selftests/bpf: Add link pinning selftests

Add selftests validating link pinning/unpinning and associated BPF link
(attachment) lifetime.

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

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+
+#include <test_progs.h>
+#include <sys/stat.h>
+
+#include "test_link_pinning.skel.h"
+
+static int duration = 0;
+
+void test_link_pinning_subtest(struct bpf_program *prog,
+			       struct test_link_pinning__bss *bss)
+{
+	const char *link_pin_path = "/sys/fs/bpf/pinned_link_test";
+	struct stat statbuf = {};
+	struct bpf_link *link;
+	int err, i;
+
+	link = bpf_program__attach(prog);
+	if (CHECK(IS_ERR(link), "link_attach", "err: %ld\n", PTR_ERR(link)))
+		goto cleanup;
+
+	bss->in = 1;
+	usleep(1);
+	CHECK(bss->out != 1, "res_check1", "exp %d, got %d\n", 1, bss->out);
+
+	/* pin link */
+	err = bpf_link__pin(link, link_pin_path);
+	if (CHECK(err, "link_pin", "err: %d\n", err))
+		goto cleanup;
+
+	CHECK(strcmp(link_pin_path, bpf_link__pin_path(link)), "pin_path1",
+	      "exp %s, got %s\n", link_pin_path, bpf_link__pin_path(link));
+
+	/* check that link was pinned */
+	err = stat(link_pin_path, &statbuf);
+	if (CHECK(err, "stat_link", "err %d errno %d\n", err, errno))
+		goto cleanup;
+
+	bss->in = 2;
+	usleep(1);
+	CHECK(bss->out != 2, "res_check2", "exp %d, got %d\n", 2, bss->out);
+
+	/* destroy link, pinned link should keep program attached */
+	bpf_link__destroy(link);
+	link = NULL;
+
+	bss->in = 3;
+	usleep(1);
+	CHECK(bss->out != 3, "res_check3", "exp %d, got %d\n", 3, bss->out);
+
+	/* re-open link from BPFFS */
+	link = bpf_link__open(link_pin_path);
+	if (CHECK(IS_ERR(link), "link_open", "err: %ld\n", PTR_ERR(link)))
+		goto cleanup;
+
+	CHECK(strcmp(link_pin_path, bpf_link__pin_path(link)), "pin_path2",
+	      "exp %s, got %s\n", link_pin_path, bpf_link__pin_path(link));
+
+	/* unpin link from BPFFS, program still attached */
+	err = bpf_link__unpin(link);
+	if (CHECK(err, "link_unpin", "err: %d\n", err))
+		goto cleanup;
+
+	/* still active, as we have FD open now */
+	bss->in = 4;
+	usleep(1);
+	CHECK(bss->out != 4, "res_check4", "exp %d, got %d\n", 4, bss->out);
+
+	bpf_link__destroy(link);
+	link = NULL;
+
+	/* Validate it's finally detached.
+	 * Actual detachment might get delayed a bit, so there is no reliable
+	 * way to validate it immediately here, let's count up for long enough
+	 * and see if eventually output stops being updated
+	 */
+	for (i = 5; i < 10000; i++) {
+		bss->in = i;
+		usleep(1);
+		if (bss->out == i - 1)
+			break;
+	}
+	CHECK(i == 10000, "link_attached", "got to iteration #%d\n", i);
+
+cleanup:
+	if (!IS_ERR(link))
+		bpf_link__destroy(link);
+}
+
+void test_link_pinning(void)
+{
+	struct test_link_pinning* skel;
+
+	skel = test_link_pinning__open_and_load();
+	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
+		return;
+
+	if (test__start_subtest("pin_raw_tp"))
+		test_link_pinning_subtest(skel->progs.raw_tp_prog, skel->bss);
+	if (test__start_subtest("pin_tp_btf"))
+		test_link_pinning_subtest(skel->progs.tp_btf_prog, skel->bss);
+
+	test_link_pinning__destroy(skel);
+}

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+
+#include <stdbool.h>
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+int in = 0;
+int out = 0;
+
+SEC("raw_tp/sys_enter")
+int raw_tp_prog(const void *ctx)
+{
+	out = in;
+	return 0;
+}
+
+SEC("tp_btf/sys_enter")
+int tp_btf_prog(const void *ctx)
+{
+	out = in;
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";