shared: reject '%' from nm_utils_ifname_valid() for kernel names

Generally, it's dangerous to reject values that were accepted
previously. This will lead to NetworkManager being unable to load
a profile from disk, which was loadable previously.

On the other hand, kernel would not have treated this setting as
it was intended. So, I would argue that the such a setting was not
working (as intended) anyway.

We can only hope that users don't configure arbitrary interface names.
It generally isn't a good idea to do, so "breaking" such things is less
of a concern.

shared/nm-glib-aux/nm-shared-utils.c

@@ -4126,8 +4126,34 @@ nm_utils_ifname_valid_kernel (const char *name, GError **error)
 	return FALSE;
 }
 
+/*****************************************************************************/
+
 static gboolean
-_nm_utils_ifname_valid_ovs (const char* name, GError **error)
+_nm_utils_ifname_valid_kernel (const char *name, GError **error)
+{
+	if (!nm_utils_ifname_valid_kernel (name, error))
+		return FALSE;
+
+	if (strchr (name, '%')) {
+		/* Kernel's dev_valid_name() accepts (almost) any binary up to 15 chars.
+		 * However, '%' is treated special as a format specifier. Try
+		 *
+		 *   ip link add 'dummy%dx' type dummy
+		 *
+		 * Don't allow that for "connection.interface-name", which either
+		 * matches an existing netdev name (thus, it cannot have a '%') or
+		 * is used to configure a name (in which case we don't want kernel
+		 * to replace the format specifier). */
+		g_set_error_literal (error, NM_UTILS_ERROR, NM_UTILS_ERROR_UNKNOWN,
+		                     _("'%%' is not allowed in interface names"));
+		return FALSE;
+	}
+
+	return TRUE;
+}
+
+static gboolean
+_nm_utils_ifname_valid_ovs (const char *name, GError **error)
 {
 	const char *ch;
 
@@ -4169,7 +4195,7 @@ nm_utils_ifname_valid (const char* name,
 
 	switch (type) {
 	case NMU_IFACE_KERNEL:
-		return nm_utils_ifname_valid_kernel (name, error);
+		return _nm_utils_ifname_valid_kernel (name, error);
 	case NMU_IFACE_OVS:
 		return _nm_utils_ifname_valid_ovs (name, error);
 	}