On Mon Jun 1, 2026 at 11:02 AM EDT, Leon Hwang wrote:
> 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 protected]>
> Signed-off-by: Leon Hwang <[email protected]>
> ---
> 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 f71ef82a5f3d..65d1dfbf3312 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 +644,10 @@ 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 (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));
Minor nit: Could we do something like
if (is_udp_tunnel) {
size_t hrdsz = ipv4 ? iph->ihl * 4 : sizeof(struct ipv6hdr);
skb_set_transport_header(skb9, skb_network_offset(skb)) + hdrsz;
}
to make the logic clearer?
> memcpy(skb_network_header(skb), hdr, len);
> bpf_compute_data_pointers(skb);
> skb_clear_hash(skb);
