selftests/bpf: Test bpf_skc_to_unix_sock() helper

Add a new test which triggers unix_listen kernel function
to test bpf_skc_to_unix_sock helper.
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211021134752.1223426-3-hengqi.chen@gmail.com

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

+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2021 Hengqi Chen */
+
+#include <test_progs.h>
+#include <sys/un.h>
+#include "test_skc_to_unix_sock.skel.h"
+
+static const char *sock_path = "@skc_to_unix_sock";
+
+void test_skc_to_unix_sock(void)
+{
+	struct test_skc_to_unix_sock *skel;
+	struct sockaddr_un sockaddr;
+	int err, sockfd = 0;
+
+	skel = test_skc_to_unix_sock__open();
+	if (!ASSERT_OK_PTR(skel, "could not open BPF object"))
+		return;
+
+	skel->rodata->my_pid = getpid();
+
+	err = test_skc_to_unix_sock__load(skel);
+	if (!ASSERT_OK(err, "could not load BPF object"))
+		goto cleanup;
+
+	err = test_skc_to_unix_sock__attach(skel);
+	if (!ASSERT_OK(err, "could not attach BPF object"))
+		goto cleanup;
+
+	/* trigger unix_listen */
+	sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
+	if (!ASSERT_GT(sockfd, 0, "socket failed"))
+		goto cleanup;
+
+	memset(&sockaddr, 0, sizeof(sockaddr));
+	sockaddr.sun_family = AF_UNIX;
+	strncpy(sockaddr.sun_path, sock_path, strlen(sock_path));
+	sockaddr.sun_path[0] = '\0';
+
+	err = bind(sockfd, (struct sockaddr *)&sockaddr, sizeof(sockaddr));
+	if (!ASSERT_OK(err, "bind failed"))
+		goto cleanup;
+
+	err = listen(sockfd, 1);
+	if (!ASSERT_OK(err, "listen failed"))
+		goto cleanup;
+
+	ASSERT_EQ(strcmp(skel->bss->path, sock_path), 0, "bpf_skc_to_unix_sock failed");
+
+cleanup:
+	if (sockfd)
+		close(sockfd);
+	test_skc_to_unix_sock__destroy(skel);
+}

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

+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2021 Hengqi Chen */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_tracing_net.h"
+
+const volatile pid_t my_pid = 0;
+char path[256] = {};
+
+SEC("fentry/unix_listen")
+int BPF_PROG(unix_listen, struct socket *sock, int backlog)
+{
+	pid_t pid = bpf_get_current_pid_tgid() >> 32;
+	struct unix_sock *unix_sk;
+	int i, len;
+
+	if (pid != my_pid)
+		return 0;
+
+	unix_sk = (struct unix_sock *)bpf_skc_to_unix_sock(sock->sk);
+	if (!unix_sk)
+		return 0;
+
+	if (!UNIX_ABSTRACT(unix_sk))
+		return 0;
+
+	len = unix_sk->addr->len - sizeof(short);
+	path[0] = '@';
+	for (i = 1; i < len; i++) {
+		if (i >= sizeof(struct sockaddr_un))
+			break;
+
+		path[i] = unix_sk->addr->name->sun_path[i];
+	}
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";