selftests/bpf: Add bpf_read_branch_records() selftest

Add a selftest to test:

* default bpf_read_branch_records() behavior
* BPF_F_GET_BRANCH_RECORDS_SIZE flag behavior
* error path on non branch record perf events
* using helper to write to stack
* using helper to write to global

On host with hardware counter support:

    # ./test_progs -t perf_branches
    #27/1 perf_branches_hw:OK
    #27/2 perf_branches_no_hw:OK
    #27 perf_branches:OK
    Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED

On host without hardware counter support (VM):

    # ./test_progs -t perf_branches
    #27/1 perf_branches_hw:OK
    #27/2 perf_branches_no_hw:OK
    #27 perf_branches:OK
    Summary: 1/2 PASSED, 1 SKIPPED, 0 FAILED

Also sync tools/include/uapi/linux/bpf.h.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200218030432.4600-3-dxu@dxuuu.xyz

tools/include/uapi/linux/bpf.h

@@ -2892,6 +2892,25 @@ union bpf_attr {
  *		Obtain the 64bit jiffies
  *	Return
  *		The 64 bit jiffies
+ *
+ * int bpf_read_branch_records(struct bpf_perf_event_data *ctx, void *buf, u32 size, u64 flags)
+ *	Description
+ *		For an eBPF program attached to a perf event, retrieve the
+ *		branch records (struct perf_branch_entry) associated to *ctx*
+ *		and store it in	the buffer pointed by *buf* up to size
+ *		*size* bytes.
+ *	Return
+ *		On success, number of bytes written to *buf*. On error, a
+ *		negative value.
+ *
+ *		The *flags* can be set to **BPF_F_GET_BRANCH_RECORDS_SIZE** to
+ *		instead	return the number of bytes required to store all the
+ *		branch entries. If this flag is set, *buf* may be NULL.
+ *
+ *		**-EINVAL** if arguments invalid or **size** not a multiple
+ *		of sizeof(struct perf_branch_entry).
+ *
+ *		**-ENOENT** if architecture does not support branch records.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -3012,7 +3031,8 @@ union bpf_attr {
 	FN(probe_read_kernel_str),	\
 	FN(tcp_send_ack),		\
 	FN(send_signal_thread),		\
-	FN(jiffies64),
+	FN(jiffies64),			\
+	FN(read_branch_records),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
@@ -3091,6 +3111,9 @@ enum bpf_func_id {
 /* BPF_FUNC_sk_storage_get flags */
 #define BPF_SK_STORAGE_GET_F_CREATE	(1ULL << 0)
 
+/* BPF_FUNC_read_branch_records flags. */
+#define BPF_F_GET_BRANCH_RECORDS_SIZE	(1ULL << 0)
+
 /* Mode for BPF_FUNC_skb_adjust_room helper. */
 enum bpf_adj_room_mode {
 	BPF_ADJ_ROOM_NET,

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

@@ -0,0 +1,170 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <pthread.h>
+#include <sched.h>
+#include <sys/socket.h>
+#include <test_progs.h>
+#include "bpf/libbpf_internal.h"
+#include "test_perf_branches.skel.h"
+
+static void check_good_sample(struct test_perf_branches *skel)
+{
+	int written_global = skel->bss->written_global_out;
+	int required_size = skel->bss->required_size_out;
+	int written_stack = skel->bss->written_stack_out;
+	int pbe_size = sizeof(struct perf_branch_entry);
+	int duration = 0;
+
+	if (CHECK(!skel->bss->valid, "output not valid",
+		 "no valid sample from prog"))
+		return;
+
+	/*
+	 * It's hard to validate the contents of the branch entries b/c it
+	 * would require some kind of disassembler and also encoding the
+	 * valid jump instructions for supported architectures. So just check
+	 * the easy stuff for now.
+	 */
+	CHECK(required_size <= 0, "read_branches_size", "err %d\n", required_size);
+	CHECK(written_stack < 0, "read_branches_stack", "err %d\n", written_stack);
+	CHECK(written_stack % pbe_size != 0, "read_branches_stack",
+	      "stack bytes written=%d not multiple of struct size=%d\n",
+	      written_stack, pbe_size);
+	CHECK(written_global < 0, "read_branches_global", "err %d\n", written_global);
+	CHECK(written_global % pbe_size != 0, "read_branches_global",
+	      "global bytes written=%d not multiple of struct size=%d\n",
+	      written_global, pbe_size);
+	CHECK(written_global < written_stack, "read_branches_size",
+	      "written_global=%d < written_stack=%d\n", written_global, written_stack);
+}
+
+static void check_bad_sample(struct test_perf_branches *skel)
+{
+	int written_global = skel->bss->written_global_out;
+	int required_size = skel->bss->required_size_out;
+	int written_stack = skel->bss->written_stack_out;
+	int duration = 0;
+
+	if (CHECK(!skel->bss->valid, "output not valid",
+		 "no valid sample from prog"))
+		return;
+
+	CHECK((required_size != -EINVAL && required_size != -ENOENT),
+	      "read_branches_size", "err %d\n", required_size);
+	CHECK((written_stack != -EINVAL && written_stack != -ENOENT),
+	      "read_branches_stack", "written %d\n", written_stack);
+	CHECK((written_global != -EINVAL && written_global != -ENOENT),
+	      "read_branches_global", "written %d\n", written_global);
+}
+
+static void test_perf_branches_common(int perf_fd,
+				      void (*cb)(struct test_perf_branches *))
+{
+	struct test_perf_branches *skel;
+	int err, i, duration = 0;
+	bool detached = false;
+	struct bpf_link *link;
+	volatile int j = 0;
+	cpu_set_t cpu_set;
+
+	skel = test_perf_branches__open_and_load();
+	if (CHECK(!skel, "test_perf_branches_load",
+		  "perf_branches skeleton failed\n"))
+		return;
+
+	/* attach perf_event */
+	link = bpf_program__attach_perf_event(skel->progs.perf_branches, perf_fd);
+	if (CHECK(IS_ERR(link), "attach_perf_event", "err %ld\n", PTR_ERR(link)))
+		goto out_destroy_skel;
+
+	/* generate some branches on cpu 0 */
+	CPU_ZERO(&cpu_set);
+	CPU_SET(0, &cpu_set);
+	err = pthread_setaffinity_np(pthread_self(), sizeof(cpu_set), &cpu_set);
+	if (CHECK(err, "set_affinity", "cpu #0, err %d\n", err))
+		goto out_destroy;
+	/* spin the loop for a while (random high number) */
+	for (i = 0; i < 1000000; ++i)
+		++j;
+
+	test_perf_branches__detach(skel);
+	detached = true;
+
+	cb(skel);
+out_destroy:
+	bpf_link__destroy(link);
+out_destroy_skel:
+	if (!detached)
+		test_perf_branches__detach(skel);
+	test_perf_branches__destroy(skel);
+}
+
+static void test_perf_branches_hw(void)
+{
+	struct perf_event_attr attr = {0};
+	int duration = 0;
+	int pfd;
+
+	/* create perf event */
+	attr.size = sizeof(attr);
+	attr.type = PERF_TYPE_HARDWARE;
+	attr.config = PERF_COUNT_HW_CPU_CYCLES;
+	attr.freq = 1;
+	attr.sample_freq = 4000;
+	attr.sample_type = PERF_SAMPLE_BRANCH_STACK;
+	attr.branch_sample_type = PERF_SAMPLE_BRANCH_USER | PERF_SAMPLE_BRANCH_ANY;
+	pfd = syscall(__NR_perf_event_open, &attr, -1, 0, -1, PERF_FLAG_FD_CLOEXEC);
+
+	/*
+	 * Some setups don't support branch records (virtual machines, !x86),
+	 * so skip test in this case.
+	 */
+	if (pfd == -1) {
+		if (errno == ENOENT || errno == EOPNOTSUPP) {
+			printf("%s:SKIP:no PERF_SAMPLE_BRANCH_STACK\n",
+			       __func__);
+			test__skip();
+			return;
+		}
+		if (CHECK(pfd < 0, "perf_event_open", "err %d errno %d\n",
+			  pfd, errno))
+			return;
+	}
+
+	test_perf_branches_common(pfd, check_good_sample);
+
+	close(pfd);
+}
+
+/*
+ * Tests negative case -- run bpf_read_branch_records() on improperly configured
+ * perf event.
+ */
+static void test_perf_branches_no_hw(void)
+{
+	struct perf_event_attr attr = {0};
+	int duration = 0;
+	int pfd;
+
+	/* create perf event */
+	attr.size = sizeof(attr);
+	attr.type = PERF_TYPE_SOFTWARE;
+	attr.config = PERF_COUNT_SW_CPU_CLOCK;
+	attr.freq = 1;
+	attr.sample_freq = 4000;
+	pfd = syscall(__NR_perf_event_open, &attr, -1, 0, -1, PERF_FLAG_FD_CLOEXEC);
+	if (CHECK(pfd < 0, "perf_event_open", "err %d\n", pfd))
+		return;
+
+	test_perf_branches_common(pfd, check_bad_sample);
+
+	close(pfd);
+}
+
+void test_perf_branches(void)
+{
+	if (test__start_subtest("perf_branches_hw"))
+		test_perf_branches_hw();
+	if (test__start_subtest("perf_branches_no_hw"))
+		test_perf_branches_no_hw();
+}

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

@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2019 Facebook
+
+#include <stddef.h>
+#include <linux/ptrace.h>
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_trace_helpers.h"
+
+int valid = 0;
+int required_size_out = 0;
+int written_stack_out = 0;
+int written_global_out = 0;
+
+struct {
+	__u64 _a;
+	__u64 _b;
+	__u64 _c;
+} fpbe[30] = {0};
+
+SEC("perf_event")
+int perf_branches(void *ctx)
+{
+	__u64 entries[4 * 3] = {0};
+	int required_size, written_stack, written_global;
+
+	/* write to stack */
+	written_stack = bpf_read_branch_records(ctx, entries, sizeof(entries), 0);
+	/* ignore spurious events */
+	if (!written_stack)
+		return 1;
+
+	/* get required size */
+	required_size = bpf_read_branch_records(ctx, NULL, 0,
+						BPF_F_GET_BRANCH_RECORDS_SIZE);
+
+	written_global = bpf_read_branch_records(ctx, fpbe, sizeof(fpbe), 0);
+	/* ignore spurious events */
+	if (!written_global)
+		return 1;
+
+	required_size_out = required_size;
+	written_stack_out = written_stack;
+	written_global_out = written_global;
+	valid = 1;
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";