dns: update detection of immutable resolv.conf

Before, we would first check whether the file is immuable before
parsing main.rc-manager setting. That means, if you configured

  [main]
  dns=default
  rc-manager=unmanged

we would still first try to detect whether the file is immutable.
The result of course is only minor, e.g. showing up in logging
as rc-manager=immutable instead of rc-manager=unmanged.
Also, an immutable resolv.conf would suppress a warning about
a bogus rc-manager setting.

Also, when selecting rc-manager=symlink and resolv.conf is a symlink
to an immutable file, we don't actually care about that. The reason is,
that if the link-target is not /var/run/NetworkManager/resolv.conf,
we anyway wouldn't modify the file.

The effect of this change is pretty minor, now in logging you would see:
  dns-mgr: init: dns=default, rc-manager=symlink
  dns-mgr: update-resolv-conf: write internal file /var/run/NetworkManager/resolv.conf succeeded but don't update /etc/resolv.conf as it points to /some/where/else
instead of
  dns-mgr: init: dns=default, rc-manager=immutable
  dns-mgr: update-resolv-conf: write internal file /var/run/NetworkManager/resolv.conf succeeded

Which feels slightly more right.

Note that symlinks cannot have file attributes.

1 files changed, 44 insertions, 10 deletions

diff --git a/src/dns-manager/nm-dns-manager.c b/src/dns-manager/nm-dns-manager.c
index 8134f2f4a4..8690be8a36 100644
--- a/src/dns-manager/nm-dns-manager.c
+++ b/src/dns-manager/nm-dns-manager.c
@@ -1508,19 +1508,53 @@ _clear_plugin (NMDnsManager *self)
 	return FALSE;
 }
 
-static bool
-_get_resconf_immutable (void)
+static NMDnsManagerResolvConfManager
+_check_resconf_immutable (NMDnsManagerResolvConfManager rc_manager)
 {
+	struct stat st;
 	int fd, flags;
 	bool immutable = FALSE;
 
-	fd = open (_PATH_RESCONF, O_RDONLY);
-	if (fd != -1) {
-		if (ioctl (fd, FS_IOC_GETFLAGS, &flags) != -1)
-			immutable = NM_FLAGS_HAS (flags, FS_IMMUTABLE_FL);
-		close (fd);
+	switch (rc_manager) {
+	case NM_DNS_MANAGER_RESOLV_CONF_MAN_UNKNOWN:
+	case NM_DNS_MANAGER_RESOLV_CONF_MAN_IMMUTABLE:
+		nm_assert_not_reached ();
+		/* fall-through */
+	case NM_DNS_MANAGER_RESOLV_CONF_MAN_UNMANAGED:
+		return NM_DNS_MANAGER_RESOLV_CONF_MAN_UNMANAGED;
+	default:
+
+		if (lstat (_PATH_RESCONF, &st) != 0)
+			return rc_manager;
+
+		if (S_ISLNK (st.st_mode)) {
+			/* only regular files and directories can have extended file attributes. */
+			switch (rc_manager) {
+			case NM_DNS_MANAGER_RESOLV_CONF_MAN_SYMLINK:
+				/* we don't care whether the link-target is immutable.
+				 * If the symlink points to another file, rc-manager=symlink anyway backs off.
+				 * Otherwise, we would only check whether our internal resolv.conf is immutable. */
+				return NM_DNS_MANAGER_RESOLV_CONF_MAN_SYMLINK;
+			case NM_DNS_MANAGER_RESOLV_CONF_MAN_UNKNOWN:
+			case NM_DNS_MANAGER_RESOLV_CONF_MAN_UNMANAGED:
+			case NM_DNS_MANAGER_RESOLV_CONF_MAN_IMMUTABLE:
+				nm_assert_not_reached ();
+				/* fall-through */
+			case NM_DNS_MANAGER_RESOLV_CONF_MAN_FILE:
+			case NM_DNS_MANAGER_RESOLV_CONF_MAN_RESOLVCONF:
+			case NM_DNS_MANAGER_RESOLV_CONF_MAN_NETCONFIG:
+				break;
+			}
+		}
+
+		fd = open (_PATH_RESCONF, O_RDONLY);
+		if (fd != -1) {
+			if (ioctl (fd, FS_IOC_GETFLAGS, &flags) != -1)
+				immutable = NM_FLAGS_HAS (flags, FS_IMMUTABLE_FL);
+			close (fd);
+		}
+		return immutable ? NM_DNS_MANAGER_RESOLV_CONF_MAN_IMMUTABLE : rc_manager;
 	}
-	return immutable;
 }
 
 NM_DEFINE_SINGLETON_GETTER (NMDnsManager, nm_dns_manager_get, NM_TYPE_DNS_MANAGER);
@@ -1537,8 +1571,6 @@ init_resolv_conf_mode (NMDnsManager *self, gboolean force_reload_plugin)
 
 	if (nm_streq0 (mode, "none"))
 		rc_manager = NM_DNS_MANAGER_RESOLV_CONF_MAN_UNMANAGED;
-	else if (_get_resconf_immutable ())
-		rc_manager = NM_DNS_MANAGER_RESOLV_CONF_MAN_IMMUTABLE;
 	else {
 		const char *man;
 
@@ -1570,6 +1602,8 @@ again:
 		}
 	}
 
+	rc_manager = _check_resconf_immutable (rc_manager);
+
 	if (nm_streq0 (mode, "dnsmasq")) {
 		if (force_reload_plugin || !NM_IS_DNS_DNSMASQ (priv->plugin)) {
 			_clear_plugin (self);