[PATCH 2/2] Use wpa conf file link target for writing

[Andrew Beltrano]

When the wpa conf file is a symbolic link, the link is clobbered when
written. This occurs because the rename() of the (regular) tmp file
used to write changes replaces the original file, thus discarding the
link.

Fix this by first resolving the conf file link target (if any), and
using the target as the defacto file path instead of the link file
itself. This ensures the tmp file is created on the same filesystem
as the link target, which is required for rename().

Signed-off-by: Andrew Beltrano <anbeltra at microsoft.com>
---
 wpa_supplicant/config_file.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/wpa_supplicant/config_file.c b/wpa_supplicant/config_file.c
index a535e3f08..fca21b9c1 100644
--- a/wpa_supplicant/config_file.c
+++ b/wpa_supplicant/config_file.c
@@ -1545,15 +1545,23 @@ int wpa_config_write(const char *name, struct wpa_config *config)
 	struct wpa_config_blob *blob;
 #endif /* CONFIG_NO_CONFIG_BLOBS */
 	int ret = 0;
-	const char *orig_name = name;
+	const char *orig_name;
 	int tmp_len;
 	char *tmp_name;
+	char *link_target;
 
 	if (!name) {
 		wpa_printf(MSG_ERROR, "No configuration file for writing");
 		return -1;
-	}
+	} else if (read_link_target(name, &link_target) < 0) {
+		wpa_printf(MSG_ERROR, "Failed to read '%s' link target for writing", name);
+		return -1;
+	} 
 
+	if (link_target)
+		name = link_target;
+
+	orig_name = name;
 	tmp_len = os_strlen(name) + 5; /* allow space for .tmp suffix */
 	tmp_name = os_malloc(tmp_len);
 	if (tmp_name) {
@@ -1567,6 +1575,7 @@ int wpa_config_write(const char *name, struct wpa_config *config)
 	if (f == NULL) {
 		wpa_printf(MSG_DEBUG, "Failed to open '%s' for writing", name);
 		os_free(tmp_name);
+		os_free(link_target);
 		return -1;
 	}
 
@@ -1619,6 +1628,8 @@ int wpa_config_write(const char *name, struct wpa_config *config)
 		os_free(tmp_name);
 	}
 
+	os_free(link_target);
+
 	wpa_printf(MSG_DEBUG, "Configuration file '%s' written %ssuccessfully",
 		   orig_name, ret ? "un" : "");
 	return ret;
-- 
2.20.1