resolved: rework dns server lifecycle logic

Previously, there was a chance of memory corruption, because when
switching to the next DNS server we didn't care whether they linked list
of DNS servers was still valid.

Clean up lifecycle of the dns server logic:

- When a DnsServer object is still in the linked list of DnsServers for
  a link or the manager, indicate so with a "linked" boolean field, and
  never follow the linked list if that boolean is not set.

- When picking a DnsServer to use for a link ot manager, always
  explicitly take a reference.

This also rearranges some logic, to make the tracking of dns servers by
link and globally more alike.

1 files changed, 75 insertions, 39 deletions

diff --git a/src/resolve/resolved-dns-server.c b/src/resolve/resolved-dns-server.c
index 207ec4c603..371594c710 100644
--- a/src/resolve/resolved-dns-server.c
+++ b/src/resolve/resolved-dns-server.c
@@ -67,6 +67,7 @@ int dns_server_new(
                 assert_not_reached("Unknown server type");
 
         s->manager = m;
+        s->linked = true;
 
         /* A new DNS server that isn't fallback is added and the one
          * we used so far was a fallback one? Then let's try to pick
@@ -87,39 +88,61 @@ DnsServer* dns_server_ref(DnsServer *s)  {
                 return NULL;
 
         assert(s->n_ref > 0);
-
         s->n_ref ++;
 
         return s;
 }
 
-static DnsServer* dns_server_free(DnsServer *s)  {
+DnsServer* dns_server_unref(DnsServer *s)  {
         if (!s)
                 return NULL;
 
-        if (s->link && s->link->current_dns_server == s)
-                link_set_dns_server(s->link, NULL);
+        assert(s->n_ref > 0);
+        s->n_ref --;
 
-        if (s->manager && s->manager->current_dns_server == s)
-                manager_set_dns_server(s->manager, NULL);
+        if (s->n_ref > 0)
+                return NULL;
 
         free(s);
-
         return NULL;
 }
 
-DnsServer* dns_server_unref(DnsServer *s)  {
-        if (!s)
-                return NULL;
+void dns_server_unlink(DnsServer *s) {
+        assert(s);
+        assert(s->manager);
 
-        assert(s->n_ref > 0);
+        /* This removes the specified server from the linked list of
+         * servers, but any server might still stay around if it has
+         * refs, for example from an ongoing transaction. */
 
-        if (s->n_ref == 1)
-                dns_server_free(s);
-        else
-                s->n_ref --;
+        if (!s->linked)
+                return;
 
-        return NULL;
+        switch (s->type) {
+
+        case DNS_SERVER_LINK:
+                assert(s->link);
+                LIST_REMOVE(servers, s->link->dns_servers, s);
+                break;
+
+        case DNS_SERVER_SYSTEM:
+                LIST_REMOVE(servers, s->manager->dns_servers, s);
+                break;
+
+        case DNS_SERVER_FALLBACK:
+                LIST_REMOVE(servers, s->manager->fallback_dns_servers, s);
+                break;
+        }
+
+        s->linked = false;
+
+        if (s->link && s->link->current_dns_server == s)
+                link_set_dns_server(s->link, NULL);
+
+        if (s->manager->current_dns_server == s)
+                manager_set_dns_server(s->manager, NULL);
+
+        dns_server_unref(s);
 }
 
 void dns_server_packet_received(DnsServer *s, usec_t rtt) {
@@ -166,34 +189,48 @@ const struct hash_ops dns_server_hash_ops = {
         .compare = dns_server_compare_func
 };
 
-void manager_flush_dns_servers(Manager *m, DnsServerType type) {
-        DnsServer **first, *s;
+DnsServer *manager_get_first_dns_server(Manager *m, DnsServerType t) {
+        assert(m);
+
+        switch (t) {
+
+        case DNS_SERVER_SYSTEM:
+                return m->dns_servers;
+
+        case DNS_SERVER_FALLBACK:
+                return m->fallback_dns_servers;
 
+        default:
+                return NULL;
+        }
+}
+
+void manager_flush_dns_servers(Manager *m, DnsServerType type) {
         assert(m);
 
-        first = type == DNS_SERVER_FALLBACK ? &m->fallback_dns_servers : &m->dns_servers;
+        for (;;) {
+                DnsServer *first;
 
-        while (*first) {
-                s = *first;
+                first = manager_get_first_dns_server(m, type);
+                if (!first)
+                        break;
 
-                LIST_REMOVE(servers, *first, s);
-                dns_server_unref(s);
+                dns_server_unlink(first);
         }
 }
 
 void manager_flush_marked_dns_servers(Manager *m, DnsServerType type) {
-        DnsServer **first, *s, *next;
+        DnsServer *first, *s, *next;
 
         assert(m);
 
-        first = type == DNS_SERVER_FALLBACK ? &m->fallback_dns_servers : &m->dns_servers;
+        first = manager_get_first_dns_server(m, type);
 
-        LIST_FOREACH_SAFE(servers, s, next, *first) {
+        LIST_FOREACH_SAFE(servers, s, next, first) {
                 if (!s->marked)
                         continue;
 
-                LIST_REMOVE(servers, *first, s);
-                dns_server_unref(s);
+                dns_server_unlink(s);
         }
 }
 
@@ -202,23 +239,20 @@ void manager_mark_dns_servers(Manager *m, DnsServerType type) {
 
         assert(m);
 
-        first = type == DNS_SERVER_FALLBACK ? m->fallback_dns_servers : m->dns_servers;
-
+        first = manager_get_first_dns_server(m, type);
         LIST_FOREACH(servers, s, first)
                 s->marked = true;
 }
 
-DnsServer* manager_find_dns_server(Manager *m, int family, const union in_addr_union *in_addr) {
-        DnsServer *s;
+DnsServer* manager_find_dns_server(Manager *m, DnsServerType type, int family, const union in_addr_union *in_addr) {
+        DnsServer *first, *s;
 
         assert(m);
         assert(in_addr);
 
-        LIST_FOREACH(servers, s, m->dns_servers)
-                if (s->family == family && in_addr_equal(family, &s->address, in_addr) > 0)
-                        return s;
+        first = manager_get_first_dns_server(m, type);
 
-        LIST_FOREACH(servers, s, m->fallback_dns_servers)
+        LIST_FOREACH(servers, s, first)
                 if (s->family == family && in_addr_equal(family, &s->address, in_addr) > 0)
                         return s;
 
@@ -238,7 +272,8 @@ DnsServer *manager_set_dns_server(Manager *m, DnsServer *s) {
                 log_info("Switching to system DNS server %s.", strna(ip));
         }
 
-        m->current_dns_server = s;
+        dns_server_unref(m->current_dns_server);
+        m->current_dns_server = dns_server_ref(s);
 
         if (m->unicast_scope)
                 dns_cache_flush(&m->unicast_scope->cache);
@@ -286,8 +321,9 @@ void manager_next_dns_server(Manager *m) {
         if (!m->current_dns_server)
                 return;
 
-        /* Change to the next one */
-        if (m->current_dns_server->servers_next) {
+        /* Change to the next one, but make sure to follow the linked
+         * list only if the server is still linked. */
+        if (m->current_dns_server->linked && m->current_dns_server->servers_next) {
                 manager_set_dns_server(m, m->current_dns_server->servers_next);
                 return;
         }