fixup! fixup! gnrc/ipv6_auto_subnets: unicast RA if upstream is 6lo

sys/net/gnrc/routing/ipv6_auto_subnets/gnrc_ipv6_auto_subnets.c

@@ -100,6 +100,7 @@
  */
 
 #include "compiler_hints.h"
+#include "macros/utils.h"
 #include "net/gnrc/ipv6.h"
 #include "net/gnrc/netif.h"
 #include "net/gnrc/netif/hdr.h"
@@ -110,12 +111,6 @@
 #include "random.h"
 #include "xtimer.h"
 
-/* If we derive the subnet from the interface's EUI, we have to use all
-   available prefix bits to ensure uniqueness */
-#if IS_USED(MODULE_GNRC_IPV6_AUTO_SUBNETS_EUI)
-#define CONFIG_GNRC_IPV6_AUTO_SUBNETS_PREFIX_MIN_LEN (64)
-#endif
-
 /**
  * @brief Port for the custom UDP sync protocol
  */
@@ -335,11 +330,18 @@ static void _init_sub_prefix_eui(ipv6_addr_t *out,
         uint8_t u8[8];
     } eui64 = {};
 
+    /* If EUI is small, we want to preserve leftover unused bits at the end */
+    uint8_t bits_total = bits + 8 * eui_len;
+    uint8_t shift = bits_total < 64
+                  ? 64 - bits_total
+                  : 0;
+
     /* treat EUI as a EUI-64 with unused bytes set to 0 */
     memcpy(&eui64.u8[sizeof(uint64_t) - eui_len], eui, eui_len);
 
     /* create downstream prefix from upstream prefix + masked EUI64 */
-    out->u64[0].u64 = prefix->u64[0].u64 | (eui64.u64 & mask);
+    out->u64[0].u64 = prefix->u64[0].u64
+                    | ((eui64.u64 & mask) >> shift);
     out->u64[1].u64 = 0;
 }
 
@@ -433,13 +435,16 @@ static void _configure_subnets(uint8_t subnets, uint8_t start_idx, gnrc_netif_t
         /* create subnet from upstream prefix */
         if (IS_USED(MODULE_GNRC_IPV6_AUTO_SUBNETS_EUI)) {
             uint8_t hwaddr[GNRC_NETIF_L2ADDR_MAXLEN];
-            int res = netif_get_opt(&downstream->netif, NETOPT_ADDRESS, 0,
-                                    hwaddr, sizeof(hwaddr));
-            if (res <= 0) {
+            int hwaddr_len = netif_get_opt(&downstream->netif, NETOPT_ADDRESS, 0,
+                                           hwaddr, sizeof(hwaddr));
+            if (hwaddr_len <= 0) {
                 DEBUG("auto_subnets: can't get l2 address from netif %u\n", downstream->pid);
                 continue;
             }
-            _init_sub_prefix_eui(&new_prefix, prefix, prefix_len, hwaddr, res);
+            _init_sub_prefix_eui(&new_prefix, prefix, prefix_len, hwaddr, hwaddr_len);
+
+            new_prefix_len = prefix_len + 8 * hwaddr_len;
+            new_prefix_len = MAX(new_prefix_len, CONFIG_GNRC_IPV6_AUTO_SUBNETS_PREFIX_MIN_LEN);
         } else {
             _init_sub_prefix(&new_prefix, prefix, prefix_len, ++start_idx, subnet_len);
         }