[PATCH net-next v6 6/8] ip_tunnel: make ip_tunnel_encap() return a drop reason

[Anton Danilov <littlesmilingcloud@gmail.com>]

ip_tunnel_encap() builds the FOU or GUE header of an IPv4 tunnel and
returns 0 or a negative error, so its callers can tell that the packet
has to be dropped, but not why.

Make it return the drop reason instead, like __iptunnel_pull_header()
and gre_parse_header() do:

 - SKB_NOT_DROPPED_YET when no encapsulation is configured or the
   header is built,
 - SKB_DROP_REASON_NOMEM when the build_header() callback fails with
   -ENOMEM, which the FOU and GUE callbacks do when the header of a GSO
   packet cannot be uncloned,
 - SKB_DROP_REASON_TUNNEL_ENCAP, a new reason, for any other failure,
   such as an encapsulation type without a registered callback.

The three callers, two in ip_tunnel.c and one in sit.c, tested the
result with "< 0". With the enum, that test would always be false and
no compiler warns about it, so they test for a non-zero value now. The
next patch reports the reason in the transmit path of ip_tunnel; sit
still frees the packet with a plain kfree_skb().

ip6_tnl_encap() keeps returning an int for now: its only caller,
ip6_tnl_xmit(), returns an int as well, and both belong to the IPv6
transmit path, which is left for a later series.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
---
 include/net/dropreason-core.h |  7 +++++++
 include/net/ip_tunnels.h      | 16 ++++++++++------
 net/ipv4/ip_tunnel.c          |  4 ++--
 net/ipv6/sit.c                |  2 +-
 4 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index 2df44745cf3b..fb6f3d8da960 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -131,6 +131,7 @@
 	FN(TUNNEL_OPT_MISMATCH)		\
 	FN(TUNNEL_OLD_SEQ)		\
 	FN(GRE_CSUM)			\
+	FN(TUNNEL_ENCAP)		\
 	FNe(MAX)
 
 /**
@@ -634,6 +635,12 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_TUNNEL_OLD_SEQ,
 	/** @SKB_DROP_REASON_GRE_CSUM: GRE checksum error */
 	SKB_DROP_REASON_GRE_CSUM,
+	/**
+	 * @SKB_DROP_REASON_TUNNEL_ENCAP: failed to build the encapsulation
+	 * header of a tunnel, e.g. an unknown or unregistered encapsulation
+	 * type.
+	 */
+	SKB_DROP_REASON_TUNNEL_ENCAP,
 	/**
 	 * @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which
 	 * shouldn't be used as a real 'reason' - only for tracing code gen
diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
index 792690ae4f26..87046ba50d45 100644
--- a/include/net/ip_tunnels.h
+++ b/include/net/ip_tunnels.h
@@ -541,18 +541,18 @@ static inline int ip_encap_hlen(struct ip_tunnel_encap *e)
 	return hlen;
 }
 
-static inline int ip_tunnel_encap(struct sk_buff *skb,
-				  struct ip_tunnel_encap *e,
-				  u8 *protocol, struct flowi4 *fl4)
+static inline enum skb_drop_reason
+ip_tunnel_encap(struct sk_buff *skb, struct ip_tunnel_encap *e,
+		u8 *protocol, struct flowi4 *fl4)
 {
 	const struct ip_tunnel_encap_ops *ops;
 	int ret = -EINVAL;
 
 	if (e->type == TUNNEL_ENCAP_NONE)
-		return 0;
+		return SKB_NOT_DROPPED_YET;
 
 	if (e->type >= MAX_IPTUN_ENCAP_OPS)
-		return -EINVAL;
+		return SKB_DROP_REASON_TUNNEL_ENCAP;
 
 	rcu_read_lock();
 	ops = rcu_dereference(iptun_encaps[e->type]);
@@ -560,7 +560,11 @@ static inline int ip_tunnel_encap(struct sk_buff *skb,
 		ret = ops->build_header(skb, e, protocol, fl4);
 	rcu_read_unlock();
 
-	return ret;
+	if (!ret)
+		return SKB_NOT_DROPPED_YET;
+	if (ret == -ENOMEM)
+		return SKB_DROP_REASON_NOMEM;
+	return SKB_DROP_REASON_TUNNEL_ENCAP;
 }
 
 /* Extract dsfield from inner protocol */
diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 7d8744ed1664..f3c5d8424925 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -619,7 +619,7 @@ void ip_md_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 	if (!tunnel_hlen)
 		tunnel_hlen = ip_encap_hlen(&tun_info->encap);
 
-	if (ip_tunnel_encap(skb, &tun_info->encap, &proto, &fl4) < 0)
+	if (ip_tunnel_encap(skb, &tun_info->encap, &proto, &fl4))
 		goto tx_error;
 
 	use_cache = ip_tunnel_dst_cache_usable(skb, tun_info);
@@ -780,7 +780,7 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 			    tunnel->net, READ_ONCE(tunnel->parms.link),
 			    tunnel->fwmark, skb_get_hash(skb), 0);
 
-	if (ip_tunnel_encap(skb, &tunnel->encap, &protocol, &fl4) < 0)
+	if (ip_tunnel_encap(skb, &tunnel->encap, &protocol, &fl4))
 		goto tx_error;
 
 	if (connected && md) {
diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index a7ec800216ad..9576d146c961 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -1096,7 +1096,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 		ttl = iph6->hop_limit;
 	tos = INET_ECN_encapsulate(tos, ipv6_get_dsfield(iph6));
 
-	if (ip_tunnel_encap(skb, &tunnel->encap, &protocol, &fl4) < 0) {
+	if (ip_tunnel_encap(skb, &tunnel->encap, &protocol, &fl4)) {
 		ip_rt_put(rt);
 		goto tx_error;
 	}
-- 
2.47.3