Subject: [PATCH bpf-next v2 2/2] selftests/bpf: test that dst is cleared on same-protocol encap

From: Jakub Kicinski <[email-masked]>
Date: Sun, 29 Mar 2026 11:04:28 -0700

Verify that bpf_skb_adjust_room() clears the routing dst even when
the encap L3 protocol matches the original packet (e.g. IPIP).
The dst selected for the inner packet is not valid for the
encapsulated result; a stale dst could lead to misrouting.

Signed-off-by: Jakub Kicinski <[email-masked]>
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---
 .../selftests/bpf/prog_tests/test_dst_clear.c | 75 +++++++++++++++++++
 .../selftests/bpf/progs/test_dst_clear.c      | 57 ++++++++++++++
 2 files changed, 132 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/test_dst_clear.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_dst_clear.c

diff --git a/tools/testing/selftests/bpf/prog_tests/test_dst_clear.c b/tools/testing/selftests/bpf/prog_tests/test_dst_clear.c
new file mode 100644
index 000000000000..8190c56556fb
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/test_dst_clear.c
@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <net/if.h>
+
+#include "test_progs.h"
+#include "network_helpers.h"
+#include "test_dst_clear.skel.h"
+
+#define NS_TEST "dst_clear_ns"
+#define IPV4_IFACE_ADDR "1.0.0.1"
+#define UDP_TEST_PORT 7777
+
+void test_dst_clear(void)
+{
+	LIBBPF_OPTS(bpf_tc_hook, qdisc_hook, .attach_point = BPF_TC_EGRESS);
+	LIBBPF_OPTS(bpf_tc_opts, tc_attach);
+	struct nstoken *nstoken = NULL;
+	struct test_dst_clear *skel;
+	struct sockaddr_in addr;
+	socklen_t addrlen;
+	char buf[128] = {};
+	int sockfd, err;
+
+	skel = test_dst_clear__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel open_and_load"))
+		return;
+
+	SYS(fail, "ip netns add %s", NS_TEST);
+	SYS(fail, "ip -net %s addr add %s/8 dev lo", NS_TEST, IPV4_IFACE_ADDR);
+	SYS(fail, "ip -net %s link set dev lo up", NS_TEST);
+
+	nstoken = open_netns(NS_TEST);
+	if (!ASSERT_OK_PTR(nstoken, "open_netns"))
+		goto fail;
+
+	qdisc_hook.ifindex = if_nametoindex("lo");
+	if (!ASSERT_GT(qdisc_hook.ifindex, 0, "if_nametoindex lo"))
+		goto fail;
+
+	err = bpf_tc_hook_create(&qdisc_hook);
+	if (!ASSERT_OK(err, "create qdisc hook"))
+		goto fail;
+
+	tc_attach.prog_fd = bpf_program__fd(skel->progs.dst_clear);
+	err = bpf_tc_attach(&qdisc_hook, &tc_attach);
+	if (!ASSERT_OK(err, "attach filter"))
+		goto fail;
+
+	addrlen = sizeof(addr);
+	err = make_sockaddr(AF_INET, IPV4_IFACE_ADDR, UDP_TEST_PORT,
+			    (void *)&addr, &addrlen);
+	if (!ASSERT_OK(err, "make_sockaddr"))
+		goto fail;
+	sockfd = socket(AF_INET, SOCK_DGRAM, 0);
+	if (!ASSERT_NEQ(sockfd, -1, "socket"))
+		goto fail;
+	err = sendto(sockfd, buf, sizeof(buf), 0, (void *)&addr, addrlen);
+	close(sockfd);
+	if (!ASSERT_EQ(err, sizeof(buf), "send"))
+		goto fail;
+
+	ASSERT_TRUE(skel->bss->had_dst, "had_dst");
+	ASSERT_TRUE(skel->bss->dst_cleared, "dst_cleared");
+
+fail:
+	if (nstoken) {
+		bpf_tc_hook_destroy(&qdisc_hook);
+		close_netns(nstoken);
+	}
+	SYS_NOFAIL("ip netns del " NS_TEST);
+	test_dst_clear__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_dst_clear.c b/tools/testing/selftests/bpf/progs/test_dst_clear.c
new file mode 100644
index 000000000000..7ac9604fd99c
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_dst_clear.c
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include "vmlinux.h"
+#include "bpf_tracing_net.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
+#define UDP_TEST_PORT 7777
+
+void *bpf_cast_to_kern_ctx(void *) __ksym;
+
+bool had_dst = false;
+bool dst_cleared = false;
+
+SEC("tc")
+int dst_clear(struct __sk_buff *skb)
+{
+	struct sk_buff *kskb;
+	struct iphdr iph;
+	struct udphdr udph;
+	int err;
+
+	if (skb->protocol != __bpf_constant_htons(ETH_P_IP))
+		return TC_ACT_OK;
+
+	if (bpf_skb_load_bytes(skb, ETH_HLEN, &iph, sizeof(iph)))
+		return TC_ACT_OK;
+
+	if (iph.protocol != IPPROTO_UDP)
+		return TC_ACT_OK;
+
+	if (bpf_skb_load_bytes(skb, ETH_HLEN + sizeof(iph), &udph, sizeof(udph)))
+		return TC_ACT_OK;
+
+	if (udph.dest != __bpf_constant_htons(UDP_TEST_PORT))
+		return TC_ACT_OK;
+
+	kskb = bpf_cast_to_kern_ctx(skb);
+	had_dst = (kskb->_skb_refdst != 0);
+
+	/* Same-protocol encap (IPIP): protocol stays IPv4, but the dst
+	 * from the original routing is no longer valid for the outer hdr.
+	 */
+	err = bpf_skb_adjust_room(skb, (s32)sizeof(struct iphdr),
+				  BPF_ADJ_ROOM_MAC,
+				  BPF_F_ADJ_ROOM_FIXED_GSO |
+				  BPF_F_ADJ_ROOM_ENCAP_L3_IPV4);
+	if (err)
+		return TC_ACT_SHOT;
+
+	dst_cleared = (kskb->_skb_refdst == 0);
+
+	return TC_ACT_SHOT;
+}
+
+char __license[] SEC("license") = "GPL";
-- 
2.53.0