ipv6 reassembly: fix detecting holes in reassembled packets

Added unit test for reassembly.

See bug #64767

src/core/ipv6/ip6_frag.c

@@ -447,6 +447,19 @@ ip6_reass(struct pbuf *p)
         }
       }
 #endif /* IP_REASS_CHECK_OVERLAP */
+      /* Check if the fragments received so far have no gaps. */
+      if (iprh_prev != NULL) {
+        if (iprh_prev->end != start) {
+          /* There is a fragment missing between the current
+           * and the previous fragment */
+          valid = 0;
+        }
+      }
+      if (end != iprh_tmp->start) {
+        /* There is a fragment missing between the current
+         * and the following fragment */
+        valid = 0;
+      }
       /* the new pbuf should be inserted before this */
       next_pbuf = q;
       if (iprh_prev != NULL) {
@@ -658,6 +671,7 @@ ip6_reass(struct pbuf *p)
     }
 
     /* Return the pbuf chain */
+    MIB2_STATS_INC(mib2.ip6reasmoks);
     return p;
   }
   /* the datagram is not (yet?) reassembled completely */

src/include/lwip/stats.h

@@ -122,7 +122,7 @@ struct stats_sys {
 
 /** SNMP MIB2 stats */
 struct stats_mib2 {
-  /* IP */
+  /* IPv4 */
   u32_t ipinhdrerrors;
   u32_t ipinaddrerrors;
   u32_t ipinunknownprotos;
@@ -140,6 +140,9 @@ struct stats_mib2 {
   u32_t ipforwdatagrams;
   u32_t ipinreceives;
 
+  /* IPv6 */
+  u32_t ip6reasmoks;
+
   /* TCP */
   u32_t tcpactiveopens;
   u32_t tcppassiveopens;