[PATCH net 2/4] ip_gre: check packet length and mtu correctly in erspan_xmit

[Xin Long]

As a ARPHRD_ETHER device, skb->len in erspan_xmit is the length
of the whole ether packet. So before checking if a packet size
exceeds the mtu, skb->len should subtract dev->hard_header_len.

Otherwise, all packets with max size according to mtu would be
trimmed to be truncated packet.

Fixes: 84e54fe0a5ea ("gre: introduce native tunnel support for ERSPAN")
Signed-off-by: Xin Long <lucien....@gmail.com>
---
 net/ipv4/ip_gre.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index b25b1e5..2a4ef9d 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -731,7 +731,7 @@ static netdev_tx_t erspan_xmit(struct sk_buff *skb,
        if (skb_cow_head(skb, dev->needed_headroom))
                goto free_skb;
 
-       if (skb->len > dev->mtu) {
+       if (skb->len - dev->hard_header_len > dev->mtu) {
                pskb_trim(skb, dev->mtu);
                truncate = true;
        }
-- 
2.1.0