Fixing fpmsyncd to handle scaled nexthops (sonic-net#3361)

What I did
Fixing fpmsyncd to handle scaled nexthops(e.g. 256 nexthops). In this scenario the max message len is increased to accommodate zebra message size. In addition initialized nlmsg_set_default_size to allow netlink to process the increased message size as well without which libnl would crash.

Why I did it
To support increased nexthops.

fpmsyncd/fpm/fpm.h

@@ -92,7 +92,7 @@
 /*
  * Largest message that can be sent to or received from the FPM.
  */
-#define FPM_MAX_MSG_LEN 4096
+#define FPM_MAX_MSG_LEN 16384
 
 /*
  * Header that precedes each fpm message to/from the FPM.

fpmsyncd/fpmsyncd.cpp

@@ -98,6 +98,7 @@ int main(int argc, char **argv)
     NetDispatcher::getInstance().registerMessageHandler(RTM_DELLINK, &sync);
 
     rtnl_route_read_protocol_names(DefaultRtProtoPath);
+    nlmsg_set_default_size(FPM_MAX_MSG_LEN);
 
     std::string suppressionEnabledStr;
     deviceMetadataTable.hget("localhost", "suppress-fib-pending", suppressionEnabledStr);

fpmsyncd/routesync.cpp

@@ -2235,12 +2235,23 @@ bool RouteSync::sendOffloadReply(struct nlmsghdr* hdr)
 bool RouteSync::sendOffloadReply(struct rtnl_route* route_obj)
 {
     SWSS_LOG_ENTER();
+    int ret = 0;
 
     nl_msg* msg{};
-    rtnl_route_build_add_request(route_obj, NLM_F_CREATE, &msg);
+    ret = rtnl_route_build_add_request(route_obj, NLM_F_CREATE, &msg);
 
+    if (ret !=0)
+    {
+        SWSS_LOG_ERROR("Route build add returned %d", ret);
+        return false;
+    }
     auto nlMsg = makeUniqueWithDestructor(msg, nlmsg_free);
 
+    if (nlMsg.get() == NULL)
+    {
+        SWSS_LOG_ERROR("Error in allocation for sending offload reply");
+        return false;
+    }
     return sendOffloadReply(nlmsg_hdr(nlMsg.get()));
 }
 

tests/mock_tests/fake_netlink.cpp

@@ -1,12 +1,15 @@
 #include <swss/linkcache.h>
 #include <swss/logger.h>
+#include <netlink/route/route.h>
 
 static rtnl_link* g_fakeLink = [](){
     auto fakeLink = rtnl_link_alloc();
     rtnl_link_set_ifindex(fakeLink, 42);
     return fakeLink;
 }();
 
+extern int rt_build_ret;
+extern bool nlmsg_alloc_ret;
 extern "C"
 {
 
@@ -15,4 +18,34 @@ struct rtnl_link* rtnl_link_get_by_name(struct nl_cache *cache, const char *name
     return g_fakeLink;
 }
 
+static int build_route_msg(struct rtnl_route *tmpl, int cmd, int flags,
+			   struct nl_msg **result)
+{
+	struct nl_msg *msg;
+	int err;
+	if (!(msg = nlmsg_alloc_simple(cmd, flags)))
+		return -NLE_NOMEM;
+	if ((err = rtnl_route_build_msg(msg, tmpl)) < 0) {
+		nlmsg_free(msg);
+		return err;
+	}
+	*result = msg;
+	return 0;
+}
+
+int rtnl_route_build_add_request(struct rtnl_route *tmpl, int flags,
+				 struct nl_msg **result)
+{
+    if (rt_build_ret != 0)
+    {
+        return rt_build_ret;
+    }
+    else if (!nlmsg_alloc_ret)
+    {
+	*result = NULL;
+        return 0;
+    }
+    return build_route_msg(tmpl, RTM_NEWROUTE, NLM_F_CREATE | flags,
+                           result);
+}
 }

tests/mock_tests/fpmsyncd/test_routesync.cpp

@@ -12,6 +12,8 @@ using namespace swss;
 
 using ::testing::_;
 
+int rt_build_ret = 0;
+bool nlmsg_alloc_ret = true;
 class MockRouteSync : public RouteSync
 {
 public:
@@ -232,3 +234,18 @@ TEST_F(FpmSyncdResponseTest, testEvpn)
     ASSERT_EQ(value.get(), "0xc8");
 
 }
+
+TEST_F(FpmSyncdResponseTest, testSendOffloadReply)
+{
+
+    rt_build_ret = 1;
+    rtnl_route* routeObject{};
+
+
+    ASSERT_EQ(m_routeSync.sendOffloadReply(routeObject), false);
+    rt_build_ret = 0;
+    nlmsg_alloc_ret = false;
+    ASSERT_EQ(m_routeSync.sendOffloadReply(routeObject), false);
+    nlmsg_alloc_ret = true;
+
+}