selftests/bpf: test eager BPF ringbuf size adjustment logic

Add test validating that libbpf adjusts (and reflects adjusted) ringbuf
size early, before bpf_object is loaded. Also make sure we can't
successfully resize ringbuf map after bpf_object is loaded.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 	if (CHECK(!skel, "skel_open", "skeleton open failed\n"))
 		return;
 
+	/* validate ringbuf size adjustment logic */
+	ASSERT_EQ(bpf_map__max_entries(skel->maps.ringbuf1), page_size, "rb1_size_before");
+	ASSERT_OK(bpf_map__set_max_entries(skel->maps.ringbuf1, page_size + 1), "rb1_resize");
+	ASSERT_EQ(bpf_map__max_entries(skel->maps.ringbuf1), 2 * page_size, "rb1_size_after");
+	ASSERT_OK(bpf_map__set_max_entries(skel->maps.ringbuf1, page_size), "rb1_reset");
+	ASSERT_EQ(bpf_map__max_entries(skel->maps.ringbuf1), page_size, "rb1_size_final");
+
 	proto_fd = bpf_map_create(BPF_MAP_TYPE_RINGBUF, NULL, 0, 0, page_size, NULL);
 	if (CHECK(proto_fd < 0, "bpf_map_create", "bpf_map_create failed\n"))
 		goto cleanup;
···
 
 	close(proto_fd);
 	proto_fd = -1;
+
+	/* make sure we can't resize ringbuf after object load */
+	if (!ASSERT_ERR(bpf_map__set_max_entries(skel->maps.ringbuf1, 3 * page_size), "rb1_resize_after_load"))
+		goto cleanup;
 
 	/* only trigger BPF program for current process */
 	skel->bss->pid = getpid();