From: Wei Wang <weibunny@fb.com>

Add nk_redirect.bpf.c, a BPF program that forwards skbs matching some IPv6
prefix received on eth0 ifindex to a specified dev ifindex.
bpf_redirect_neigh() is used to make sure neighbor lookup is performed
and proper MAC addr is being used.

Signed-off-by: Wei Wang <weibunny@fb.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Tested-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
 .../drivers/net/hw/nk_redirect.bpf.c          | 60 +++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 tools/testing/selftests/drivers/net/hw/nk_redirect.bpf.c

diff --git a/tools/testing/selftests/drivers/net/hw/nk_redirect.bpf.c b/tools/testing/selftests/drivers/net/hw/nk_redirect.bpf.c
new file mode 100644
index 000000000000..7ac9ffd50f15
--- /dev/null
+++ b/tools/testing/selftests/drivers/net/hw/nk_redirect.bpf.c
@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * BPF program for redirecting traffic using bpf_redirect_neigh().
+ * Unlike bpf_redirect() which preserves L2 headers, bpf_redirect_neigh()
+ * performs neighbor lookup and fills in the correct L2 addresses for the
+ * target interface. This is necessary when redirecting across different
+ * device types (e.g., from netdevsim to netkit).
+ */
+#include <linux/bpf.h>
+#include <linux/pkt_cls.h>
+#include <linux/if_ether.h>
+#include <linux/ipv6.h>
+#include <linux/in6.h>
+#include <bpf/bpf_endian.h>
+#include <bpf/bpf_helpers.h>
+
+#define TC_ACT_OK 0
+#define ETH_P_IPV6 0x86DD
+
+#define ctx_ptr(field)		((void *)(long)(field))
+
+#define v6_p64_equal(a, b)	(a.s6_addr32[0] == b.s6_addr32[0] && \
+				 a.s6_addr32[1] == b.s6_addr32[1])
+
+volatile __u32 redirect_ifindex;
+volatile __u8 ipv6_prefix[16];
+
+SEC("tc/ingress")
+int tc_redirect(struct __sk_buff *skb)
+{
+	void *data_end = ctx_ptr(skb->data_end);
+	void *data = ctx_ptr(skb->data);
+	struct in6_addr *match_prefix;
+	struct ipv6hdr *ip6h;
+	struct ethhdr *eth;
+
+	match_prefix = (struct in6_addr *)ipv6_prefix;
+
+	if (skb->protocol != bpf_htons(ETH_P_IPV6))
+		return TC_ACT_OK;
+
+	eth = data;
+	if ((void *)(eth + 1) > data_end)
+		return TC_ACT_OK;
+
+	ip6h = data + sizeof(struct ethhdr);
+	if ((void *)(ip6h + 1) > data_end)
+		return TC_ACT_OK;
+
+	if (!v6_p64_equal(ip6h->daddr, (*match_prefix)))
+		return TC_ACT_OK;
+
+	/*
+	 * Use bpf_redirect_neigh() to perform neighbor lookup and fill in
+	 * correct L2 addresses for the target interface.
+	 */
+	return bpf_redirect_neigh(redirect_ifindex, NULL, 0, 0);
+}
+
+char __license[] SEC("license") = "GPL";
-- 
2.52.0