coverity: fix dns_sd.c Read from pointer after free

Fix CID 355434 (#1-2 of 2): Read from pointer after free (USE_AFTER_FREE)
deref_after_free: Dereferencing freed pointer ndata.
that Coverity pointed out.

We were freeing the pointer and then using it at the top of
the loop, so change the logic to fix this.

Signed-off-by: Robin Getz <[email protected]>

dns_sd.c

@@ -30,7 +30,6 @@
 /* Some functions for handling common linked list operations */
 static void dnssd_remove_node(struct dns_sd_discovery_data **ddata, int n)
 {
-
 	struct dns_sd_discovery_data *d, *ndata, *ldata, *tdata;
 	int i;
 
@@ -54,6 +53,7 @@ static void dnssd_remove_node(struct dns_sd_discovery_data **ddata, int n)
 			ldata = ndata;
 			i++;
 		}
+		ERROR("dnssd_remove_node call when %i exceeds list length (%i)\n", n, i);
 	}
 
 	*ddata = d;
@@ -158,10 +158,11 @@ void port_knock_discovery_data(struct dns_sd_discovery_data **ddata)
 
 	d = *ddata;
 	iio_mutex_lock(d->lock);
-	for (i = 0, ndata = d; ndata->next != NULL; ndata = ndata->next) {
+	for (i = 0, ndata = d; ndata->next != NULL; ) {
 		char port_str[6];
 		struct addrinfo hints, *res, *rp;
 		int fd;
+		bool found = false;
 
 		memset(&hints, 0, sizeof(hints));
 		hints.ai_family = AF_UNSPEC;
@@ -175,7 +176,6 @@ void port_knock_discovery_data(struct dns_sd_discovery_data **ddata)
 			DEBUG("Unable to find host ('%s'): %s\n",
 					ndata->hostname,
 					gai_strerror(ret));
-			dnssd_remove_node(&d, i);
 		} else {
 			for (rp = res; rp != NULL; rp = rp->ai_next) {
 				fd = create_socket(rp, DEFAULT_TIMEOUT_MS);
@@ -184,18 +184,23 @@ void port_knock_discovery_data(struct dns_sd_discovery_data **ddata)
 							rp->ai_family == AF_INET ? "ipv4" : "",
 							rp->ai_family == AF_INET6? "ipv6" : "",
 					ndata->hostname, ndata->port, ndata->addr_str);
-					dnssd_remove_node(&d, i);
 				} else {
 					close(fd);
 					DEBUG("Something %s%s at '%s:%d' %s)\n",
 							rp->ai_family == AF_INET ? "ipv4" : "",
 							rp->ai_family == AF_INET6? "ipv6" : "",
 							ndata->hostname, ndata->port, ndata->addr_str);
-					i++;
+					found = true;
 				}
 			}
 		}
 		freeaddrinfo(res);
+		ndata = ndata->next;
+		if (found) {
+			i++;
+		} else {
+			dnssd_remove_node(&d, i);
+		}
 	}
 	iio_mutex_unlock(d->lock);
 	*ddata = d;