Subject: [PATCH bpf v4 1/2] bpf: Update transport_header when encapsulating UDP tunnel in lwt

From: Leon Hwang <[email-masked]>
Date: Tue, 2 Jun 2026 23:09:30 +0800

Currently, bpf_lwt_push_ip_encap() does not update skb->transport_header.
When a driver, e.g. ice, reuses the stale skb->transport_header to
offload checksum computation to NIC hardware, VxLAN packets encapsulated
by bpf_lwt_push_encap() helper may be dropped due to incorrect checksum.

Update skb->transport_header in bpf_lwt_push_ip_encap() whenever the
encapsulated packet uses UDP, so checksum offload works correctly.

Fixes: 52f278774e79 ("bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap")
Cc: Leon Hwang <[email-masked]>
Signed-off-by: Leon Hwang <[email-masked]>
---
 net/core/lwt_bpf.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index f71ef82a5f3d..bf588f508b79 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -599,6 +599,7 @@ static int handle_gso_encap(struct sk_buff *skb, bool ipv4, int encap_len)
 
 int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
 {
+	bool is_udp_tunnel;
 	struct iphdr *iph;
 	bool ipv4;
 	int err;
@@ -612,10 +613,16 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
 		ipv4 = true;
 		if (unlikely(len < iph->ihl * 4))
 			return -EINVAL;
+		is_udp_tunnel = iph->protocol == IPPROTO_UDP;
+		if (unlikely(is_udp_tunnel && len < iph->ihl * 4 + sizeof(struct udphdr)))
+			return -EINVAL;
 	} else if (iph->version == 6) {
 		ipv4 = false;
 		if (unlikely(len < sizeof(struct ipv6hdr)))
 			return -EINVAL;
+		is_udp_tunnel = ((struct ipv6hdr *)iph)->nexthdr == NEXTHDR_UDP;
+		if (unlikely(is_udp_tunnel && len < sizeof(struct ipv6hdr) + sizeof(struct udphdr)))
+			return -EINVAL;
 	} else {
 		return -EINVAL;
 	}
@@ -637,6 +644,11 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
 	if (ingress)
 		skb_postpush_rcsum(skb, iph, len);
 	skb_reset_network_header(skb);
+	if (is_udp_tunnel) {
+		size_t iph_sz = ipv4 ? iph->ihl * 4 : sizeof(struct ipv6hdr);
+
+		skb_set_transport_header(skb, skb_network_offset(skb) + iph_sz);
+	}
 	memcpy(skb_network_header(skb), hdr, len);
 	bpf_compute_data_pointers(skb);
 	skb_clear_hash(skb);
-- 
2.54.0