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

[Leon Hwang <leon.hwang@linux.dev>]

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 <leon.huangfu@shopee.com>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 net/core/lwt_bpf.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index f71ef82a5f3d..cbb2803367c6 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -637,6 +637,10 @@ 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 && iph->protocol == IPPROTO_UDP /* UDP tunnel */)
+		skb_set_transport_header(skb, skb_network_offset(skb) + iph->ihl * 4);
+	else if (!ipv4 && ((struct ipv6hdr *)hdr)->nexthdr == NEXTHDR_UDP /* UDP tunnel */)
+		skb_set_transport_header(skb, skb_network_offset(skb) + sizeof(struct ipv6hdr));
 	memcpy(skb_network_header(skb), hdr, len);
 	bpf_compute_data_pointers(skb);
 	skb_clear_hash(skb);
-- 
2.54.0