cifs: Add support for mounting Windows 2008 DFS shares

Windows 2008 CIFS servers do not always return PATH_NOT_COVERED when
attempting to access a DFS share.  Therefore, when checking for remote
shares, unconditionally ask for a DFS referral for the UNC (w/out prepath)
before continuing with previous behavior of attempting to access the UNC +
prepath and checking for PATH_NOT_COVERED.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=31092

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Sean Finney <seanius@seanius.net>
Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/connect.c

@@ -2937,6 +2937,24 @@ cifs_mount(struct super_block *sb, struct cifs_sb_info *cifs_sb,
 			       (tcon->ses->server->maxBuf - MAX_CIFS_HDR_SIZE));
 
 remote_path_check:
+#ifdef CONFIG_CIFS_DFS_UPCALL
+	/*
+	 * Perform an unconditional check for whether there are DFS
+	 * referrals for this path without prefix, to provide support
+	 * for DFS referrals from w2k8 servers which don't seem to respond
+	 * with PATH_NOT_COVERED to requests that include the prefix.
+	 * Chase the referral if found, otherwise continue normally.
+	 */
+	if (referral_walks_count == 0) {
+		int refrc = expand_dfs_referral(xid, pSesInfo, volume_info,
+						cifs_sb, &mount_data, false);
+		if (!refrc) {
+			referral_walks_count++;
+			goto try_mount_again;
+		}
+	}
+#endif
+
 	/* check if a whole path (including prepath) is not remote */
 	if (!rc && tcon) {
 		/* build_path_to_root works only when we have a valid tcon */