[PATCH net-next v4 02/10] ip6_tunnel: add drop reasons to the generic RX path

[Anton Danilov <littlesmilingcloud@gmail.com>]

__ip6_tnl_rcv() mirrors its IPv4 counterpart: all of its failures share
a single plain kfree_skb(). Reuse the drop reasons that ip_tunnel_rcv()
now reports and the ones the length helpers already return.

Note that skb_vlan_inet_prepare() returns an enum skb_drop_reason that
has so far been discarded, and that the ETH_HLEN check gets one by
calling pskb_may_pull_reason() instead of pskb_may_pull().

__ip6_tnl_rcv() is reached two ways: ip6_gre (ip6gre, ip6gretap,
ip6erspan) goes through the exported ip6_tnl_rcv(), while the
ip6_tunnel encapsulations (ip4ip6, ip6ip6, mplsip6) reach it from
ipxip6_rcv(). Only ip6_gre sets the checksum and sequence number bits:
tpi_v4, tpi_v6 and tpi_mpls carry nothing but .proto, and unlike ipip
and sit, ip6_tunnel stores IFLA_IPTUN_FLAGS in parms.flags, not in
parms.i_flags. So the option mismatch and the old sequence reasons are
reachable through ip6_gre alone.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
---
 net/ipv6/ip6_tunnel.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index d5ff50a2ac01..458ce328311b 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -813,6 +813,7 @@ static int __ip6_tnl_rcv(struct ip6_tnl *tunnel, struct sk_buff *skb,
 						struct sk_buff *skb),
 			 bool log_ecn_err)
 {
+	enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
 	const struct ipv6hdr *ipv6h;
 	int nh, err;
 
@@ -820,15 +821,22 @@ static int __ip6_tnl_rcv(struct ip6_tnl *tunnel, struct sk_buff *skb,
 	    test_bit(IP_TUNNEL_CSUM_BIT, tpi->flags)) {
 		DEV_STATS_INC(tunnel->dev, rx_crc_errors);
 		DEV_STATS_INC(tunnel->dev, rx_errors);
+		reason = SKB_DROP_REASON_TNL_OPT_MISMATCH;
 		goto drop;
 	}
 
 	if (test_bit(IP_TUNNEL_SEQ_BIT, tunnel->parms.i_flags)) {
-		if (!test_bit(IP_TUNNEL_SEQ_BIT, tpi->flags) ||
-		    (tunnel->i_seqno &&
-		     (s32)(ntohl(tpi->seq) - tunnel->i_seqno) < 0)) {
+		if (!test_bit(IP_TUNNEL_SEQ_BIT, tpi->flags)) {
 			DEV_STATS_INC(tunnel->dev, rx_fifo_errors);
 			DEV_STATS_INC(tunnel->dev, rx_errors);
+			reason = SKB_DROP_REASON_TNL_OPT_MISMATCH;
+			goto drop;
+		}
+		if (tunnel->i_seqno &&
+		    (s32)(ntohl(tpi->seq) - tunnel->i_seqno) < 0) {
+			DEV_STATS_INC(tunnel->dev, rx_fifo_errors);
+			DEV_STATS_INC(tunnel->dev, rx_errors);
+			reason = SKB_DROP_REASON_TNL_OLD_SEQ;
 			goto drop;
 		}
 		tunnel->i_seqno = ntohl(tpi->seq) + 1;
@@ -838,7 +846,8 @@ static int __ip6_tnl_rcv(struct ip6_tnl *tunnel, struct sk_buff *skb,
 
 	/* Warning: All skb pointers will be invalidated! */
 	if (tunnel->dev->type == ARPHRD_ETHER) {
-		if (!pskb_may_pull(skb, ETH_HLEN)) {
+		reason = pskb_may_pull_reason(skb, ETH_HLEN);
+		if (reason) {
 			DEV_STATS_INC(tunnel->dev, rx_length_errors);
 			DEV_STATS_INC(tunnel->dev, rx_errors);
 			goto drop;
@@ -859,7 +868,8 @@ static int __ip6_tnl_rcv(struct ip6_tnl *tunnel, struct sk_buff *skb,
 
 	skb_reset_network_header(skb);
 
-	if (skb_vlan_inet_prepare(skb, true)) {
+	reason = skb_vlan_inet_prepare(skb, true);
+	if (reason) {
 		DEV_STATS_INC(tunnel->dev, rx_length_errors);
 		DEV_STATS_INC(tunnel->dev, rx_errors);
 		goto drop;
@@ -881,6 +891,7 @@ static int __ip6_tnl_rcv(struct ip6_tnl *tunnel, struct sk_buff *skb,
 		if (err > 1) {
 			DEV_STATS_INC(tunnel->dev, rx_frame_errors);
 			DEV_STATS_INC(tunnel->dev, rx_errors);
+			reason = SKB_DROP_REASON_IP_TUNNEL_ECN;
 			goto drop;
 		}
 	}
@@ -898,7 +909,7 @@ static int __ip6_tnl_rcv(struct ip6_tnl *tunnel, struct sk_buff *skb,
 drop:
 	if (tun_dst)
 		dst_release((struct dst_entry *)tun_dst);
-	kfree_skb(skb);
+	kfree_skb_reason(skb, reason);
 	return 0;
 }
 
-- 
2.47.3