packet: do skb_probe_transport_header when we actually have data

In tpacket_fill_skb() commit c1aad275b029 ("packet: set transport
header before doing xmit") and later on 40893fd0fd4e ("net: switch
to use skb_probe_transport_header()") was probing for a transport
header on the skb from a ring buffer slot, but at a time, where
the skb has _not even_ been filled with data yet. So that call into
the flow dissector is pretty useless. Lets do it after we've set
up the skb frags.

Fixes: c1aad275b029 ("packet: set transport header before doing xmit")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index af399ca..80c36c0 100644 (file)
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2368,8 +2368,6 @@ static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
        skb_reserve(skb, hlen);
        skb_reset_network_header(skb);
 
-       if (!packet_use_direct_xmit(po))
-               skb_probe_transport_header(skb, 0);
        if (unlikely(po->tp_tx_has_off)) {
                int off_min, off_max, off;
                off_min = po->tp_hdrlen - sizeof(struct sockaddr_ll);
@@ -2449,6 +2447,9 @@ static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
                len = ((to_write > len_max) ? len_max : to_write);
        }
 
+       if (!packet_use_direct_xmit(po))
+               skb_probe_transport_header(skb, 0);
+
        return tp_len;
 }