Truncating a file and then deciding not to change its

contents doesn't do what you think it does.

Restore "no dns servers, no search, means don't touch
resolv.conf" behaviour unintentionally changed at t2k17.

Noticed by ajacoutot@ during an upgrade using a
non-OpenBSD dhcpd server that only provides the
options it is asked for.

1 files changed, 14 insertions, 11 deletions

diff --git a/sbin/dhclient/kroute.c b/sbin/dhclient/kroute.c
index 6db356bf9ef..d9d6b16b871 100644
--- a/sbin/dhclient/kroute.c
+++ b/sbin/dhclient/kroute.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kroute.c,v 1.138 2017/08/14 22:12:59 krw Exp $	*/
+/*	$OpenBSD: kroute.c,v 1.139 2017/08/18 15:06:11 krw Exp $	*/
 
 /*
  * Copyright 2012 Kenneth R Westerback <krw@openbsd.org>
@@ -553,6 +553,9 @@ priv_write_resolv_conf(char *contents)
 	size_t		 sz;
 	int		 fd;
 
+	if (contents == NULL)
+		return;
+
 	fd = open(path, O_WRONLY | O_CREAT | O_TRUNC,
 	    S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
 
@@ -561,15 +564,13 @@ priv_write_resolv_conf(char *contents)
 		return;
 	}
 
-	if (contents != NULL) {
-		sz = strlen(contents);
-		n = write(fd, contents, sz);
-		if (n == -1)
-			log_warn("Couldn't write contents to '%s'", path);
-		else if ((size_t)n < sz)
-			log_warnx("Short contents write to '%s' (%zd vs %zu)",
-			    path, n, sz);
-	}
+	sz = strlen(contents);
+	n = write(fd, contents, sz);
+	if (n == -1)
+		log_warn("Couldn't write contents to '%s'", path);
+	else if ((size_t)n < sz)
+		log_warnx("Short contents write to '%s' (%zd vs %zu)",
+		    path, n, sz);
 
 	close(fd);
 }
@@ -704,7 +705,8 @@ set_resolv_conf(char *name, uint8_t *rtsearch, unsigned int rtsearch_len,
 
 	if (len == 0) {
 		free(dn);
-		return;
+		contents = NULL;
+		goto done;
 	}
 
 	rslt = asprintf(&courtesy, "# Generated by %s dhclient\n", name);
@@ -733,6 +735,7 @@ set_resolv_conf(char *name, uint8_t *rtsearch, unsigned int rtsearch_len,
 	if (config->resolv_tail != NULL)
 		strlcat(contents, config->resolv_tail, len);
 
+done:
 	rslt = imsg_compose(unpriv_ibuf, IMSG_SET_RESOLV_CONF,
 	    0, 0, -1, contents, len);
 	if (rslt == -1)