net: sched: report if filter is too large to dump

[ Upstream commit 5ae437ad ]

So far, if the filter was too large to fit in the allocated skb, the
kernel did not return any error and stopped dumping. Modify the dumper
so that it returns -EMSGSIZE when a filter fails to dump and it is the
first filter in the skb. If we are not first, we will get a next chance
with more room.

I understand this is pretty near to being an API change, but the
original design (silent truncation) can be considered a bug.

Note: The error case can happen pretty easily if you create a filter
with 32 actions and have 4kb pages. Also recent versions of iproute try
to be clever with their buffer allocation size, which in turn leads to
Signed-off-by: Roman Kapl <code@rkapl.cz>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/sched/cls_api.c

@@ -871,13 +871,18 @@ static int tc_dump_tfilter(struct sk_buff *skb, struct netlink_callback *cb)
 		if (tca[TCA_CHAIN] &&
 		    nla_get_u32(tca[TCA_CHAIN]) != chain->index)
 			continue;
-		if (!tcf_chain_dump(chain, skb, cb, index_start, &index))
+		if (!tcf_chain_dump(chain, skb, cb, index_start, &index)) {
+			err = -EMSGSIZE;
 			break;
+		}
 	}
 
 	cb->args[0] = index;
 
 out:
+	/* If we did no progress, the error (EMSGSIZE) is real */
+	if (skb->len == 0 && err)
+		return err;
 	return skb->len;
 }