selftests/bpf: Add test for skb_load_bytes

Use bpf_prog_test_run_opts to test the skb_load_bytes function. Tests
the behavior when offset is greater than INT_MAX or a normal value.
Signed-off-by: Liu Jian <liujian56@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20220416105801.88708-4-liujian56@huawei.com

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

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <network_helpers.h>
+#include "skb_load_bytes.skel.h"
+
+void test_skb_load_bytes(void)
+{
+	struct skb_load_bytes *skel;
+	int err, prog_fd, test_result;
+	struct __sk_buff skb = { 0 };
+
+	LIBBPF_OPTS(bpf_test_run_opts, tattr,
+		.data_in = &pkt_v4,
+		.data_size_in = sizeof(pkt_v4),
+		.ctx_in = &skb,
+		.ctx_size_in = sizeof(skb),
+	);
+
+	skel = skb_load_bytes__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	prog_fd = bpf_program__fd(skel->progs.skb_process);
+	if (!ASSERT_GE(prog_fd, 0, "prog_fd"))
+		goto out;
+
+	skel->bss->load_offset = (uint32_t)(-1);
+	err = bpf_prog_test_run_opts(prog_fd, &tattr);
+	if (!ASSERT_OK(err, "bpf_prog_test_run_opts"))
+		goto out;
+	test_result = skel->bss->test_result;
+	if (!ASSERT_EQ(test_result, -EFAULT, "offset -1"))
+		goto out;
+
+	skel->bss->load_offset = (uint32_t)10;
+	err = bpf_prog_test_run_opts(prog_fd, &tattr);
+	if (!ASSERT_OK(err, "bpf_prog_test_run_opts"))
+		goto out;
+	test_result = skel->bss->test_result;
+	if (!ASSERT_EQ(test_result, 0, "offset 10"))
+		goto out;
+
+out:
+	skb_load_bytes__destroy(skel);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+__u32 load_offset = 0;
+int test_result = 0;
+
+SEC("tc")
+int skb_process(struct __sk_buff *skb)
+{
+	char buf[16];
+
+	test_result = bpf_skb_load_bytes(skb, load_offset, buf, 10);
+
+	return 0;
+}