selftests/bpf: Test bpf_skb_change_tail() in TC ingress

Similarly to the previous test, we also need a test case to cover
positive offsets as well, TC is an excellent hook for this.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Zijian Zhang <zijianzhang@bytedance.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241213034057.246437-5-xiyou.wangcong@gmail.com

Author: Cong Wang

tools/testing/selftests/bpf/prog_tests/tc_change_tail.c

@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <error.h>
+#include <test_progs.h>
+#include <linux/pkt_cls.h>
+
+#include "test_tc_change_tail.skel.h"
+#include "socket_helpers.h"
+
+#define LO_IFINDEX 1
+
+void test_tc_change_tail(void)
+{
+	LIBBPF_OPTS(bpf_tcx_opts, tcx_opts);
+	struct test_tc_change_tail *skel = NULL;
+	struct bpf_link *link;
+	int c1, p1;
+	char buf[2];
+	int ret;
+
+	skel = test_tc_change_tail__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "test_tc_change_tail__open_and_load"))
+		return;
+
+	link = bpf_program__attach_tcx(skel->progs.change_tail, LO_IFINDEX,
+				     &tcx_opts);
+	if (!ASSERT_OK_PTR(link, "bpf_program__attach_tcx"))
+		goto destroy;
+
+	skel->links.change_tail = link;
+	ret = create_pair(AF_INET, SOCK_DGRAM, &c1, &p1);
+	if (!ASSERT_OK(ret, "create_pair"))
+		goto destroy;
+
+	ret = xsend(p1, "Tr", 2, 0);
+	ASSERT_EQ(ret, 2, "xsend(p1)");
+	ret = recv(c1, buf, 2, 0);
+	ASSERT_EQ(ret, 2, "recv(c1)");
+	ASSERT_EQ(skel->data->change_tail_ret, 0, "change_tail_ret");
+
+	ret = xsend(p1, "G", 1, 0);
+	ASSERT_EQ(ret, 1, "xsend(p1)");
+	ret = recv(c1, buf, 2, 0);
+	ASSERT_EQ(ret, 1, "recv(c1)");
+	ASSERT_EQ(skel->data->change_tail_ret, 0, "change_tail_ret");
+
+	ret = xsend(p1, "E", 1, 0);
+	ASSERT_EQ(ret, 1, "xsend(p1)");
+	ret = recv(c1, buf, 1, 0);
+	ASSERT_EQ(ret, 1, "recv(c1)");
+	ASSERT_EQ(skel->data->change_tail_ret, -EINVAL, "change_tail_ret");
+
+	ret = xsend(p1, "Z", 1, 0);
+	ASSERT_EQ(ret, 1, "xsend(p1)");
+	ret = recv(c1, buf, 1, 0);
+	ASSERT_EQ(ret, 1, "recv(c1)");
+	ASSERT_EQ(skel->data->change_tail_ret, -EINVAL, "change_tail_ret");
+
+	close(c1);
+	close(p1);
+destroy:
+	test_tc_change_tail__destroy(skel);
+}

tools/testing/selftests/bpf/progs/test_tc_change_tail.c

@@ -0,0 +1,106 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <linux/if_ether.h>
+#include <linux/in.h>
+#include <linux/ip.h>
+#include <linux/udp.h>
+#include <linux/pkt_cls.h>
+
+long change_tail_ret = 1;
+
+static __always_inline struct iphdr *parse_ip_header(struct __sk_buff *skb, int *ip_proto)
+{
+	void *data_end = (void *)(long)skb->data_end;
+	void *data = (void *)(long)skb->data;
+	struct ethhdr *eth = data;
+	struct iphdr *iph;
+
+	/* Verify Ethernet header */
+	if ((void *)(data + sizeof(*eth)) > data_end)
+		return NULL;
+
+	/* Skip Ethernet header to get to IP header */
+	iph = (void *)(data + sizeof(struct ethhdr));
+
+	/* Verify IP header */
+	if ((void *)(data + sizeof(struct ethhdr) + sizeof(*iph)) > data_end)
+		return NULL;
+
+	/* Basic IP header validation */
+	if (iph->version != 4)  /* Only support IPv4 */
+		return NULL;
+
+	if (iph->ihl < 5)  /* Minimum IP header length */
+		return NULL;
+
+	*ip_proto = iph->protocol;
+	return iph;
+}
+
+static __always_inline struct udphdr *parse_udp_header(struct __sk_buff *skb, struct iphdr *iph)
+{
+	void *data_end = (void *)(long)skb->data_end;
+	void *hdr = (void *)iph;
+	struct udphdr *udp;
+
+	/* Calculate UDP header position */
+	udp = hdr + (iph->ihl * 4);
+	hdr = (void *)udp;
+
+	/* Verify UDP header bounds */
+	if ((void *)(hdr + sizeof(*udp)) > data_end)
+		return NULL;
+
+	return udp;
+}
+
+SEC("tc/ingress")
+int change_tail(struct __sk_buff *skb)
+{
+	int len = skb->len;
+	struct udphdr *udp;
+	struct iphdr *iph;
+	void *data_end;
+	char *payload;
+	int ip_proto;
+
+	bpf_skb_pull_data(skb, len);
+
+	data_end = (void *)(long)skb->data_end;
+	iph = parse_ip_header(skb, &ip_proto);
+	if (!iph)
+		return TCX_PASS;
+
+	if (ip_proto != IPPROTO_UDP)
+		return TCX_PASS;
+
+	udp = parse_udp_header(skb, iph);
+	if (!udp)
+		return TCX_PASS;
+
+	payload = (char *)udp + (sizeof(struct udphdr));
+	if (payload + 1 > (char *)data_end)
+		return TCX_PASS;
+
+	if (payload[0] == 'T') { /* Trim the packet */
+		change_tail_ret = bpf_skb_change_tail(skb, len - 1, 0);
+		if (!change_tail_ret)
+			bpf_skb_change_tail(skb, len, 0);
+		return TCX_PASS;
+	} else if (payload[0] == 'G') { /* Grow the packet */
+		change_tail_ret = bpf_skb_change_tail(skb, len + 1, 0);
+		if (!change_tail_ret)
+			bpf_skb_change_tail(skb, len, 0);
+		return TCX_PASS;
+	} else if (payload[0] == 'E') { /* Error */
+		change_tail_ret = bpf_skb_change_tail(skb, 65535, 0);
+		return TCX_PASS;
+	} else if (payload[0] == 'Z') { /* Zero */
+		change_tail_ret = bpf_skb_change_tail(skb, 0, 0);
+		return TCX_PASS;
+	}
+	return TCX_DROP;
+}
+
+char _license[] SEC("license") = "GPL";