Subject: [PATCH bpf-next v4 4/6] selftests/bpf: add icmp_send_unreach kfunc tests

From: Mahe Tardy <[email-masked]>
Date: Mon, 20 Apr 2026 10:58:14 +0000

This test opens a server and client, enters a new cgroup, attach a
cgroup_skb program on egress and calls the icmp_send_unreach function
from the client egress so that an ICMP unreach control message is sent
back to the client.  It then fetches the message from the error queue to
confirm the correct ICMP unreach code has been sent.

Note that, for the client, we have to connect in non-blocking mode to
let the test execute faster. Otherwise, we need to wait for the TCP
three-way handshake to timeout in the kernel before reading the errno.

Also note that we don't set IP_RECVERR on the socket in
connect_to_fd_nonblock since the error will be transferred anyway in our
test because the connection is rejected at the beginning of the TCP
handshake. See in net/ipv4/tcp_ipv4.c:tcp_v4_err line 615 to 655 for
more details.

Signed-off-by: Mahe Tardy <[email-masked]>
---
 .../bpf/prog_tests/icmp_send_unreach_kfunc.c  | 136 ++++++++++++++++++
 .../selftests/bpf/progs/icmp_send_unreach.c   |  36 +++++
 2 files changed, 172 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/icmp_send_unreach_kfunc.c
 create mode 100644 tools/testing/selftests/bpf/progs/icmp_send_unreach.c

diff --git a/tools/testing/selftests/bpf/prog_tests/icmp_send_unreach_kfunc.c b/tools/testing/selftests/bpf/prog_tests/icmp_send_unreach_kfunc.c
new file mode 100644
index 000000000000..24d5e01cfe80
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/icmp_send_unreach_kfunc.c
@@ -0,0 +1,136 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <network_helpers.h>
+#include <linux/errqueue.h>
+#include "icmp_send_unreach.skel.h"
+
+#define TIMEOUT_MS 1000
+#define SRV_PORT 54321
+
+#define ICMP_DEST_UNREACH 3
+
+#define ICMP_FRAG_NEEDED 4
+#define NR_ICMP_UNREACH 15
+
+static int connect_to_fd_nonblock(int server_fd)
+{
+	struct sockaddr_storage addr;
+	socklen_t len = sizeof(addr);
+	int fd, err;
+
+	if (getsockname(server_fd, (struct sockaddr *)&addr, &len))
+		return -1;
+
+	fd = socket(addr.ss_family, SOCK_STREAM | SOCK_NONBLOCK, 0);
+	if (fd < 0)
+		return -1;
+
+	err = connect(fd, (struct sockaddr *)&addr, len);
+	if (err < 0 && errno != EINPROGRESS) {
+		close(fd);
+		return -1;
+	}
+
+	return fd;
+}
+
+static void read_icmp_errqueue(int sockfd, int expected_code)
+{
+	ssize_t n;
+	struct sock_extended_err *sock_err;
+	struct cmsghdr *cm;
+	char ctrl_buf[512];
+	struct msghdr msg = {
+		.msg_control = ctrl_buf,
+		.msg_controllen = sizeof(ctrl_buf),
+	};
+
+	n = recvmsg(sockfd, &msg, MSG_ERRQUEUE);
+	if (!ASSERT_GE(n, 0, "recvmsg_errqueue"))
+		return;
+
+	cm = CMSG_FIRSTHDR(&msg);
+	if (!ASSERT_NEQ(cm, NULL, "cm_firsthdr_null"))
+		return;
+
+	for (; cm; cm = CMSG_NXTHDR(&msg, cm)) {
+		if (!ASSERT_EQ(cm->cmsg_level, IPPROTO_IP, "cmsg_type") ||
+		    !ASSERT_EQ(cm->cmsg_type, IP_RECVERR, "cmsg_level"))
+			continue;
+
+		sock_err = (struct sock_extended_err *)CMSG_DATA(cm);
+
+		if (!ASSERT_EQ(sock_err->ee_origin, SO_EE_ORIGIN_ICMP,
+			       "sock_err_origin_icmp"))
+			return;
+		if (!ASSERT_EQ(sock_err->ee_type, ICMP_DEST_UNREACH,
+			       "sock_err_type_dest_unreach"))
+			return;
+		ASSERT_EQ(sock_err->ee_code, expected_code, "sock_err_code");
+	}
+}
+
+static void trigger_prog_read_icmp_errqueue(int *code)
+{
+	int srv_fd = -1, client_fd = -1;
+
+	srv_fd = start_server(AF_INET, SOCK_STREAM, "127.0.0.1", SRV_PORT,
+			      TIMEOUT_MS);
+	if (!ASSERT_GE(srv_fd, 0, "start_server"))
+		return;
+
+	client_fd = connect_to_fd_nonblock(srv_fd);
+	if (!ASSERT_GE(client_fd, 0, "client_connect_nonblock")) {
+		close(srv_fd);
+		return;
+	}
+
+	/* Skip reading ICMP error queue if code is invalid */
+	if (*code >= 0 && *code <= NR_ICMP_UNREACH)
+		read_icmp_errqueue(client_fd, *code);
+
+	close(srv_fd);
+	close(client_fd);
+}
+
+void test_icmp_send_unreach_kfunc(void)
+{
+	struct icmp_send_unreach *skel;
+	int cgroup_fd = -1;
+	int *code;
+
+	skel = icmp_send_unreach__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		goto cleanup;
+
+	cgroup_fd = test__join_cgroup("/icmp_send_unreach_cgroup");
+	if (!ASSERT_GE(cgroup_fd, 0, "join_cgroup"))
+		goto cleanup;
+
+	skel->links.egress =
+		bpf_program__attach_cgroup(skel->progs.egress, cgroup_fd);
+	if (!ASSERT_OK_PTR(skel->links.egress, "prog_attach_cgroup"))
+		goto cleanup;
+
+	code = &skel->bss->unreach_code;
+
+	for (*code = 0; *code <= NR_ICMP_UNREACH; (*code)++) {
+		/* The TCP stack reacts differently when asking for
+		 * fragmentation, let's ignore it for now.
+		 */
+		if (*code == ICMP_FRAG_NEEDED)
+			continue;
+
+		trigger_prog_read_icmp_errqueue(code);
+		ASSERT_EQ(skel->data->kfunc_ret, 0, "kfunc_ret");
+	}
+
+	/* Test an invalid code */
+	*code = -1;
+	trigger_prog_read_icmp_errqueue(code);
+	ASSERT_EQ(skel->data->kfunc_ret, -EINVAL, "kfunc_ret");
+
+cleanup:
+	icmp_send_unreach__destroy(skel);
+	close(cgroup_fd);
+}
diff --git a/tools/testing/selftests/bpf/progs/icmp_send_unreach.c b/tools/testing/selftests/bpf/progs/icmp_send_unreach.c
new file mode 100644
index 000000000000..6fc5595f08aa
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/icmp_send_unreach.c
@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
+#define SERVER_PORT 54321
+/* 127.0.0.1 in network byte order */
+#define SERVER_IP 0x7F000001
+
+int unreach_code = 0;
+int kfunc_ret = -1;
+
+SEC("cgroup_skb/egress")
+int egress(struct __sk_buff *skb)
+{
+	void *data = (void *)(long)skb->data;
+	void *data_end = (void *)(long)skb->data_end;
+	struct iphdr *iph;
+	struct tcphdr *tcph;
+
+	iph = data;
+	if ((void *)(iph + 1) > data_end || iph->version != 4 ||
+	    iph->protocol != IPPROTO_TCP || iph->daddr != bpf_htonl(SERVER_IP))
+		return SK_PASS;
+
+	tcph = (void *)iph + iph->ihl * 4;
+	if ((void *)(tcph + 1) > data_end ||
+	    tcph->dest != bpf_htons(SERVER_PORT))
+		return SK_PASS;
+
+	kfunc_ret = bpf_icmp_send_unreach(skb, unreach_code);
+
+	return SK_DROP;
+}
+
+char LICENSE[] SEC("license") = "Dual BSD/GPL";
--
2.34.1