)NLMSG_DATA(nh);
+		struct rtattr *rta;
 		size_t na;
 
-	if (!MAC_IS_ZERO(mac)) {
-		req.nlh.nlmsg_len = sizeof(req);
-		memcpy(req.set.mac, mac, ETH_ALEN);
-		req.rta.rta_type = IFLA_ADDRESS;
-		req.rta.rta_len = RTA_LENGTH(ETH_ALEN);
-		if (nl_req(ns, buf, &req, req.nlh.nlmsg_len) < 0)
-			return;
-
-		up = 0;
-	}
-
-	if (mtu) {
-		req.nlh.nlmsg_len = offsetof(struct req_t, set.mtu)
-			+ sizeof(req.set.mtu);
-		req.set.mtu.mtu = mtu;
-		req.rta.rta_type = IFLA_MTU;
-		req.rta.rta_len = RTA_LENGTH(sizeof(unsigned int));
-		if (nl_req(ns, buf, &req, req.nlh.nlmsg_len) < 0)
-			return;
-
-		up = 0;
-	}
-
-	if (up && nl_req(ns, buf, &req, req.nlh.nlmsg_len) < 0)
-		return;
-
-	if (change)
-		return;
-
-	if ((n = nl_req(ns, buf, &req, req.nlh.nlmsg_len)) < 0)
-		return;
-
-	nh = (struct nlmsghdr *)buf;
-	for ( ; NLMSG_OK(nh, n); nh = NLMSG_NEXT(nh, n)) {
 		if (nh->nlmsg_type != RTM_NEWLINK)
-			goto next;
+			continue;
 
-		ifm = (struct ifinfomsg *)NLMSG_DATA(nh);
+		for (rta = IFLA_RTA(ifm), na = RTM_PAYLOAD(nh);
-
+		     RTA_OK(rta, na);
-		for (rta = IFLA_RTA(ifm), na = RTM_PAYLOAD(nh); RTA_OK(rta, na);
 		     rta = RTA_NEXT(rta, na)) {
 			if (rta->rta_type != IFLA_ADDRESS)
 				continue;
@@ -570,8 +531,70 @@ void nl_link(int ns, unsigned int ifi, void *mac, int up, int mtu)
 			memcpy(mac, RTA_DATA(rta), ETH_ALEN);
 			break;
 		}
-next:
-		if (nh->nlmsg_type == NLMSG_DONE)
-			break;
 	}
 }
+
+/**
+ * nl_link_set_mac() - Set link MAC address
+ * @ns:		Use netlink socket in namespace
+ * @ifi:	Interface index
+ * @mac:	MAC address to set
+ */
+void nl_link_set_mac(int ns, unsigned int ifi, void *mac)
+{
+	struct req_t {
+		struct nlmsghdr nlh;
+		struct ifinfomsg ifm;
+		struct rtattr rta;
+		unsigned char mac[ETH_ALEN];
+	} req = {
+		.nlh.nlmsg_type	  = RTM_NEWLINK,
+		.nlh.nlmsg_len	  = sizeof(req),
+		.nlh.nlmsg_flags  = NLM_F_REQUEST | NLM_F_ACK,
+		.nlh.nlmsg_seq	  = nl_seq++,
+		.ifm.ifi_family	  = AF_UNSPEC,
+		.ifm.ifi_index	  = ifi,
+		.rta.rta_type	  = IFLA_ADDRESS,
+		.rta.rta_len	  = RTA_LENGTH(ETH_ALEN),
+	};
+	char buf[NLBUFSIZ];
+
+	memcpy(req.mac, mac, ETH_ALEN);
+
+	nl_req(ns, buf, &req, sizeof(req));
+}
+
+/**
+ * nl_link_up() - Bring link up
+ * @ns:		Use netlink socket in namespace
+ * @ifi:	Interface index
+ * @mtu:	If non-zero, set interface MTU
+ */
+void nl_link_up(int ns, unsigned int ifi, int mtu)
+{
+	struct req_t {
+		struct nlmsghdr nlh;
+		struct ifinfomsg ifm;
+		struct rtattr rta;
+		unsigned int mtu;
+	} req = {
+		.nlh.nlmsg_type   = RTM_NEWLINK,
+		.nlh.nlmsg_len    = sizeof(req),
+		.nlh.nlmsg_flags  = NLM_F_REQUEST | NLM_F_ACK,
+		.nlh.nlmsg_seq	  = nl_seq++,
+		.ifm.ifi_family	  = AF_UNSPEC,
+		.ifm.ifi_index	  = ifi,
+		.ifm.ifi_flags	  = IFF_UP,
+		.ifm.ifi_change	  = IFF_UP,
+		.rta.rta_type	  = IFLA_MTU,
+		.rta.rta_len	  = RTA_LENGTH(sizeof(unsigned int)),
+		.mtu		  = mtu,
+	};
+	char buf[NLBUFSIZ];
+
+	if (!mtu)
+		/* Shorten request to drop MTU attribute */
+		req.nlh.nlmsg_len = offsetof(struct req_t, rta);
+
+	nl_req(ns, buf, &req, req.nlh.nlmsg_len);
+}

netlink.h

@@ -18,6 +18,8 @@ void nl_route(enum nl_op op, unsigned int ifi, unsigned int ifi_ns,
 	      sa_family_t af, void *gw);
 void nl_addr(enum nl_op op, unsigned int ifi, unsigned int ifi_ns,
 	     sa_family_t af, void *addr, int *prefix_len, void *addr_l);
-void nl_link(int ns, unsigned int ifi, void *mac, int up, int mtu);
+void nl_link_get_mac(int ns, unsigned int ifi, void *mac);
+void nl_link_set_mac(int ns, unsigned int ifi, void *mac);
+void nl_link_up(int ns, unsigned int ifi, int mtu);
 
 #endif /* NETLINK_H */

pasta.c

@@ -272,13 +272,19 @@ void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid,
  */
 void pasta_ns_conf(struct ctx *c)
 {
-	nl_link(1, 1 /* lo */, MAC_ZERO, 1, 0);
+	nl_link_up(1, 1 /* lo */, 0);
+
+	/* Get or set MAC in target namespace */
+	if (MAC_IS_ZERO(c->mac_guest))
+		nl_link_get_mac(1, c->pasta_ifi, c->mac_guest);
+	else
+		nl_link_set_mac(1, c->pasta_ifi, c->mac_guest);
 
 	if (c->pasta_conf_ns) {
 		enum nl_op op_routes = c->no_copy_routes ? NL_SET : NL_DUP;
 		enum nl_op op_addrs =  c->no_copy_addrs  ? NL_SET : NL_DUP;
 
-		nl_link(1, c->pasta_ifi, c->mac_guest, 1, c->mtu);
+		nl_link_up(1, c->pasta_ifi, c->mtu);
 
 		if (c->ifi4) {
 			nl_addr(op_addrs, c->ifi4, c->pasta_ifi, AF_INET,
@@ -294,8 +300,6 @@ void pasta_ns_conf(struct ctx *c)
 			nl_route(op_routes, c->ifi6, c->pasta_ifi, AF_INET6,
 				 &c->ip6.gw);
 		}
-	} else {
-		nl_link(1, c->pasta_ifi, c->mac_guest, 0, 0);
 	}
 
 	proto_update_l2_buf(c->mac_guest, NULL, NULL);