selftests/bpf: Add a BPF selftest for bpf_skb_change_tail()

As requested by Daniel, we need to add a selftest to cover
bpf_skb_change_tail() cases in skb_verdict. Here we test trimming,
growing and error cases, and validate its expected return values and the
expected sizes of the payload.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241213034057.246437-3-xiyou.wangcong@gmail.com

Author: Cong Wang

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

@@ -12,6 +12,7 @@
 #include "test_sockmap_progs_query.skel.h"
 #include "test_sockmap_pass_prog.skel.h"
 #include "test_sockmap_drop_prog.skel.h"
+#include "test_sockmap_change_tail.skel.h"
 #include "bpf_iter_sockmap.skel.h"
 
 #include "sockmap_helpers.h"
@@ -643,6 +644,54 @@ static void test_sockmap_skb_verdict_fionread(bool pass_prog)
 		test_sockmap_drop_prog__destroy(drop);
 }
 
+static void test_sockmap_skb_verdict_change_tail(void)
+{
+	struct test_sockmap_change_tail *skel;
+	int err, map, verdict;
+	int c1, p1, sent, recvd;
+	int zero = 0;
+	char buf[2];
+
+	skel = test_sockmap_change_tail__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+	verdict = bpf_program__fd(skel->progs.prog_skb_verdict);
+	map = bpf_map__fd(skel->maps.sock_map_rx);
+
+	err = bpf_prog_attach(verdict, map, BPF_SK_SKB_STREAM_VERDICT, 0);
+	if (!ASSERT_OK(err, "bpf_prog_attach"))
+		goto out;
+	err = create_pair(AF_INET, SOCK_STREAM, &c1, &p1);
+	if (!ASSERT_OK(err, "create_pair()"))
+		goto out;
+	err = bpf_map_update_elem(map, &zero, &c1, BPF_NOEXIST);
+	if (!ASSERT_OK(err, "bpf_map_update_elem(c1)"))
+		goto out_close;
+	sent = xsend(p1, "Tr", 2, 0);
+	ASSERT_EQ(sent, 2, "xsend(p1)");
+	recvd = recv(c1, buf, 2, 0);
+	ASSERT_EQ(recvd, 1, "recv(c1)");
+	ASSERT_EQ(skel->data->change_tail_ret, 0, "change_tail_ret");
+
+	sent = xsend(p1, "G", 1, 0);
+	ASSERT_EQ(sent, 1, "xsend(p1)");
+	recvd = recv(c1, buf, 2, 0);
+	ASSERT_EQ(recvd, 2, "recv(c1)");
+	ASSERT_EQ(skel->data->change_tail_ret, 0, "change_tail_ret");
+
+	sent = xsend(p1, "E", 1, 0);
+	ASSERT_EQ(sent, 1, "xsend(p1)");
+	recvd = recv(c1, buf, 1, 0);
+	ASSERT_EQ(recvd, 1, "recv(c1)");
+	ASSERT_EQ(skel->data->change_tail_ret, -EINVAL, "change_tail_ret");
+
+out_close:
+	close(c1);
+	close(p1);
+out:
+	test_sockmap_change_tail__destroy(skel);
+}
+
 static void test_sockmap_skb_verdict_peek_helper(int map)
 {
 	int err, c1, p1, zero = 0, sent, recvd, avail;
@@ -1058,6 +1107,8 @@ void test_sockmap_basic(void)
 		test_sockmap_skb_verdict_fionread(true);
 	if (test__start_subtest("sockmap skb_verdict fionread on drop"))
 		test_sockmap_skb_verdict_fionread(false);
+	if (test__start_subtest("sockmap skb_verdict change tail"))
+		test_sockmap_skb_verdict_change_tail();
 	if (test__start_subtest("sockmap skb_verdict msg_f_peek"))
 		test_sockmap_skb_verdict_peek();
 	if (test__start_subtest("sockmap skb_verdict msg_f_peek with link"))

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

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 ByteDance */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+struct {
+	__uint(type, BPF_MAP_TYPE_SOCKMAP);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, int);
+} sock_map_rx SEC(".maps");
+
+long change_tail_ret = 1;
+
+SEC("sk_skb")
+int prog_skb_verdict(struct __sk_buff *skb)
+{
+	char *data, *data_end;
+
+	bpf_skb_pull_data(skb, 1);
+	data = (char *)(unsigned long)skb->data;
+	data_end = (char *)(unsigned long)skb->data_end;
+
+	if (data + 1 > data_end)
+		return SK_PASS;
+
+	if (data[0] == 'T') { /* Trim the packet */
+		change_tail_ret = bpf_skb_change_tail(skb, skb->len - 1, 0);
+		return SK_PASS;
+	} else if (data[0] == 'G') { /* Grow the packet */
+		change_tail_ret = bpf_skb_change_tail(skb, skb->len + 1, 0);
+		return SK_PASS;
+	} else if (data[0] == 'E') { /* Error */
+		change_tail_ret = bpf_skb_change_tail(skb, 65535, 0);
+		return SK_PASS;
+	}
+	return SK_PASS;
+}
+
+char _license[] SEC("license") = "GPL";