diff -urpa postgresql-10.5.orig/src/bin/pg_upgrade/option.c postgresql-10.5/src/bin/pg_upgrade/option.c
--- postgresql-10.5.orig/src/bin/pg_upgrade/option.c	2018-08-22 19:57:55.237373015 +0000
+++ postgresql-10.5/src/bin/pg_upgrade/option.c	2018-08-22 19:59:10.944372163 +0000
@@ -460,6 +460,11 @@ get_sock_dir(ClusterInfo *cluster, bool
 			cluster->sockdir = pg_malloc(MAXPGPATH);
 			if (!getcwd(cluster->sockdir, MAXPGPATH))
 				pg_fatal("could not determine 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
 		{
diff -urpa postgresql-10.5.orig/src/include/pg_config_manual.h postgresql-10.5/src/include/pg_config_manual.h
--- postgresql-10.5.orig/src/include/pg_config_manual.h	2018-08-22 19:57:55.095373017 +0000
+++ postgresql-10.5/src/include/pg_config_manual.h	2018-08-22 19:58:30.664372616 +0000
@@ -181,7 +181,7 @@
  * 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.