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Miller" , "Eric Dumazet" , "Jakub Kicinski" , "Paolo Abeni" , "Simon Horman" , "Andrii Nakryiko" , "Eduard Zingerman" , "Alexei Starovoitov" , "Daniel Borkmann" , "Martin KaFai Lau" , "Kumar Kartikeya Dwivedi" , "Song Liu" , "Yonghong Song" , "Jiri Olsa" , "Shuah Khan" , "Guillaume Nault" , "Ido Schimmel" , "Fernando Fernandez Mancera" , "Peter Oskolkov" , , , , , "Leon Hwang" Subject: Re: [PATCH bpf v3 1/2] bpf: Update transport_header when encapsulating UDP tunnel in lwt From: "Emil Tsalapatis" To: "Leon Hwang" , X-Mailer: aerc 0.21.0-0-g5549850facc2 References: <20260601150203.20352-1-leon.hwang@linux.dev> <20260601150203.20352-2-leon.hwang@linux.dev> In-Reply-To: <20260601150203.20352-2-leon.hwang@linux.dev> 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_pus= h_encap") > Cc: Leon Hwang > Signed-off-by: Leon Hwang > --- > 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) > =20 > 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 =3D true; > if (unlikely(len < iph->ihl * 4)) > return -EINVAL; > + is_udp_tunnel =3D iph->protocol =3D=3D IPPROTO_UDP; > + if (unlikely(is_udp_tunnel && len < iph->ihl * 4 + sizeof(struct udphd= r))) > + return -EINVAL; > } else if (iph->version =3D=3D 6) { > ipv4 =3D false; > if (unlikely(len < sizeof(struct ipv6hdr))) > return -EINVAL; > + is_udp_tunnel =3D ((struct ipv6hdr *)iph)->nexthdr =3D=3D NEXTHDR_UDP; > + if (unlikely(is_udp_tunnel && len < sizeof(struct ipv6hdr) + sizeof(st= ruct 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 =3D 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);