bpf, sockmap: Test FIONREAD returns correct bytes in rx buffer

A bug was reported where ioctl(FIONREAD) returned zero even though the
socket with a SK_SKB verdict program attached had bytes in the msg
queue. The result is programs may hang or more likely try to recover,
but use suboptimal buffer sizes.

Add a test to check that ioctl(FIONREAD) returns the correct number of
bytes.
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20230523025618.113937-13-john.fastabend@gmail.com

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

@@ -410,6 +410,52 @@ static void test_sockmap_skb_verdict_shutdown(void)
 	test_sockmap_pass_prog__destroy(skel);
 }
 
+static void test_sockmap_skb_verdict_fionread(void)
+{
+	int err, map, verdict, s, c0, c1, p0, p1;
+	struct test_sockmap_pass_prog *skel;
+	int zero = 0, sent, recvd, avail;
+	char buf[256] = "0123456789";
+
+	skel = test_sockmap_pass_prog__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;
+
+	s = socket_loopback(AF_INET, SOCK_STREAM);
+	if (!ASSERT_GT(s, -1, "socket_loopback(s)"))
+		goto out;
+	err = create_socket_pairs(s, AF_INET, SOCK_STREAM, &c0, &c1, &p0, &p1);
+	if (!ASSERT_OK(err, "create_socket_pairs(s)"))
+		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, &buf, sizeof(buf), 0);
+	ASSERT_EQ(sent, sizeof(buf), "xsend(p0)");
+	err = ioctl(c1, FIONREAD, &avail);
+	ASSERT_OK(err, "ioctl(FIONREAD) error");
+	ASSERT_EQ(avail, sizeof(buf), "ioctl(FIONREAD)");
+	recvd = recv_timeout(c1, &buf, sizeof(buf), SOCK_NONBLOCK, IO_TIMEOUT_SEC);
+	ASSERT_EQ(recvd, sizeof(buf), "recv_timeout(c0)");
+
+out_close:
+	close(c0);
+	close(p0);
+	close(c1);
+	close(p1);
+out:
+	test_sockmap_pass_prog__destroy(skel);
+}
+
 void test_sockmap_basic(void)
 {
 	if (test__start_subtest("sockmap create_update_free"))
@@ -446,4 +492,6 @@ void test_sockmap_basic(void)
 		test_sockmap_progs_query(BPF_SK_SKB_VERDICT);
 	if (test__start_subtest("sockmap skb_verdict shutdown"))
 		test_sockmap_skb_verdict_shutdown();
+	if (test__start_subtest("sockmap skb_verdict fionread"))
+		test_sockmap_skb_verdict_fionread();
 }