netdev - [PATCH net v3 4/5] packet: infer protocol from ethernet header if unset

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Date:	Wed, 11 Nov 2015 23:25:43 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	davem@...emloft.net
Cc:	edumazet@...gle.com, willemb@...gle.com, tklauser@...tanz.ch,
	netdev@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH net v3 4/5] packet: infer protocol from ethernet header if unset

In case no struct sockaddr_ll has been passed to packet
socket's sendmsg() when doing a TX_RING flush run, then
skb->protocol is set to po->num instead, which is the protocol
passed via socket(2)/bind(2).

Applications only xmitting can go the path of allocating the
socket as socket(PF_PACKET, <mode>, 0) and do a bind(2) on the
TX_RING with sll_protocol of 0. That way, register_prot_hook()
is neither called on creation nor on bind time, which saves
cycles when there's no interest in capturing anyway.

That leaves us however with po->num 0 instead and therefore
the TX_RING flush run sets skb->protocol to 0 as well. Eric
reported that this leads to problems when using tools like
trafgen over bonding device. I.e. the bonding's hash function
could invoke the kernel's flow dissector, which depends on
skb->protocol being properly set. In the current situation, all
the traffic is then directed to a single slave.

Fix it up by inferring skb->protocol from the Ethernet header
when not set and we have ARPHRD_ETHER device type. This is only
done in case of SOCK_RAW and where we have a dev->hard_header_len
length. In case of ARPHRD_ETHER devices, this is guaranteed to
cover ETH_HLEN, and therefore being accessed on the skb after
the skb_store_bits().

Reported-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
---
 net/packet/af_packet.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 8795b0f..0066da2 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2338,6 +2338,15 @@ static bool ll_header_truncated(const struct net_device *dev, int len)
 	return false;
 }

+static void tpacket_set_protocol(const struct net_device *dev,
+				 struct sk_buff *skb)
+{
+	if (dev->type == ARPHRD_ETHER) {
+		skb_reset_mac_header(skb);
+		skb->protocol = eth_hdr(skb)->h_proto;
+	}
+}
+
 static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
 		void *frame, struct net_device *dev, int size_max,
 		__be16 proto, unsigned char *addr, int hlen)
@@ -2419,6 +2428,8 @@ static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
 				dev->hard_header_len);
 		if (unlikely(err))
 			return err;
+		if (!skb->protocol)
+			tpacket_set_protocol(dev, skb);

 		data += dev->hard_header_len;
 		to_write -= dev->hard_header_len;
-- 
1.9.3

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