netlink: Add support to fetch default gateway from multipath routes

If the default route for a given IP version is a multipath one,
instead of refusing to start because there's no RTA_GATEWAY attribute
in the set returned by the kernel, we can just pick one of the paths.

To make this somewhat less arbitrary, pick the path with the highest
weight, if weights differ.

Reported-by: Ed Santiago <santiago@redhat.com>
Link: https://github.com/containers/podman/issues/20927
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 48 insertions, 3 deletions

diff --git a/netlink.c b/netlink.c
index bf79dd4..f0b04cb 100644
--- a/netlink.c
+++ b/netlink.c
@@ -271,12 +271,17 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
 
 		for (rta = RTM_RTA(rtm), na = RTM_PAYLOAD(nh); RTA_OK(rta, na);
 		     rta = RTA_NEXT(rta, na)) {
-			if (rta->rta_type != RTA_OIF)
-				continue;
+			if (rta->rta_type == RTA_OIF) {
+				ifi = *(unsigned int *)RTA_DATA(rta);
+			} else if (rta->rta_type == RTA_MULTIPATH) {
+				struct rtnexthop *rtnh;
 
-			ifi = *(unsigned int *)RTA_DATA(rta);
+				rtnh = (struct rtnexthop *)RTA_DATA(rta);
+				ifi = rtnh->rtnh_ifindex;
+			}
 		}
 	}
+
 	if (status < 0)
 		warn("netlink: RTM_GETROUTE failed: %s", strerror(-status));
 
@@ -284,6 +289,43 @@ unsigned int nl_get_ext_if(int s, sa_family_t af)
 }
 
 /**
+ * nl_route_get_def_multipath() - Get lowest-weight route from nexthop list
+ * @rta:	Routing netlink attribute with type RTA_MULTIPATH
+ * @gw:		Default gateway to fill
+ *
+ * Return: true if a gateway was found, false otherwise
+ */
+bool nl_route_get_def_multipath(struct rtattr *rta, void *gw)
+{
+	struct rtnexthop *rtnh;
+	bool found = false;
+	int hops = -1;
+
+	for (rtnh = (struct rtnexthop *)RTA_DATA(rta);
+	     RTNH_OK(rtnh, RTA_PAYLOAD(rta)); rtnh = RTNH_NEXT(rtnh)) {
+		size_t len = rtnh->rtnh_len - sizeof(*rtnh);
+		struct rtattr *rta_inner;
+
+		if (rtnh->rtnh_hops < hops)
+			continue;
+
+		hops = rtnh->rtnh_hops;
+
+		for (rta_inner = RTNH_DATA(rtnh); RTA_OK(rta_inner, len);
+		     rta_inner = RTA_NEXT(rta_inner, len)) {
+
+			if (rta_inner->rta_type != RTA_GATEWAY)
+				continue;
+
+			memcpy(gw, RTA_DATA(rta_inner), RTA_PAYLOAD(rta_inner));
+			found = true;
+		}
+	}
+
+	return found;
+}
+
+/**
  * nl_route_get_def() - Get default route for given interface and address family
  * @s:		Netlink socket
  * @ifi:	Interface index
@@ -326,6 +368,9 @@ int nl_route_get_def(int s, unsigned int ifi, sa_family_t af, void *gw)
 
 		for (rta = RTM_RTA(rtm), na = RTM_PAYLOAD(nh); RTA_OK(rta, na);
 		     rta = RTA_NEXT(rta, na)) {
+			if (rta->rta_type == RTA_MULTIPATH)
+				found = nl_route_get_def_multipath(rta, gw);
+
 			if (rta->rta_type != RTA_GATEWAY)
 				continue;