From ebd5751cdeb4c753c6c9df31b82dcd6afee2cd39 Mon Sep 17 00:00:00 2001
From: Maria Matejka <mq@ucw.cz>
Date: Sun, 30 May 2021 13:07:16 +0200
Subject: [PATCH] Babel: Seqno requests are properly decoupled from neighbors
 when the underlying interface disappears

When an interface disappears, all the neighbors are freed as well. Seqno
requests were anyway not decoupled from them, leading to strange
segfaults. This fix adds a proper seqno request list inside neighbors to
make sure that no pointer to neighbor is kept after free.
---
 lib/lists.h         |  2 ++
 proto/babel/babel.c | 30 ++++++++++++++++++------------
 proto/babel/babel.h |  3 ++-
 3 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/lib/lists.h b/lib/lists.h
index 066eafbb..479f4ed1 100644
--- a/lib/lists.h
+++ b/lib/lists.h
@@ -59,6 +59,8 @@ typedef union list {			/* In fact two overlayed nodes */
 /* WALK_LIST_FIRST supposes that called code removes each processed node */
 #define WALK_LIST_FIRST(n,list) \
      while(n=HEAD(list), (NODE (n))->next)
+#define WALK_LIST_FIRST2(n,pos,list) \
+     while(n=SKIP_BACK(typeof(*n),pos,HEAD(list)), (n)->pos.next)
 #define WALK_LIST_BACKWARDS(n,list) for(n=TAIL(list);(NODE (n))->prev; \
 				n=(void *)((NODE (n))->prev))
 #define WALK_LIST_BACKWARDS_DELSAFE(n,prv,list) \
diff --git a/proto/babel/babel.c b/proto/babel/babel.c
index 25d8e330..fdebc352 100644
--- a/proto/babel/babel.c
+++ b/proto/babel/babel.c
@@ -56,13 +56,6 @@ static void babel_update_cost(struct babel_neighbor *n);
 static inline void babel_kick_timer(struct babel_proto *p);
 static inline void babel_iface_kick_timer(struct babel_iface *ifa);
 
-static inline void babel_lock_neighbor(struct babel_neighbor *nbr)
-{ if (nbr) nbr->uc++; }
-
-static inline void babel_unlock_neighbor(struct babel_neighbor *nbr)
-{ if (nbr && !--nbr->uc) mb_free(nbr); }
-
-
 /*
  *	Functions to maintain data structures
  */
@@ -316,8 +309,9 @@ babel_add_seqno_request(struct babel_proto *p, struct babel_entry *e,
 	return;
 
       /* Found older */
-      babel_unlock_neighbor(sr->nbr);
       rem_node(NODE sr);
+      rem_node(&sr->nbr_node);
+
       goto found;
     }
 
@@ -330,7 +324,10 @@ found:
   sr->hop_count = hop_count;
   sr->count = 0;
   sr->expires = current_time() + BABEL_SEQNO_REQUEST_EXPIRY;
-  babel_lock_neighbor(sr->nbr = nbr);
+
+  if (sr->nbr = nbr)
+    add_tail(&nbr->requests, &sr->nbr_node);
+
   add_tail(&e->requests, NODE sr);
 
   babel_send_seqno_request(p, e, sr);
@@ -339,7 +336,9 @@ found:
 static void
 babel_remove_seqno_request(struct babel_proto *p, struct babel_seqno_request *sr)
 {
-  babel_unlock_neighbor(sr->nbr);
+  if (sr->nbr)
+    rem_node(&sr->nbr_node);
+
   rem_node(NODE sr);
   sl_free(p->seqno_slab, sr);
 }
@@ -427,7 +426,7 @@ babel_get_neighbor(struct babel_iface *ifa, ip_addr addr)
   nbr->txcost = BABEL_INFINITY;
   nbr->cost = BABEL_INFINITY;
   init_list(&nbr->routes);
-  babel_lock_neighbor(nbr);
+  init_list(&nbr->requests);
   add_tail(&ifa->neigh_list, NODE nbr);
 
   return nbr;
@@ -448,9 +447,16 @@ babel_flush_neighbor(struct babel_proto *p, struct babel_neighbor *nbr)
     babel_flush_route(p, r);
   }
 
+  struct babel_seqno_request *sr;
+  WALK_LIST_FIRST2(sr, nbr_node, nbr->requests)
+  {
+    sr->nbr = NULL;
+    rem_node(&sr->nbr_node);
+  }
+
   nbr->ifa = NULL;
   rem_node(NODE nbr);
-  babel_unlock_neighbor(nbr);
+  mb_free(nbr);
 }
 
 static void
diff --git a/proto/babel/babel.h b/proto/babel/babel.h
index e075024c..09bf530c 100644
--- a/proto/babel/babel.h
+++ b/proto/babel/babel.h
@@ -198,7 +198,6 @@ struct babel_neighbor {
   struct babel_iface *ifa;
 
   ip_addr addr;
-  uint uc;				/* Reference counter for seqno requests */
   u16 rxcost;				/* Sent in last IHU */
   u16 txcost;				/* Received in last IHU */
   u16 cost;				/* Computed neighbor cost */
@@ -212,6 +211,7 @@ struct babel_neighbor {
   btime ihu_expiry;
 
   list routes;				/* Routes this neighbour has sent us (struct babel_route) */
+  list requests;			/* Seqno requests bound to this neighbor */
 };
 
 struct babel_source {
@@ -241,6 +241,7 @@ struct babel_route {
 
 struct babel_seqno_request {
   node n;
+  node nbr_node;
   u64 router_id;
   u16 seqno;
   u8 hop_count;