resolved: detect and handle mDNS race condition upon probing

As discussed in RFC 6762, Section 8.2 a race condition may
happen when two hosts are probing for the same name simultaniously.

Detect and handle such race conditions.

src/resolve/resolved-dns-transaction.c

@@ -365,7 +365,7 @@ void dns_transaction_complete(DnsTransaction *t, DnsTransactionState state) {
         SET_FOREACH_MOVE(z, t->notify_zone_items_done, t->notify_zone_items)
                 dns_zone_item_notify(z);
         SWAP_TWO(t->notify_zone_items, t->notify_zone_items_done);
-        if (t->probing)
+        if (t->probing && t->state == DNS_TRANSACTION_ATTEMPTS_MAX_REACHED)
                 (void) dns_scope_announce(t->scope, false);
 
         SET_FOREACH_MOVE(d, t->notify_transactions_done, t->notify_transactions)

src/resolve/resolved-dns-zone.c

@@ -95,7 +95,7 @@ void dns_zone_flush(DnsZone *z) {
         z->by_name = hashmap_free(z->by_name);
 }
 
-static DnsZoneItem* dns_zone_get(DnsZone *z, DnsResourceRecord *rr) {
+DnsZoneItem* dns_zone_get(DnsZone *z, DnsResourceRecord *rr) {
         DnsZoneItem *i;
 
         assert(z);

src/resolve/resolved-dns-zone.h

@@ -67,6 +67,7 @@ struct DnsZoneItem {
 void dns_zone_flush(DnsZone *z);
 
 int dns_zone_put(DnsZone *z, DnsScope *s, DnsResourceRecord *rr, bool probe);
+DnsZoneItem* dns_zone_get(DnsZone *z, DnsResourceRecord *rr);
 void dns_zone_remove_rr(DnsZone *z, DnsResourceRecord *rr);
 int dns_zone_remove_rrs_by_key(DnsZone *z, DnsResourceKey *key);
 

src/resolve/resolved-mdns.c

@@ -22,10 +22,13 @@
 #include <netinet/in.h>
 #include <arpa/inet.h>
 
+#include "alloc-util.h"
 #include "fd-util.h"
 #include "resolved-manager.h"
 #include "resolved-mdns.h"
 
+#define CLEAR_CACHE_FLUSH(x) (~MDNS_RR_CACHE_FLUSH & (x))
+
 void manager_mdns_stop(Manager *m) {
         assert(m);
 
@@ -68,10 +71,145 @@ eaddrinuse:
         return 0;
 }
 
+static int mdns_rr_compare(const void *a, const void *b) {
+        DnsResourceRecord **x = (DnsResourceRecord**) a, **y = (DnsResourceRecord**) b;
+        size_t m;
+        int r;
+
+        assert(x);
+        assert(*x);
+        assert(y);
+        assert(*y);
+
+        if (CLEAR_CACHE_FLUSH((*x)->key->class) < CLEAR_CACHE_FLUSH((*y)->key->class))
+                return -1;
+        else if (CLEAR_CACHE_FLUSH((*x)->key->class) > CLEAR_CACHE_FLUSH((*y)->key->class))
+                return 1;
+
+        if ((*x)->key->type < (*y)->key->type)
+                return -1;
+        else if ((*x)->key->type > (*y)->key->type)
+                return 1;
+
+        r = dns_resource_record_to_wire_format(*x, false);
+        if (r < 0) {
+                log_warning_errno(r, "Can't wire-format RR: %m");
+                return 0;
+        }
+
+        r = dns_resource_record_to_wire_format(*y, false);
+        if (r < 0) {
+                log_warning_errno(r, "Can't wire-format RR: %m");
+                return 0;
+        }
+
+        m = MIN(DNS_RESOURCE_RECORD_RDATA_SIZE(*x), DNS_RESOURCE_RECORD_RDATA_SIZE(*y));
+
+        r = memcmp(DNS_RESOURCE_RECORD_RDATA(*x), DNS_RESOURCE_RECORD_RDATA(*y), m);
+        if (r != 0)
+                return r;
+
+        if (DNS_RESOURCE_RECORD_RDATA_SIZE(*x) < DNS_RESOURCE_RECORD_RDATA_SIZE(*y))
+                return -1;
+        else if (DNS_RESOURCE_RECORD_RDATA_SIZE(*x) > DNS_RESOURCE_RECORD_RDATA_SIZE(*y))
+                return 1;
+
+        return 0;
+}
+
+static int proposed_rrs_cmp(DnsResourceRecord **x, unsigned x_size, DnsResourceRecord **y, unsigned y_size) {
+        unsigned m;
+        int r;
+
+        m = MIN(x_size, y_size);
+        for (unsigned i = 0; i < m; i++) {
+                r = mdns_rr_compare(&x[i], &y[i]);
+                if (r != 0)
+                        return r;
+        }
+
+        if (x_size < y_size)
+                return -1;
+        if (x_size > y_size)
+                return 1;
+
+        return 0;
+}
+
+static int mdns_packet_extract_matching_rrs(DnsPacket *p, DnsResourceKey *key, DnsResourceRecord ***ret_rrs) {
+        _cleanup_free_ DnsResourceRecord **list = NULL;
+        unsigned n = 0, size = 0;
+        int r;
+
+        assert(p);
+        assert(key);
+        assert(ret_rrs);
+        assert_return(DNS_PACKET_NSCOUNT(p) > 0, -EINVAL);
+
+        for (unsigned i = DNS_PACKET_ANCOUNT(p); i < (DNS_PACKET_ANCOUNT(p) + DNS_PACKET_NSCOUNT(p)); i++) {
+                r = dns_resource_key_match_rr(key, p->answer->items[i].rr, NULL);
+                if (r < 0)
+                        return r;
+                if (r > 0)
+                        size++;
+        }
+
+        if (size == 0)
+                return 0;
+
+        list = new(DnsResourceRecord *, size);
+        if (!list)
+                return -ENOMEM;
+
+        for (unsigned i = DNS_PACKET_ANCOUNT(p); i < (DNS_PACKET_ANCOUNT(p) + DNS_PACKET_NSCOUNT(p)); i++) {
+                r = dns_resource_key_match_rr(key, p->answer->items[i].rr, NULL);
+                if (r < 0)
+                        return r;
+                if (r > 0)
+                        list[n++] = p->answer->items[i].rr;
+        }
+        assert(n == size);
+        qsort_safe(list, size, sizeof(DnsResourceRecord*), mdns_rr_compare);
+
+        *ret_rrs = list;
+        list = NULL;
+
+        return size;
+}
+
+static int mdns_do_tiebreak(DnsResourceKey *key, DnsAnswer *answer, DnsPacket *p) {
+        _cleanup_free_ DnsResourceRecord **our = NULL, **remote = NULL;
+        DnsResourceRecord *rr;
+        unsigned i = 0;
+        unsigned size;
+        int r;
+
+        size = dns_answer_size(answer);
+        our = new(DnsResourceRecord *, size);
+        if (!our)
+                return -ENOMEM;
+
+        DNS_ANSWER_FOREACH(rr, answer)
+                our[i++] = rr;
+        qsort_safe(our, size, sizeof(DnsResourceRecord*), mdns_rr_compare);
+
+        r = mdns_packet_extract_matching_rrs(p, key, &remote);
+        if (r < 0)
+                return r;
+
+        assert(r > 0);
+
+        if (proposed_rrs_cmp(remote, r, our, size) > 0)
+                return 1;
+
+        return 0;
+}
+
 static int mdns_scope_process_query(DnsScope *s, DnsPacket *p) {
         _cleanup_(dns_answer_unrefp) DnsAnswer *full_answer = NULL;
         _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
         DnsResourceKey *key = NULL;
+        DnsResourceRecord *rr;
         bool tentative = false;
         int r;
 
@@ -91,6 +229,31 @@ static int mdns_scope_process_query(DnsScope *s, DnsPacket *p) {
                 if (r < 0)
                         return log_debug_errno(r, "Failed to lookup key: %m");
 
+                if (tentative && DNS_PACKET_NSCOUNT(p) > 0) {
+                        /*
+                         * A race condition detected with the probe packet from
+                         * a remote host.
+                         * Do simultaneous probe tiebreaking as described in
+                         * RFC 6762, Section 8.2. In case we lost don't reply
+                         * the question and withdraw conflicting RRs.
+                         */
+                        r = mdns_do_tiebreak(key, answer, p);
+                        if (r < 0)
+                                return log_debug_errno(r, "Failed to do tiebreaking");
+
+                        if (r > 0) { /* we lost */
+                                DNS_ANSWER_FOREACH(rr, answer) {
+                                        DnsZoneItem *i;
+
+                                        i = dns_zone_get(&s->zone, rr);
+                                        if (i)
+                                                dns_zone_item_conflict(i);
+                                }
+
+                                continue;
+                        }
+                }
+
                 r = dns_answer_extend(&full_answer, answer);
                 if (r < 0)
                         return log_debug_errno(r, "Failed to extend answer: %m");