cifs: do not share tcp sessions of dfs connections

Make sure that we do not share tcp sessions of dfs mounts when
mounting regular shares that connect to same server.  DFS connections
rely on a single instance of tcp in order to do failover properly in
cifs_reconnect().
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifsglob.h

@@ -693,6 +693,9 @@ struct TCP_Server_Info {
 	bool use_swn_dstaddr;
 	struct sockaddr_storage swn_dstaddr;
 #endif
+#ifdef CONFIG_CIFS_DFS_UPCALL
+	bool is_dfs_conn; /* if a dfs connection */
+#endif
 };
 
 struct cifs_credits {

fs/cifs/connect.c

@@ -1268,6 +1268,16 @@ cifs_find_tcp_session(struct smb3_fs_context *ctx)
 
 	spin_lock(&cifs_tcp_ses_lock);
 	list_for_each_entry(server, &cifs_tcp_ses_list, tcp_ses_list) {
+#ifdef CONFIG_CIFS_DFS_UPCALL
+		/*
+		 * DFS failover implementation in cifs_reconnect() requires unique tcp sessions for
+		 * DFS connections to do failover properly, so avoid sharing them with regular
+		 * shares or even links that may connect to same server but having completely
+		 * different failover targets.
+		 */
+		if (server->is_dfs_conn)
+			continue;
+#endif
 		/*
 		 * Skip ses channels since they're only handled in lower layers
 		 * (e.g. cifs_send_recv).
@@ -2968,6 +2978,23 @@ static int mount_setup_tlink(struct cifs_sb_info *cifs_sb, struct cifs_ses *ses,
 }
 
 #ifdef CONFIG_CIFS_DFS_UPCALL
+static int mount_get_dfs_conns(struct smb3_fs_context *ctx, struct cifs_sb_info *cifs_sb,
+			       unsigned int *xid, struct TCP_Server_Info **nserver,
+			       struct cifs_ses **nses, struct cifs_tcon **ntcon)
+{
+	int rc;
+
+	ctx->nosharesock = true;
+	rc = mount_get_conns(ctx, cifs_sb, xid, nserver, nses, ntcon);
+	if (*nserver) {
+		cifs_dbg(FYI, "%s: marking tcp session as a dfs connection\n", __func__);
+		spin_lock(&cifs_tcp_ses_lock);
+		(*nserver)->is_dfs_conn = true;
+		spin_unlock(&cifs_tcp_ses_lock);
+	}
+	return rc;
+}
+
 /*
  * cifs_build_path_to_root returns full path to root when we do not have an
  * existing connection (tcon)
@@ -3163,7 +3190,7 @@ static int do_dfs_failover(const char *path, const char *full_path, struct cifs_
 			 tmp_ctx.prepath);
 
 		mount_put_conns(cifs_sb, *xid, *server, *ses, *tcon);
-		rc = mount_get_conns(&tmp_ctx, cifs_sb, xid, server, ses, tcon);
+		rc = mount_get_dfs_conns(&tmp_ctx, cifs_sb, xid, server, ses, tcon);
 		if (!rc || (*server && *ses)) {
 			/*
 			 * We were able to connect to new target server. Update current context with
@@ -3462,7 +3489,12 @@ int cifs_mount(struct cifs_sb_info *cifs_sb, struct smb3_fs_context *ctx)
 			goto error;
 	}
 
-	ctx->nosharesock = true;
+	mount_put_conns(cifs_sb, xid, server, ses, tcon);
+	/*
+	 * Ignore error check here because we may failover to other targets from cached a
+	 * referral.
+	 */
+	(void)mount_get_dfs_conns(ctx, cifs_sb, &xid, &server, &ses, &tcon);
 
 	/* Get path of DFS root */
 	ref_path = build_unc_path_to_root(ctx, cifs_sb, false);
@@ -3491,7 +3523,7 @@ int cifs_mount(struct cifs_sb_info *cifs_sb, struct smb3_fs_context *ctx)
 		/* Connect to new DFS target only if we were redirected */
 		if (oldmnt != cifs_sb->ctx->mount_options) {
 			mount_put_conns(cifs_sb, xid, server, ses, tcon);
-			rc = mount_get_conns(ctx, cifs_sb, &xid, &server, &ses, &tcon);
+			rc = mount_get_dfs_conns(ctx, cifs_sb, &xid, &server, &ses, &tcon);
 		}
 		if (rc && !server && !ses) {
 			/* Failed to connect. Try to connect to other targets in the referral. */