[PATCH net] amt: do not offer software GSO on the amt device

[Omar Ramadan <omar@blockcast.net>]

amt_link_setup() puts NETIF_F_GSO_SOFTWARE in the device features, so
the core hands amt_dev_xmit() a GSO skb unsegmented: a UDP_SEGMENT
sender, or a UDP packet GRO merged on a link with UDP GRO forwarding
or fraglist GRO, that is routed to the amt device. The relay copies
the skb for each tunnel in amt_send_multicast_data() and pushes the AMT
and outer headers, but it never marks the copy for UDP tunnel
segmentation, so skb->encapsulation stays clear and gso_type is still
the inner SKB_GSO_UDP_L4. When the outer IP layer segments it,
__udp_gso_segment() cuts the outer UDP payload at the inner gso_size:
the first segment carries the AMT header, the inner headers and part
of the data, and every later one carries raw inner payload behind a
new outer UDP header, which a gateway takes for a malformed AMT
message.

The amt device cannot offload segmentation to anything, so stop
offering it and let validate_xmit_skb() segment a GSO skb before
amt_dev_xmit() replicates it. Each gateway then gets one AMT message
per inner packet. Neither mode needs GSO: a gateway only sends IGMP
and MLD reports.

Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface")
Signed-off-by: Omar Ramadan <omar@blockcast.net>
---
 drivers/net/amt.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index bddc24e18..6eb148c9f 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c
@@ -3155,10 +3155,8 @@ static void amt_link_setup(struct net_device *dev)
 	dev->priv_flags		|= IFF_NO_QUEUE;
 	dev->lltx		= true;
 	dev->netns_immutable	= true;
-	dev->features		|= NETIF_F_GSO_SOFTWARE;
 	dev->hw_features	|= NETIF_F_SG | NETIF_F_HW_CSUM;
 	dev->hw_features	|= NETIF_F_FRAGLIST | NETIF_F_RXCSUM;
-	dev->hw_features	|= NETIF_F_GSO_SOFTWARE;
 	dev->pcpu_stat_type	= NETDEV_PCPU_STAT_TSTATS;
 	eth_hw_addr_random(dev);
 	eth_zero_addr(dev->broadcast);

base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7
-- 
2.47.3