[PATCH] s2io: only arm hardware LSO for GSO skbs

[Michael Pratte <slatoncomputers@gmail.com>]

s2io_xmit() enables the Xframe/Xframe-II hardware LSO (TCP segmentation)
engine whenever the skb's gso_type carries SKB_GSO_TCPV4/TCPV6, and
programs the segment size from gso_size:

	offload_type = s2io_offload_type(skb);
	if (offload_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)) {
		txdp->Control_1 |= TXD_TCP_LSO_EN;
		txdp->Control_1 |= TXD_TCP_LSO_MSS(s2io_tcp_mss(skb));
	}

Since commit 51466a7545b7 ("tcp: fill shinfo->gso_type at last moment")
tcp_transmit_skb() sets skb_shinfo(skb)->gso_type unconditionally on
every TCP skb, including non-GSO frames where gso_size is 0. The driver
therefore arms the LSO engine with MSS == 0 for ordinary TCP segments
such as the connection's SYN. The Xframe-II LSO engine treats an MSS of
0 as an illegal descriptor and aborts the transmit (lso_err_reg reports
LSO6_ABORT), so the frame is dropped before it reaches the MAC. The
result is that no TCP can be transmitted on these adapters since v4.2;
UDP and ICMP (which never carry SKB_GSO_TCPV4) are unaffected.

Only arm the LSO engine when the skb is actually GSO (gso_size > 0),
restoring the pre-4.2 behaviour. Non-GSO TCP frames take the normal
transmit path.

Reproduced and fixed on Linux 6.6.67 with an Xframe-II adapter
(PCI 17d5:5832); bisected to good v4.1.6 / bad v4.2.2.

Fixes: 51466a7545b7 ("tcp: fill shinfo->gso_type at last moment")
Signed-off-by: Michael Pratte <slatoncomputers@gmail.com>
---
[ Not upstream and cannot be: the s2io driver was removed from mainline in
  commit aba0138eb7d7 ("net: ethernet: neterion: s2io: remove unused
  driver"). It still ships in the 6.6.y and 6.12.y stable trees, where this
  bug is present and the patch applies cleanly. Please apply there. ]

 drivers/net/ethernet/neterion/s2io.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/neterion/s2io.c b/drivers/net/ethernet/neterion/s2io.c
index 1e55ccb..9988ff9 100644
--- a/drivers/net/ethernet/neterion/s2io.c
+++ b/drivers/net/ethernet/neterion/s2io.c
@@ -4105,7 +4105,7 @@ static netdev_tx_t s2io_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	offload_type = s2io_offload_type(skb);
-	if (offload_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)) {
+	if ((offload_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6)) && skb_is_gso(skb)) {
 		txdp->Control_1 |= TXD_TCP_LSO_EN;
 		txdp->Control_1 |= TXD_TCP_LSO_MSS(s2io_tcp_mss(skb));
 	}
-- 
2.54.0