Subject: [PATCH bpf 2/2] selftests/bpf: Add tests to verify the fix of encapsulating VxLAN in lwt

From: Leon Hwang <[email-masked]>
Date: Mon, 25 May 2026 22:26:50 +0800

Add two tests to verify the transport header of skb has been set when
encapsulating VxLAN using bpf_lwt_push_encap() helper.

1. VxLAN over IPv4.
2. VxLAN over IPv6.

Without the fix, the tests would fail:

 lwt_ip_encap_vxlan:FAIL:transport_hdr offset unexpected transport_hdr offset: actual 70 != expected 20
 #208     lwt_ip_encap_vxlan_ipv4:FAIL
 lwt_ip_encap_vxlan:FAIL:transport_hdr offset unexpected transport_hdr offset: actual 110 != expected 40
 #209     lwt_ip_encap_vxlan_ipv6:FAIL

The unexpected offsets are: outer encap headers
(IPv4: iphdr+udp+vxlan+eth = 50 bytes, IPv6: ipv6hdr+udp+vxlan+eth = 70 bytes)
plus the inner IP header (20 or 40 bytes), because without the fix
transport_header still points at the inner transport layer instead of the
outer UDP header.

Assisted-by: Claude:claude-sonnet-4-6
Cc: Leon Hwang <[email-masked]>
Signed-off-by: Leon Hwang <[email-masked]>
---
 .../selftests/bpf/prog_tests/lwt_ip_encap.c   | 153 ++++++++++++++++++
 .../selftests/bpf/progs/test_lwt_ip_encap.c   | 112 +++++++++++++
 .../bpf/progs/test_lwt_ip_encap_fix.c         |  36 +++++
 3 files changed, 301 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/test_lwt_ip_encap_fix.c

diff --git a/tools/testing/selftests/bpf/prog_tests/lwt_ip_encap.c b/tools/testing/selftests/bpf/prog_tests/lwt_ip_encap.c
index b6391af5f6f9..04fc35470f21 100644
--- a/tools/testing/selftests/bpf/prog_tests/lwt_ip_encap.c
+++ b/tools/testing/selftests/bpf/prog_tests/lwt_ip_encap.c
@@ -1,8 +1,11 @@
 // SPDX-License-Identifier: GPL-2.0-only
+#include <linux/ip.h>
+#include <linux/ipv6.h>
 #include <netinet/in.h>
 
 #include "network_helpers.h"
 #include "test_progs.h"
+#include "test_lwt_ip_encap_fix.skel.h"
 
 #define BPF_FILE "test_lwt_ip_encap.bpf.o"
 
@@ -35,6 +38,10 @@
 #define IP6_ADDR_SRC IP6_ADDR_1
 #define IP6_ADDR_DST IP6_ADDR_4
 
+/* VxLAN tunnel endpoints, reachable via the bottom route (veth5/6/7/8). */
+#define IP4_ADDR_VXLAN  "172.16.17.100"
+#define IP6_ADDR_VXLAN  "fb20::1"
+
 /* Setup/topology:
  *
  *    NS1             NS2             NS3
@@ -538,3 +545,149 @@ void test_lwt_ip_encap_ipv4(void)
 	if (test__start_subtest("ingress"))
 		lwt_ip_encap(IPV4_ENCAP, INGRESS, "");
 }
+
+/* VxLAN Setup/topology:
+ *
+ * NS1 (IP*_ADDR_1)                NS2                  NS3 (IP*_ADDR_4)
+ *       [ping src]
+ *           |                          top route
+ *         veth1 (LWT encap)  <<-- veth2        veth3  -X-  veth4 (ping dst)
+ *           |                                                ^
+ *       (bottom route)                                       | (inner pkt)
+ *           v                        bottom route            |
+ *         veth5              -->> veth6        veth7  -->> veth8 (vxlan decap)
+ *                                                          (IP*_ADDR_VXLAN)
+ *
+ * Add the VxLAN endpoint addresses to NS3's veth8, create standard
+ * VxLAN decap devices bound to those addresses, and install routes so
+ * NS1/NS2 can reach the endpoints via the bottom route.
+ */
+static int setup_vxlan_routes(const char *ns3, const char *ns1, const char *ns2,
+			      const char *vrf)
+{
+	struct nstoken *nstoken;
+
+	nstoken = open_netns(ns3);
+	if (!ASSERT_OK_PTR(nstoken, "open ns3 for vxlan"))
+		return -1;
+
+	SYS(fail_close, "ip    a add %s/32  dev veth8", IP4_ADDR_VXLAN);
+	SYS(fail_close, "ip -6 a add %s/128 dev veth8", IP6_ADDR_VXLAN);
+	/* Standard VxLAN devices to decap the encapsulated packets.  The inner
+	 * Ethernet frame uses a broadcast dst MAC so the IP stack accepts it
+	 * without ARP or FDB configuration.
+	 */
+	SYS(fail_close, "ip link add vxlan4 type vxlan id 1 dstport 4789 local %s dev veth8 nolearning noudpcsum",
+	    IP4_ADDR_VXLAN);
+	SYS(fail_close, "ip link set vxlan4 up");
+	SYS(fail_close, "ip link add vxlan6 type vxlan id 1 dstport 4789 local %s dev veth8 nolearning udp6zerocsumrx",
+	    IP6_ADDR_VXLAN);
+	SYS(fail_close, "ip link set vxlan6 up");
+	close_netns(nstoken);
+
+	SYS(fail, "ip -n %s    route add %s/32  dev veth5 via %s %s",
+	    ns1, IP4_ADDR_VXLAN, IP4_ADDR_6, vrf);
+	SYS(fail, "ip -n %s    route add %s/32  dev veth7 via %s %s",
+	    ns2, IP4_ADDR_VXLAN, IP4_ADDR_8, vrf);
+	SYS(fail, "ip -n %s -6 route add %s/128 dev veth5 via %s %s",
+	    ns1, IP6_ADDR_VXLAN, IP6_ADDR_6, vrf);
+	SYS(fail, "ip -n %s -6 route add %s/128 dev veth7 via %s %s",
+	    ns2, IP6_ADDR_VXLAN, IP6_ADDR_8, vrf);
+	return 0;
+
+fail_close:
+	close_netns(nstoken);
+fail:
+	return -1;
+}
+
+/* VxLAN encap tests (IPv4-outer and IPv6-outer variants).
+ *
+ * Test 1 - functional: the BPF LWT xmit program encapsulates the packet
+ *   (protocol=UDP, port=4789) and re-routes it without dropping it.
+ *   Verified by ping success.
+ *
+ * Test 2 - fix verification: after bpf_lwt_push_ip_encap() the
+ *   skb->transport_header must point at the outer UDP header, i.e.
+ *   transport_header - network_header == sizeof(outer IP header).
+ *   Without the fix the transport_header still points at the inner
+ *   transport layer, giving a wrong (larger) offset.
+ */
+static void lwt_ip_encap_vxlan(bool ipv4_encap)
+{
+	char ns1[NETNS_NAME_SIZE] = NETNS_BASE "-1-";
+	char ns2[NETNS_NAME_SIZE] = NETNS_BASE "-2-";
+	char ns3[NETNS_NAME_SIZE] = NETNS_BASE "-3-";
+	const char *sec = ipv4_encap ? "encap_vxlan" : "encap_vxlan6";
+	int expected_offset = ipv4_encap ? (int)sizeof(struct iphdr)
+					 : (int)sizeof(struct ipv6hdr);
+	struct test_lwt_ip_encap_fix *skel = NULL;
+	int thdr_offset;
+
+	if (!ASSERT_OK(create_ns(ns1, NETNS_NAME_SIZE), "create ns1"))
+		goto out;
+	if (!ASSERT_OK(create_ns(ns2, NETNS_NAME_SIZE), "create ns2"))
+		goto out;
+	if (!ASSERT_OK(create_ns(ns3, NETNS_NAME_SIZE), "create ns3"))
+		goto out;
+
+	if (!ASSERT_OK(setup_network(ns1, ns2, ns3, ""), "setup network"))
+		goto out;
+
+	if (!ASSERT_OK(setup_vxlan_routes(ns3, ns1, ns2, ""), "setup vxlan routes"))
+		goto out;
+
+	/* Attach fexit to bpf_lwt_push_ip_encap() before installing the
+	 * LWT route so we don't miss the first encap call.
+	 */
+	skel = test_lwt_ip_encap_fix__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "test_lwt_ip_encap_fix__open_and_load"))
+		goto out;
+
+	if (!ASSERT_OK(test_lwt_ip_encap_fix__attach(skel), "test_lwt_ip_encap_fix__attach"))
+		goto out;
+
+	/* Remove the direct NS2->DST route so packets must go via LWT encap. */
+	SYS(out, "ip -n %s    route del %s/32  dev veth3", ns2, IP4_ADDR_DST);
+	SYS(out, "ip -n %s -6 route del %s/128 dev veth3", ns2, IP6_ADDR_DST);
+
+	/* Install the VxLAN BPF LWT xmit route. */
+	if (ipv4_encap)
+		SYS(out, "ip -n %s route add %s encap bpf xmit obj %s sec %s dev veth1",
+		    ns1, IP4_ADDR_DST, BPF_FILE, sec);
+	else
+		SYS(out, "ip -n %s -6 route add %s encap bpf xmit obj %s sec %s dev veth1",
+		    ns1, IP6_ADDR_DST, BPF_FILE, sec);
+
+	skel->bss->fexit_triggered = false;
+	if (ipv4_encap)
+		SYS(out, "ip netns exec %s ping  -c 1 -W1 %s", ns1, IP4_ADDR_DST);
+	else
+		SYS(out, "ip netns exec %s ping6 -c 1 -W1 %s", ns1, IP6_ADDR_DST);
+
+	/* Test 1: fexit triggered means bpf_lwt_push_ip_encap() succeeded. */
+	if (!ASSERT_TRUE(skel->bss->fexit_triggered, "fexit_triggered"))
+		goto out;
+
+	/* Test 2: transport_header must sit immediately after the outer IP
+	 * header, pointing at the UDP header of the VxLAN encap.
+	 */
+	thdr_offset = (int)skel->bss->transport_hdr - (int)skel->bss->network_hdr;
+	ASSERT_EQ(thdr_offset, expected_offset, "transport_hdr offset");
+
+out:
+	test_lwt_ip_encap_fix__destroy(skel);
+	SYS_NOFAIL("ip netns del %s", ns1);
+	SYS_NOFAIL("ip netns del %s", ns2);
+	SYS_NOFAIL("ip netns del %s", ns3);
+}
+
+void test_lwt_ip_encap_vxlan_ipv4(void)
+{
+	lwt_ip_encap_vxlan(IPV4_ENCAP);
+}
+
+void test_lwt_ip_encap_vxlan_ipv6(void)
+{
+	lwt_ip_encap_vxlan(IPV6_ENCAP);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_lwt_ip_encap.c b/tools/testing/selftests/bpf/progs/test_lwt_ip_encap.c
index d6cb986e7533..36f0fc682ffb 100644
--- a/tools/testing/selftests/bpf/progs/test_lwt_ip_encap.c
+++ b/tools/testing/selftests/bpf/progs/test_lwt_ip_encap.c
@@ -2,8 +2,10 @@
 #include <stddef.h>
 #include <string.h>
 #include <linux/bpf.h>
+#include <linux/if_ether.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>
+#include <linux/udp.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_endian.h>
 
@@ -82,4 +84,114 @@ int bpf_lwt_encap_gre6(struct __sk_buff *skb)
 	return BPF_LWT_REROUTE;
 }
 
+struct vxlanhdr {
+	__be32 vx_flags;  /* I flag = 0x08000000 (valid VNI) */
+	__be32 vx_vni;    /* VNI in top 24 bits */
+};
+
+#define VXLAN_PORT  4789
+#define VXLAN_FLAGS 0x08000000
+#define VXLAN_VNI   1
+
+static const __u8 bcast[ETH_ALEN] = {
+	0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+};
+
+static const __u8 srcmac[ETH_ALEN] = {
+	0x02, 0x00, 0x00, 0x00, 0x00, 0x01,
+};
+
+SEC("encap_vxlan")
+int bpf_lwt_encap_vxlan(struct __sk_buff *skb)
+{
+	struct encap_hdr {
+		struct iphdr    iph;
+		struct udphdr   udph;
+		struct vxlanhdr vxh;
+		struct ethhdr   eth;
+	} __attribute__((__packed__)) /* packed is required to avoid padding */ hdr;
+	int err;
+
+	memset(&hdr, 0, sizeof(hdr));
+
+	hdr.iph.ihl      = 5;
+	hdr.iph.version  = 4;
+	hdr.iph.ttl      = 0x40;
+	hdr.iph.protocol = 17; /* IPPROTO_UDP */
+	hdr.iph.tot_len  = bpf_htons(skb->len + sizeof(hdr));
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	hdr.iph.saddr = 0x640510ac;  /* 172.16.5.100  */
+	hdr.iph.daddr = 0x641110ac;  /* 172.16.17.100 */
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+	hdr.iph.saddr = 0xac100564;  /* 172.16.5.100 */
+	hdr.iph.daddr = 0xac101164;  /* 172.16.17.100 */
+#else
+#error "Fix your compiler's __BYTE_ORDER__?!"
+#endif
+
+	hdr.udph.source = bpf_htons(VXLAN_PORT);
+	hdr.udph.dest   = bpf_htons(VXLAN_PORT);
+	hdr.udph.len    = bpf_htons(skb->len + sizeof(hdr.udph) + sizeof(hdr.vxh) +
+				    sizeof(hdr.eth));
+
+	hdr.vxh.vx_flags = bpf_htonl(VXLAN_FLAGS);
+	hdr.vxh.vx_vni   = bpf_htonl(VXLAN_VNI << 8);
+
+	__builtin_memcpy(hdr.eth.h_dest, bcast, ETH_ALEN);
+	__builtin_memcpy(hdr.eth.h_source, srcmac, ETH_ALEN);
+	hdr.eth.h_proto = bpf_htons(ETH_P_IP);
+
+	err = bpf_lwt_push_encap(skb, BPF_LWT_ENCAP_IP, &hdr, sizeof(hdr));
+	if (err)
+		return BPF_DROP;
+
+	return BPF_LWT_REROUTE;
+}
+
+SEC("encap_vxlan6")
+int bpf_lwt_encap_vxlan6(struct __sk_buff *skb)
+{
+	struct encap_hdr {
+		struct ipv6hdr  ip6hdr;
+		struct udphdr   udph;
+		struct vxlanhdr vxh;
+		struct ethhdr   eth;
+	} __attribute__((__packed__)) /* packed is required to avoid padding */ hdr;
+	int err;
+
+	memset(&hdr, 0, sizeof(hdr));
+
+	hdr.ip6hdr.version     = 6;
+	hdr.ip6hdr.nexthdr     = 17; /* IPPROTO_UDP */
+	hdr.ip6hdr.hop_limit   = 0x40;
+	hdr.ip6hdr.payload_len = bpf_htons(skb->len + sizeof(hdr.udph) + sizeof(hdr.vxh) +
+					   sizeof(hdr.eth));
+	/* fb05::1 */
+	hdr.ip6hdr.saddr.s6_addr[0]  = 0xfb;
+	hdr.ip6hdr.saddr.s6_addr[1]  = 0x05;
+	hdr.ip6hdr.saddr.s6_addr[15] = 1;
+	/* fb20::1 */
+	hdr.ip6hdr.daddr.s6_addr[0]  = 0xfb;
+	hdr.ip6hdr.daddr.s6_addr[1]  = 0x20;
+	hdr.ip6hdr.daddr.s6_addr[15] = 1;
+
+	hdr.udph.source = bpf_htons(VXLAN_PORT);
+	hdr.udph.dest   = bpf_htons(VXLAN_PORT);
+	hdr.udph.len    = bpf_htons(skb->len + sizeof(hdr.udph) + sizeof(hdr.vxh) +
+				    sizeof(hdr.eth));
+
+	hdr.vxh.vx_flags = bpf_htonl(VXLAN_FLAGS);
+	hdr.vxh.vx_vni   = bpf_htonl(VXLAN_VNI << 8);
+
+	__builtin_memcpy(hdr.eth.h_dest, bcast, ETH_ALEN);
+	__builtin_memcpy(hdr.eth.h_source, srcmac, ETH_ALEN);
+	hdr.eth.h_proto = bpf_htons(ETH_P_IPV6);
+
+	err = bpf_lwt_push_encap(skb, BPF_LWT_ENCAP_IP, &hdr, sizeof(hdr));
+	if (err)
+		return BPF_DROP;
+
+	return BPF_LWT_REROUTE;
+}
+
 char _license[] SEC("license") = "GPL";
diff --git a/tools/testing/selftests/bpf/progs/test_lwt_ip_encap_fix.c b/tools/testing/selftests/bpf/progs/test_lwt_ip_encap_fix.c
new file mode 100644
index 000000000000..e9043fe654eb
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_lwt_ip_encap_fix.c
@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * fexit on bpf_lwt_push_ip_encap() to verify skb->transport_header is
+ * correctly updated when a UDP-based tunnel (e.g. VxLAN) is pushed.
+ */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+
+/* Written by fexit, read by the user-space test via skeleton BSS. */
+__u16 transport_hdr = 0;
+__u16 network_hdr = 0;
+bool fexit_triggered = false;
+
+/*
+ * bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
+ *
+ * After a successful push the transport_header must point at the outer
+ * transport header (UDP for VxLAN), i.e.
+ *   transport_header - network_header == sizeof(outer IP header)
+ */
+SEC("fexit/bpf_lwt_push_ip_encap")
+int BPF_PROG(fexit_lwt_push_ip_encap, struct sk_buff *skb, void *hdr, u32 len, bool ingress,
+	     int retval)
+{
+	if (retval || fexit_triggered)
+		return 0;
+
+	fexit_triggered = true;
+	transport_hdr = BPF_CORE_READ(skb, transport_header);
+	network_hdr   = BPF_CORE_READ(skb, network_header);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.54.0