resolved: split dns_query_process_cname() into two separate functions

This does some refactoring: the dns_query_process_cname() function
becomes two: dns_query_process_cname_one() and
dns_query_process_cname_many(). The former will process exactly one
CNAME chain element, the latter will follow a chain for as long as
possible within the current packet.

dns_query_process_cname_many() is mostly identical to the old
dns_query_process_cname(), and all existing code is moved over to using
that.

This is mostly preparation for the next commit, where we make direct use
of dns_query_process_cname_one().

This also renames the DNS_QUERY_RESTARTED return value to
DNS_QUERY_CNAME. That's because in the dns_query_process_cname_many()
case as before if we return this we restarted the query in case we
reached the end of the chain without a conclusive answer, as before. But
in dns_query_process_cname_one() we'll only go one step anyway, and
leave restarting if needed to the caller. Hence DNS_QUERY_RESTARTED is a
bit of a misnomer in that case.

This also gets rid of the weird tail recursion in
dns_query_process_cname() and replaces it with an explicit loop in
dns_query_process_cname_many(). The old recursion wasn't a security
issue since we put a limit on the number of CNAMEs we follow anyway, but
it's still icky to scale stack use by that.

1 files changed, 10 insertions, 10 deletions

diff --git a/src/resolve/resolved-bus.c b/src/resolve/resolved-bus.c
index 032ed0256b..c3624669ce 100644
--- a/src/resolve/resolved-bus.c
+++ b/src/resolve/resolved-bus.c
@@ -195,14 +195,14 @@ static void bus_method_resolve_hostname_complete(DnsQuery *q) {
                 goto finish;
         }
 
-        r = dns_query_process_cname(q);
+        r = dns_query_process_cname_many(q);
         if (r == -ELOOP) {
                 r = sd_bus_reply_method_errorf(q->bus_request, BUS_ERROR_CNAME_LOOP, "CNAME loop detected, or CNAME resolving disabled on '%s'", dns_query_string(q));
                 goto finish;
         }
         if (r < 0)
                 goto finish;
-        if (r == DNS_QUERY_RESTARTED) /* This was a cname, and the query was restarted. */
+        if (r == DNS_QUERY_CNAME) /* This was a cname, and the query was restarted. */
                 return;
 
         r = sd_bus_message_new_method_return(q->bus_request, &reply);
@@ -486,14 +486,14 @@ static void bus_method_resolve_address_complete(DnsQuery *q) {
                 goto finish;
         }
 
-        r = dns_query_process_cname(q);
+        r = dns_query_process_cname_many(q);
         if (r == -ELOOP) {
                 r = sd_bus_reply_method_errorf(q->bus_request, BUS_ERROR_CNAME_LOOP, "CNAME loop detected, or CNAME resolving disabled on '%s'", dns_query_string(q));
                 goto finish;
         }
         if (r < 0)
                 goto finish;
-        if (r == DNS_QUERY_RESTARTED) /* This was a cname, and the query was restarted. */
+        if (r == DNS_QUERY_CNAME) /* This was a cname, and the query was restarted. */
                 return;
 
         r = sd_bus_message_new_method_return(q->bus_request, &reply);
@@ -660,14 +660,14 @@ static void bus_method_resolve_record_complete(DnsQuery *q) {
                 goto finish;
         }
 
-        r = dns_query_process_cname(q);
+        r = dns_query_process_cname_many(q);
         if (r == -ELOOP) {
                 r = sd_bus_reply_method_errorf(q->bus_request, BUS_ERROR_CNAME_LOOP, "CNAME loop detected, or CNAME resolving disabled on '%s'", dns_query_string(q));
                 goto finish;
         }
         if (r < 0)
                 goto finish;
-        if (r == DNS_QUERY_RESTARTED) /* This was a cname, and the query was restarted. */
+        if (r == DNS_QUERY_CNAME) /* This was a cname, and the query was restarted. */
                 return;
 
         r = sd_bus_message_new_method_return(q->bus_request, &reply);
@@ -1107,8 +1107,8 @@ static void resolve_service_hostname_complete(DnsQuery *q) {
                 return;
         }
 
-        r = dns_query_process_cname(q);
-        if (r == DNS_QUERY_RESTARTED) /* This was a cname, and the query was restarted. */
+        r = dns_query_process_cname_many(q);
+        if (r == DNS_QUERY_CNAME) /* This was a cname, and the query was restarted. */
                 return;
 
         /* This auxiliary lookup is finished or failed, let's see if all are finished now. */
@@ -1180,14 +1180,14 @@ static void bus_method_resolve_service_complete(DnsQuery *q) {
                 goto finish;
         }
 
-        r = dns_query_process_cname(q);
+        r = dns_query_process_cname_many(q);
         if (r == -ELOOP) {
                 r = sd_bus_reply_method_errorf(q->bus_request, BUS_ERROR_CNAME_LOOP, "CNAME loop detected, or CNAME resolving disabled on '%s'", dns_query_string(q));
                 goto finish;
         }
         if (r < 0)
                 goto finish;
-        if (r == DNS_QUERY_RESTARTED) /* This was a cname, and the query was restarted. */
+        if (r == DNS_QUERY_CNAME) /* This was a cname, and the query was restarted. */
                 return;
 
         question = dns_query_question_for_protocol(q, q->answer_protocol);