Subject: [PATCH bpf v2 3/4] bpf: Update transport_header when encapsulating UDP tunnel in lwt

From: Leon Hwang <[email-masked]>
Date: Fri, 29 May 2026 23:13:50 +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 | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index c306120e11d2..1d556dec94b4 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -600,6 +600,7 @@ 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)
 {
 	u8 buff[LWT_BPF_MAX_HEADROOM];
+	bool is_udp_tunnel;
 	struct iphdr *iph;
 	bool ipv4;
 	int err;
@@ -615,10 +616,16 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
 		ipv4 = true;
 		if (unlikely(iph->ihl < 5 || 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;
 	}
@@ -641,6 +648,10 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
 		skb_postpush_rcsum(skb, iph, len);
 	skb_reset_network_header(skb);
 	memcpy(skb_network_header(skb), buff, len);
+	if (ipv4 && is_udp_tunnel)
+		skb_set_transport_header(skb, skb_network_offset(skb) + iph->ihl * 4);
+	else if (!ipv4 && is_udp_tunnel)
+		skb_set_transport_header(skb, skb_network_offset(skb) + sizeof(struct ipv6hdr));
 	bpf_compute_data_pointers(skb);
 	skb_clear_hash(skb);
 
-- 
2.54.0