cifs: Always resolve hostname before reconnecting

BugLink: https://bugs.launchpad.net/bugs/1818803

commit 28eb24ff upstream.

In case a hostname resolves to a different IP address (e.g. long
running mounts), make sure to resolve it every time prior to calling
generic_ip_connect() in reconnect.
Suggested-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Paulo Alcantara <palcantara@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

fs/cifs/connect.c

@@ -48,6 +48,7 @@
 #include "cifs_unicode.h"
 #include "cifs_debug.h"
 #include "cifs_fs_sb.h"
+#include "dns_resolve.h"
 #include "ntlmssp.h"
 #include "nterr.h"
 #include "rfc1002pdu.h"
@@ -307,6 +308,53 @@ static void cifs_prune_tlinks(struct work_struct *work);
 static int cifs_setup_volume_info(struct smb_vol *volume_info, char *mount_data,
 					const char *devname);
 
+/*
+ * Resolve hostname and set ip addr in tcp ses. Useful for hostnames that may
+ * get their ip addresses changed at some point.
+ *
+ * This should be called with server->srv_mutex held.
+ */
+#ifdef CONFIG_CIFS_DFS_UPCALL
+static int reconn_set_ipaddr(struct TCP_Server_Info *server)
+{
+	int rc;
+	int len;
+	char *unc, *ipaddr = NULL;
+
+	if (!server->hostname)
+		return -EINVAL;
+
+	len = strlen(server->hostname) + 3;
+
+	unc = kmalloc(len, GFP_KERNEL);
+	if (!unc) {
+		cifs_dbg(FYI, "%s: failed to create UNC path\n", __func__);
+		return -ENOMEM;
+	}
+	snprintf(unc, len, "\\\\%s", server->hostname);
+
+	rc = dns_resolve_server_name_to_ip(unc, &ipaddr);
+	kfree(unc);
+
+	if (rc < 0) {
+		cifs_dbg(FYI, "%s: failed to resolve server part of %s to IP: %d\n",
+			 __func__, server->hostname, rc);
+		return rc;
+	}
+
+	rc = cifs_convert_address((struct sockaddr *)&server->dstaddr, ipaddr,
+				  strlen(ipaddr));
+	kfree(ipaddr);
+
+	return !rc ? -1 : 0;
+}
+#else
+static inline int reconn_set_ipaddr(struct TCP_Server_Info *server)
+{
+	return 0;
+}
+#endif
+
 /*
  * cifs tcp session reconnection
  *
@@ -404,6 +452,11 @@ cifs_reconnect(struct TCP_Server_Info *server)
 		rc = generic_ip_connect(server);
 		if (rc) {
 			cifs_dbg(FYI, "reconnect error %d\n", rc);
+			rc = reconn_set_ipaddr(server);
+			if (rc) {
+				cifs_dbg(FYI, "%s: failed to resolve hostname: %d\n",
+					 __func__, rc);
+			}
 			mutex_unlock(&server->srv_mutex);
 			msleep(3000);
 		} else {