diff -urpa postgresql-9.6.10.orig/src/bin/pg_upgrade/option.c postgresql-9.6.10/src/bin/pg_upgrade/option.c
--- postgresql-9.6.10.orig/src/bin/pg_upgrade/option.c	2018-08-22 20:58:08.789332330 +0000
+++ postgresql-9.6.10/src/bin/pg_upgrade/option.c	2018-08-22 21:05:48.468327154 +0000
@@ -462,6 +462,11 @@ get_sock_dir(ClusterInfo *cluster, bool
 			cluster->sockdir = pg_malloc(MAXPGPATH);
 			if (!getcwd(cluster->sockdir, MAXPGPATH))
 				pg_fatal("cannot find current directory\n");
+#ifndef UNIX_PATH_MAX
+#define UNIX_PATH_MAX 108
+#endif
+			if (strlen(cluster->sockdir) >= UNIX_PATH_MAX - sizeof(".s.PGSQL.50432"))
+				strcpy(cluster->sockdir, "/tmp"); /* fall back to tmp */
 		}
 		else
 		{
@@ -487,7 +492,6 @@ get_sock_dir(ClusterInfo *cluster, bool
 			{
 				if (fgets(line, sizeof(line), fp) == NULL)
 					pg_fatal("Cannot read line %d from %s: %m\n", lineno, filename);
-
 				/* potentially overwrite user-supplied value */
 				if (lineno == LOCK_FILE_LINE_PORT)
 					sscanf(line, "%hu", &old_cluster.port);
diff -urpa postgresql-9.6.10.orig/src/include/pg_config_manual.h postgresql-9.6.10/src/include/pg_config_manual.h
--- postgresql-9.6.10.orig/src/include/pg_config_manual.h	2018-08-22 20:58:08.658332331 +0000
+++ postgresql-9.6.10/src/include/pg_config_manual.h	2018-08-22 20:58:43.807331936 +0000
@@ -182,7 +182,7 @@
  * here's where to twiddle it.  You can also override this at runtime
  * with the postmaster's -k switch.
  */
-#define DEFAULT_PGSOCKET_DIR  "/tmp"
+#define DEFAULT_PGSOCKET_DIR  "/var/run/postgresql"
 
 /*
  * This is the default event source for Windows event log.