Merge branch 'bpf_link'

Andrii Nakryiko says:

====================
This patch series adds bpf_link abstraction, analogous to libbpf's already
existing bpf_link abstraction. This formalizes and makes more uniform existing
bpf_link-like BPF program link (attachment) types (raw tracepoint and tracing
links), which are FD-based objects that are automatically detached when last
file reference is closed. These types of BPF program links are switched to
using bpf_link framework.

FD-based bpf_link approach provides great safety guarantees, by ensuring there
is not going to be an abandoned BPF program attached, if user process suddenly
exits or forgets to clean up after itself. This is especially important in
production environment and is what all the recent new BPF link types followed.

One of the previously existing  inconveniences of FD-based approach, though,
was the scenario in which user process wants to install BPF link and exit, but
let attached BPF program run. Now, with bpf_link abstraction in place, it's
easy to support pinning links in BPF FS, which is done as part of the same
patch #1. This allows FD-based BPF program links to survive exit of a user
process and original file descriptor being closed, by creating an file entry
in BPF FS. This provides great safety by default, with simple way to opt out
for cases where it's needed.

Corresponding libbpf APIs are added in the same patch set, as well as
selftests for this functionality.

Other types of BPF program attachments (XDP, cgroup, perf_event, etc) are
going to be converted in subsequent patches to follow similar approach.

v1->v2:
- use bpf_link_new_fd() uniformly (Alexei).
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 130 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/link_pinning.c b/tools/testing/selftests/bpf/prog_tests/link_pinning.c
new file mode 100644
index 000000000000..a743288cf384
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/link_pinning.c
@@ -0,0 +1,105 @@
+// 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);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_link_pinning.c b/tools/testing/selftests/bpf/progs/test_link_pinning.c
new file mode 100644
index 000000000000..bbf2a5264dc0
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_link_pinning.c
@@ -0,0 +1,25 @@
+// 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";