[PATCH net-next v4 1/2] l2tp: account for IP version in SKB headroom

Account for the IP version of the tunnel when accounting skb headroom on
xmit. This avoids having to potentially copy the skb a second time down
the stack due to allocating not enough space for IPv6 headers in case
the tunnel uses IPv6.

Signed-off-by: David Bauer <[email protected]>
---
 v4:
  - Update comment to reflect new code behavior

 net/l2tp/l2tp_core.c |  3 ++-
 net/l2tp/l2tp_core.h |  1 +
 net/l2tp/l2tp_eth.c  | 14 ++++----------
 3 files changed, 7 insertions(+), 11 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 4712cc41881a3..d6fa0f7436629 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1237,7 +1237,7 @@ static int l2tp_xmit_core(struct l2tp_session *session, struct sk_buff *skb, uns
 	 * make room. Adjust truesize.
 	 */
 	uhlen = (tunnel->encap == L2TP_ENCAPTYPE_UDP) ? sizeof(*uh) : 0;
-	headroom = NET_SKB_PAD + sizeof(struct iphdr) + uhlen + session->hdr_len;
+	headroom = NET_SKB_PAD + tunnel->l3_overhead + uhlen + session->hdr_len;
 	if (skb_cow_head(skb, headroom)) {
 		kfree_skb(skb);
 		return NET_XMIT_DROP;
@@ -1688,6 +1688,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
 	}
 
 	sk->sk_allocation = GFP_ATOMIC;
+	tunnel->l3_overhead = kernel_sock_ip_overhead(sk);
 	release_sock(sk);
 
 	sock_hold(sk);
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index ffd8ced3a51ff..aab574376d95f 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -167,6 +167,7 @@ struct l2tp_tunnel {
 	u32			tunnel_id;
 	u32			peer_tunnel_id;
 	int			version;	/* 2=>L2TPv2, 3=>L2TPv3 */
+	int			l3_overhead;	/* IP header overhead */
 
 	char			name[L2TP_TUNNEL_NAME_MAX]; /* for logging */
 	enum l2tp_encap_type	encap;
diff --git a/net/l2tp/l2tp_eth.c b/net/l2tp/l2tp_eth.c
index a4956ef9574cc..66d1fc3ad409a 100644
--- a/net/l2tp/l2tp_eth.c
+++ b/net/l2tp/l2tp_eth.c
@@ -188,7 +188,6 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel,
 				struct net_device *dev)
 {
 	unsigned int overhead = 0;
-	u32 l3_overhead = 0;
 	u32 mtu;
 
 	/* if the encap is UDP, account for UDP header size */
@@ -197,22 +196,17 @@ static void l2tp_eth_adjust_mtu(struct l2tp_tunnel *tunnel,
 		dev->needed_headroom += sizeof(struct udphdr);
 	}
 
-	lock_sock(tunnel->sock);
-	l3_overhead = kernel_sock_ip_overhead(tunnel->sock);
-	release_sock(tunnel->sock);
-
-	if (l3_overhead == 0) {
+	if (tunnel->l3_overhead == 0) {
 		/* L3 Overhead couldn't be identified, this could be
-		 * because tunnel->sock was NULL or the socket's
-		 * address family was not IPv4 or IPv6,
-		 * dev mtu stays at 1500.
+		 * because the socket's address family was not IPv4
+		 * or IPv6 - dev mtu stays at 1500.
 		 */
 		return;
 	}
 	/* Adjust MTU, factor overhead - underlay L3, overlay L2 hdr
 	 * UDP overhead, if any, was already factored in above.
 	 */
-	overhead += session->hdr_len + ETH_HLEN + l3_overhead;
+	overhead += session->hdr_len + ETH_HLEN + tunnel->l3_overhead;
 
 	mtu = l2tp_tunnel_dst_mtu(tunnel) - overhead;
 	if (mtu < dev->min_mtu || mtu > dev->max_mtu)
-- 
2.53.0