[dplane_fpm_sonic]: Fix for SRv6 SIDs learnt from the kernel (#557)

The current SRv6 test cases are implemented by installing the SRv6 SIDs in the kernel. Then the kernel notifies FRR about these SIDs. The current Netlink messages from the kernel does not provide the SID information (block length, node length, function length, argument length).

This PR extends the `dplane_fpm_sonic` to handle this issue by setting the SID information to the locator defaults.

src/sonic-frr/dplane_fpm_sonic/dplane_fpm_sonic.c

@@ -69,6 +69,12 @@
  */
 #define FPM_HEADER_SIZE 4
 
+/* Default SRv6 SID format values */
+DEFAULT_SRV6_LOCALSID_FORMAT_BLOCK_LEN = 32;
+DEFAULT_SRV6_LOCALSID_FORMAT_NODE_LEN = 16;
+DEFAULT_SRV6_LOCALSID_FORMAT_FUNCTION_LEN = 16;
+DEFAULT_SRV6_LOCALSID_FORMAT_ARGUMENT_LEN = 0;
+
 /**
  * Custom Netlink TLVs
 */
@@ -951,6 +957,7 @@ static ssize_t netlink_srv6_localsid_msg_encode(int cmd,
 	vrf_id_t vrf_id;
 	uint32_t table_id;
 	uint32_t action;
+	uint32_t block_len, node_len, func_len, arg_len;
 
 	struct {
 		struct nlmsghdr n;
@@ -1035,28 +1042,44 @@ static ssize_t netlink_srv6_localsid_msg_encode(int cmd,
 		nl_attr_nest(&req->n, datalen, 
 					FPM_SRV6_LOCALSID_FORMAT);
 
+	block_len = nexthop->nh_srv6->seg6local_ctx.block_len;
+	node_len = nexthop->nh_srv6->seg6local_ctx.node_len;
+	func_len = nexthop->nh_srv6->seg6local_ctx.function_len;
+	arg_len = nexthop->nh_srv6->seg6local_ctx.argument_len;
+
+	/*
+	 * If block/node/func/arg length are not provided by the srv6 nexthop,
+	 * then we use the default values
+	 */
+	if (block_len == 0 && node_len == 0 && func_len == 0 && arg_len == 0) {
+		block_len = DEFAULT_SRV6_LOCALSID_FORMAT_BLOCK_LEN;
+		node_len = DEFAULT_SRV6_LOCALSID_FORMAT_NODE_LEN;
+		func_len = DEFAULT_SRV6_LOCALSID_FORMAT_FUNCTION_LEN;
+		arg_len = DEFAULT_SRV6_LOCALSID_FORMAT_ARGUMENT_LEN;
+	}
+
 	if (!nl_attr_put8(
 			&req->n, datalen, 
 			FPM_SRV6_LOCALSID_FORMAT_BLOCK_LEN,
-			nexthop->nh_srv6->seg6local_ctx.block_len))
+			block_len))
 		return -1;
 
 	if (!nl_attr_put8(
 			&req->n, datalen, 
 			FPM_SRV6_LOCALSID_FORMAT_NODE_LEN,
-			nexthop->nh_srv6->seg6local_ctx.node_len))
+			node_len))
 		return -1;
 
 	if (!nl_attr_put8(
 			&req->n, datalen, 
 			FPM_SRV6_LOCALSID_FORMAT_FUNC_LEN,
-			nexthop->nh_srv6->seg6local_ctx.function_len))
+			func_len))
 		return -1;
 
 	if (!nl_attr_put8(
 			&req->n, datalen, 
 			FPM_SRV6_LOCALSID_FORMAT_ARG_LEN,
-			nexthop->nh_srv6->seg6local_ctx.argument_len))
+			arg_len))
 		return -1;
 
 	nl_attr_nest_end(&req->n, nest);