From d102bf90eaed8020064a2a5bc107b4cf7949d60b Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benpicco@beuth-hochschule.de>
Date: Thu, 9 Jun 2022 16:54:30 +0200
Subject: [PATCH] gnrc/ipv6_auto_subnets: always send RIO to upstream network

In a881af8b08f39 we would return early if the subnet did not change,
but we must still send the router advertisement with the route information
option to the upstream network, otherwise hosts in that network will not
consider the downstream subnet off-link.
---
 .../gnrc_ipv6_auto_subnets.c                  | 33 +++++++++----------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/sys/net/gnrc/routing/ipv6_auto_subnets/gnrc_ipv6_auto_subnets.c b/sys/net/gnrc/routing/ipv6_auto_subnets/gnrc_ipv6_auto_subnets.c
index 05d0a53491..a012128d0d 100644
--- a/sys/net/gnrc/routing/ipv6_auto_subnets/gnrc_ipv6_auto_subnets.c
+++ b/sys/net/gnrc/routing/ipv6_auto_subnets/gnrc_ipv6_auto_subnets.c
@@ -255,6 +255,7 @@ static void _init_sub_prefix(ipv6_addr_t *out,
     out->u8[bytes] |= idx << shift;
 }
 
+/* returns true if a new prefix was added, false if nothing changed */
 static bool _remove_old_prefix(gnrc_netif_t *netif,
                                const ipv6_addr_t *pfx, uint8_t pfx_len,
                                gnrc_pktsnip_t **ext_opts)
@@ -271,7 +272,7 @@ static bool _remove_old_prefix(gnrc_netif_t *netif,
 
         /* The prefix did not change - nothing to do here */
         if (match_len >= pfx_len && pfx_len == entry.pfx_len) {
-            return true;
+            return false;
         }
 
         /* find prefix that is closest to the new prefix */
@@ -283,7 +284,7 @@ static bool _remove_old_prefix(gnrc_netif_t *netif,
 
     /* no prefix found */
     if (old_pfx_len == CONFIG_GNRC_IPV6_AUTO_SUBNETS_PREFIX_FIX_LEN) {
-        return false;
+        return true;
     }
 
     DEBUG("auto_subnets: remove old prefix %s/%u\n",
@@ -299,7 +300,7 @@ static bool _remove_old_prefix(gnrc_netif_t *netif,
     /* remove the prefix */
     gnrc_ipv6_nib_pl_del(netif->pid, &old_pfx, old_pfx_len);
 
-    return false;
+    return true;
 }
 
 static void _configure_subnets(uint8_t subnets, uint8_t start_idx, gnrc_netif_t *upstream,
@@ -346,20 +347,21 @@ static void _configure_subnets(uint8_t subnets, uint8_t start_idx, gnrc_netif_t
 
         /* first remove old prefix if the prefix changed */
         if (_remove_old_prefix(downstream, &new_prefix, new_prefix_len, &ext_opts)) {
-            /* if the prefix did not change, there is nothing to do here */
-            continue;
-        }
 
-        /* configure subnet on downstream interface */
-        idx = gnrc_netif_ipv6_add_prefix(downstream, &new_prefix, new_prefix_len,
+            /* configure subnet on downstream interface */
+            idx = gnrc_netif_ipv6_add_prefix(downstream, &new_prefix, new_prefix_len,
                                          valid_ltime, pref_ltime);
-        if (idx < 0) {
-            DEBUG("auto_subnets: adding prefix to %u failed\n", downstream->pid);
-            continue;
-        }
+            if (idx < 0) {
+                DEBUG("auto_subnets: adding prefix to %u failed\n", downstream->pid);
+                continue;
+            }
 
-        /* start advertising subnet */
-        gnrc_ipv6_nib_change_rtr_adv_iface(downstream, true);
+            /* start advertising subnet */
+            gnrc_ipv6_nib_change_rtr_adv_iface(downstream, true);
+
+            /* configure RPL root if applicable */
+            gnrc_rpl_configure_root(downstream, &downstream->ipv6.addrs[idx]);
+        }
 
         /* add route information option with new subnet */
         tmp = gnrc_ndp_opt_ri_build(&new_prefix, new_prefix_len, valid_ltime,
@@ -369,9 +371,6 @@ static void _configure_subnets(uint8_t subnets, uint8_t start_idx, gnrc_netif_t
         } else {
             ext_opts = tmp;
         }
-
-        /* configure RPL root if applicable */
-        gnrc_rpl_configure_root(downstream, &downstream->ipv6.addrs[idx]);
     }
 
     /* immediately send an RA with RIO */