Fix for cgroup filter addition problem.

Currently, due to the incomplete netlink datagram sent by libnl, cgroup
filter addition is not fully functional. The datagram generated by `tc`
command includes an empty attribute section, which is stripped off
in the libnl counterpart.

In this commit, a new `interface nla_nest_end_keep_empty()` is added.
This function closes attribute without stripping off empty attribute.

4 files changed, 55 insertions, 1 deletions

diff --git a/include/netlink/attr.h b/include/netlink/attr.h
index da9eee4..e47aa5d 100644
--- a/include/netlink/attr.h
+++ b/include/netlink/attr.h
@@ -143,6 +143,7 @@ extern int		nla_put_nested(struct nl_msg *, int,
 				       const struct nl_msg *);
 extern struct nlattr *	nla_nest_start(struct nl_msg *, int);
 extern int		nla_nest_end(struct nl_msg *, struct nlattr *);
+extern int		nla_nest_end_keep_empty(struct nl_msg *, struct nlattr *);
 extern void		nla_nest_cancel(struct nl_msg *, const struct nlattr *);
 extern int		nla_parse_nested(struct nlattr **, int, struct nlattr *,
 					 const struct nla_policy *);
diff --git a/lib/attr.c b/lib/attr.c
index b81e550..d90fa47 100644
--- a/lib/attr.c
+++ b/lib/attr.c
@@ -962,6 +962,55 @@ int nla_nest_end(struct nl_msg *msg, struct nlattr *start)
 }
 
 /**
+ * Finalize nesting of attributes without stripping off empty attributes.
+ * @arg msg		Netlink message.
+ * @arg start		Container attribute as returned from nla_nest_start().
+ *
+ * Corrects the container attribute header to include the appeneded attributes.
+ * Keep empty attribute if NO actual attribute payload exists.
+ *
+ * @return 0 on success or a negative error code.
+ */
+int nla_nest_end_keep_empty(struct nl_msg *msg, struct nlattr *start)
+{
+	size_t pad, len;
+
+	len = (void *) nlmsg_tail(msg->nm_nlh) - (void *) start;
+
+	if (len > USHRT_MAX) {
+		/*
+		 * Max nlattr size is exceeded, trim the attribute header again
+		 */
+		nla_nest_cancel(msg, start);
+
+		/* Return error only if nlattr size was exceeded */
+		return -NLE_ATTRSIZE;
+	}
+
+	start->nla_len = len;
+
+	pad = NLMSG_ALIGN(msg->nm_nlh->nlmsg_len) - msg->nm_nlh->nlmsg_len;
+	if (pad > 0) {
+		/*
+		 * Data inside attribute does not end at a alignment boundry.
+		 * Pad accordingly and accoun for the additional space in
+		 * the message. nlmsg_reserve() may never fail in this situation,
+		 * the allocate message buffer must be a multiple of NLMSG_ALIGNTO.
+		 */
+		if (!nlmsg_reserve(msg, pad, 0))
+			BUG();
+
+		NL_DBG(2, "msg %p: attr <%p> %d: added %zu bytes of padding\n",
+			msg, start, start->nla_type, pad);
+	}
+
+	NL_DBG(2, "msg %p: attr <%p> %d: closing nesting, len=%u\n",
+		msg, start, start->nla_type, start->nla_len);
+
+	return 0;
+}
+
+/**
  * Cancel the addition of a nested attribute
  * @arg msg		Netlink message
  * @arg attr		Nested netlink attribute
diff --git a/lib/route/tc.c b/lib/route/tc.c
index 5dc43e1..62efd35 100644
--- a/lib/route/tc.c
+++ b/lib/route/tc.c
@@ -226,7 +226,10 @@ int rtnl_tc_msg_build(struct rtnl_tc *tc, int type, int flags,
 			if ((err = ops->to_msg_fill(tc, data, msg)) < 0)
 				goto nla_put_failure;
 
-			nla_nest_end(msg, opts);
+			if (strcmp("cgroup", tc->tc_kind))
+				nla_nest_end(msg, opts);
+			else
+				nla_nest_end_keep_empty(msg, opts);
 		} else if ((err = ops->to_msg_fill_raw(tc, data, msg)) < 0)
 			goto nla_put_failure;
 	}
diff --git a/libnl-3.sym b/libnl-3.sym
index 4546a40..42b38b7 100644
--- a/libnl-3.sym
+++ b/libnl-3.sym
@@ -260,6 +260,7 @@ global:
 	nla_memcpy;
 	nla_nest_cancel;
 	nla_nest_end;
+	nla_nest_end_keep_empty;
 	nla_nest_start;
 	nla_next;
 	nla_ok;