[PATCH v6 bpf-next 0/2] Add bpf_read_branch_records() helper

[PATCH v6 bpf-next 0/2] Add bpf_read_branch_records() helper

[Daniel Xu]

Branch records are a CPU feature that can be configured to record
certain branches that are taken during code execution. This data is
particularly interesting for profile guided optimizations. perf has had
branch record support for a while but the data collection can be a bit
coarse grained.

We (Facebook) have seen in experiments that associating metadata with
branch records can improve results (after postprocessing). We generally
use bpf_probe_read_*() to get metadata out of userspace. That's why bpf
support for branch records is useful.

Aside from this particular use case, having branch data available to bpf
progs can be useful to get stack traces out of userspace applications
that omit frame pointers.

Changes in v6:
- Move #ifdef a little to avoid unused variable warnings on !x86
- Test negative condition in selftest (-EINVAL on improperly configured
  perf event)
- Skip positive condition selftest on setups that don't support branch
  records

Changes in v5:
- Rename bpf_perf_prog_read_branches() -> bpf_read_branch_records()
- Rename BPF_F_GET_BR_SIZE -> BPF_F_GET_BRANCH_RECORDS_SIZE
- Squash tools/ bpf.h sync into selftest commit

Changes in v4:
- Add BPF_F_GET_BR_SIZE flag
- Return -ENOENT on unsupported architectures
- Only accept initialized memory in helper
- Check buffer size is multiple of sizeof(struct perf_branch_entry)
- Use bpf skeleton in selftest
- Add commit messages
- Spelling and formatting

Changes in v3:
- Document filling unused buffer with zero
- Formatting fixes
- Rebase

Changes in v2:
- Change to a bpf helper instead of context access
- Avoid mentioning Intel specific things

Daniel Xu (2):
  bpf: Add bpf_read_branch_records() helper
  selftests/bpf: add bpf_read_branch_records() selftest

 include/uapi/linux/bpf.h                      |  25 ++-
 kernel/trace/bpf_trace.c                      |  41 ++++
 tools/include/uapi/linux/bpf.h                |  25 ++-
 .../selftests/bpf/prog_tests/perf_branches.c  | 182 ++++++++++++++++++
 .../selftests/bpf/progs/test_perf_branches.c  |  74 +++++++
 5 files changed, 345 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/perf_branches.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_perf_branches.c

-- 
2.21.1

[PATCH v6 bpf-next 1/2] bpf: Add bpf_read_branch_records() helper

[Daniel Xu]

Branch records are a CPU feature that can be configured to record
certain branches that are taken during code execution. This data is
particularly interesting for profile guided optimizations. perf has had
branch record support for a while but the data collection can be a bit
coarse grained.

We (Facebook) have seen in experiments that associating metadata with
branch records can improve results (after postprocessing). We generally
use bpf_probe_read_*() to get metadata out of userspace. That's why bpf
support for branch records is useful.

Aside from this particular use case, having branch data available to bpf
progs can be useful to get stack traces out of userspace applications
that omit frame pointers.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
 include/uapi/linux/bpf.h | 25 +++++++++++++++++++++++-
 kernel/trace/bpf_trace.c | 41 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index f1d74a2bd234..332aa433d045 100644
--- a/include/uapi/linux/bpf.h
+++ b/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 buf_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
+ *		*buf_size* bytes.
+ *
+ *		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.
+ *	Return
+ *		On success, number of bytes written to *buf*. On error, a
+ *		negative value.
+ *
+ *		**-EINVAL** if arguments invalid or **buf_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,
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 19e793aa441a..efd119de95b8 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1028,6 +1028,45 @@ static const struct bpf_func_proto bpf_perf_prog_read_value_proto = {
          .arg3_type      = ARG_CONST_SIZE,
 };
 
+BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
+	   void *, buf, u32, size, u64, flags)
+{
+#ifndef CONFIG_X86
+	return -ENOENT;
+#else
+	struct perf_branch_stack *br_stack = ctx->data->br_stack;
+	u32 br_entry_size = sizeof(struct perf_branch_entry);
+	u32 to_copy;
+
+	if (unlikely(flags & ~BPF_F_GET_BRANCH_RECORDS_SIZE))
+		return -EINVAL;
+
+	if (unlikely(!br_stack))
+		return -EINVAL;
+
+	if (flags & BPF_F_GET_BRANCH_RECORDS_SIZE)
+		return br_stack->nr * br_entry_size;
+
+	if (!buf || (size % br_entry_size != 0))
+		return -EINVAL;
+
+	to_copy = min_t(u32, br_stack->nr * br_entry_size, size);
+	memcpy(buf, br_stack->entries, to_copy);
+
+	return to_copy;
+#endif
+}
+
+static const struct bpf_func_proto bpf_read_branch_records_proto = {
+	.func           = bpf_read_branch_records,
+	.gpl_only       = true,
+	.ret_type       = RET_INTEGER,
+	.arg1_type      = ARG_PTR_TO_CTX,
+	.arg2_type      = ARG_PTR_TO_MEM_OR_NULL,
+	.arg3_type      = ARG_CONST_SIZE_OR_ZERO,
+	.arg4_type      = ARG_ANYTHING,
+};
+
 static const struct bpf_func_proto *
 pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
@@ -1040,6 +1079,8 @@ pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_get_stack_proto_tp;
 	case BPF_FUNC_perf_prog_read_value:
 		return &bpf_perf_prog_read_value_proto;
+	case BPF_FUNC_read_branch_records:
+		return &bpf_read_branch_records_proto;
 	default:
 		return tracing_func_proto(func_id, prog);
 	}
-- 
2.21.1

[PATCH v6 bpf-next 2/2] selftests/bpf: add bpf_read_branch_records() selftest

[Daniel Xu]

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 map

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>
---
 tools/include/uapi/linux/bpf.h                |  25 ++-
 .../selftests/bpf/prog_tests/perf_branches.c  | 182 ++++++++++++++++++
 .../selftests/bpf/progs/test_perf_branches.c  |  74 +++++++
 3 files changed, 280 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/perf_branches.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_perf_branches.c

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index f1d74a2bd234..332aa433d045 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/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 buf_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
+ *		*buf_size* bytes.
+ *
+ *		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.
+ *	Return
+ *		On success, number of bytes written to *buf*. On error, a
+ *		negative value.
+ *
+ *		**-EINVAL** if arguments invalid or **buf_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,
diff --git a/tools/testing/selftests/bpf/prog_tests/perf_branches.c b/tools/testing/selftests/bpf/prog_tests/perf_branches.c
new file mode 100644
index 000000000000..bceefaea85b9
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/perf_branches.c
@@ -0,0 +1,182 @@
+// 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"
+
+struct output {
+	int required_size;
+	int written_stack;
+	int written_map;
+};
+
+static void on_good_sample(void *ctx, int cpu, void *data, __u32 size)
+{
+	int required_size = ((struct output *)data)->required_size;
+	int written_stack = ((struct output *)data)->written_stack;
+	int written_map = ((struct output *)data)->written_map;
+	int pbe_size = sizeof(struct perf_branch_entry);
+	int duration = 0;
+
+	/*
+	 * 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_map < 0, "read_branches_map", "err %d\n", written_map);
+	CHECK(written_map % pbe_size != 0, "read_branches_map",
+	      "map bytes written=%d not multiple of struct size=%d\n",
+	      written_map, pbe_size);
+	CHECK(written_map < written_stack, "read_branches_size",
+	      "written_map=%d < written_stack=%d\n", written_map, written_stack);
+
+	*(int *)ctx = 1;
+}
+
+static void on_bad_sample(void *ctx, int cpu, void *data, __u32 size)
+{
+	int required_size = ((struct output *)data)->required_size;
+	int written_stack = ((struct output *)data)->written_stack;
+	int written_map = ((struct output *)data)->written_map;
+	int duration = 0;
+
+	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_map != -EINVAL && written_map != -ENOENT),
+	      "read_branches_map", "written %d\n", written_map);
+
+	*(int *)ctx = 1;
+}
+
+static void test_perf_branches_common(int perf_fd, void *sample_cb)
+{
+	struct perf_buffer_opts pb_opts = {};
+	int err, i, duration = 0, ok = 0;
+	struct test_perf_branches *skel;
+	struct perf_buffer *pb;
+	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"))
+		goto out_destroy;
+
+	/* 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;
+
+	/* set up perf buffer */
+	pb_opts.sample_cb = sample_cb;
+	pb_opts.ctx = &ok;
+	pb = perf_buffer__new(bpf_map__fd(skel->maps.perf_buf_map), 1, &pb_opts);
+	if (CHECK(IS_ERR(pb), "perf_buf__new", "err %ld\n", PTR_ERR(pb)))
+		goto out_detach;
+
+	/* 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_free_pb;
+	/* spin the loop for a while (random high number) */
+	for (i = 0; i < 1000000; ++i)
+		++j;
+
+	/* read perf buffer */
+	err = perf_buffer__poll(pb, 500);
+	if (CHECK(err < 0, "perf_buffer__poll", "err %d\n", err))
+		goto out_free_pb;
+
+	if (CHECK(!ok, "ok", "not ok\n"))
+		goto out_free_pb;
+
+out_free_pb:
+	perf_buffer__free(pb);
+out_detach:
+	bpf_link__destroy(link);
+out_destroy:
+	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) {
+			printf("%s:SKIP:no PERF_SAMPLE_BRANCH_STACK\n",
+			       __func__);
+			test__skip();
+			return;
+		}
+		if (CHECK(pfd < 0, "perf_event_open", "err %d\n", pfd))
+			return;
+	}
+
+	test_perf_branches_common(pfd, on_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, on_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();
+}
diff --git a/tools/testing/selftests/bpf/progs/test_perf_branches.c b/tools/testing/selftests/bpf/progs/test_perf_branches.c
new file mode 100644
index 000000000000..60327d512400
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_perf_branches.c
@@ -0,0 +1,74 @@
+// 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"
+
+struct fake_perf_branch_entry {
+	__u64 _a;
+	__u64 _b;
+	__u64 _c;
+};
+
+struct output {
+	int required_size;
+	int written_stack;
+	int written_map;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
+	__uint(key_size, sizeof(int));
+	__uint(value_size, sizeof(int));
+} perf_buf_map SEC(".maps");
+
+typedef struct fake_perf_branch_entry fpbe_t[30];
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, fpbe_t);
+} scratch_map SEC(".maps");
+
+SEC("perf_event")
+int perf_branches(void *ctx)
+{
+	struct fake_perf_branch_entry entries[4] = {0};
+	struct output output = {0};
+	__u32 key = 0, *value;
+
+	/* write to stack */
+	output.written_stack = bpf_read_branch_records(ctx, entries,
+						       sizeof(entries), 0);
+	/* ignore spurious events */
+	if (!output.written_stack)
+		return 1;
+
+	/* get required size */
+	output.required_size =
+		bpf_read_branch_records(ctx, NULL, 0,
+					BPF_F_GET_BRANCH_RECORDS_SIZE);
+
+	/* write to map */
+	value = bpf_map_lookup_elem(&scratch_map, &key);
+	if (value)
+		output.written_map =
+			bpf_read_branch_records(ctx,
+						value,
+						30 * sizeof(struct fake_perf_branch_entry),
+						0);
+
+	/* ignore spurious events */
+	if (!output.written_map)
+		return 1;
+
+	bpf_perf_event_output(ctx, &perf_buf_map, BPF_F_CURRENT_CPU,
+			      &output, sizeof(output));
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.21.1

Re: [PATCH v6 bpf-next 1/2] bpf: Add bpf_read_branch_records() helper

[Daniel Borkmann]

On 1/27/20 12:35 AM, Daniel Xu wrote:
> Branch records are a CPU feature that can be configured to record
> certain branches that are taken during code execution. This data is
> particularly interesting for profile guided optimizations. perf has had
> branch record support for a while but the data collection can be a bit
> coarse grained.
> 
> We (Facebook) have seen in experiments that associating metadata with
> branch records can improve results (after postprocessing). We generally
> use bpf_probe_read_*() to get metadata out of userspace. That's why bpf
> support for branch records is useful.
> 
> Aside from this particular use case, having branch data available to bpf
> progs can be useful to get stack traces out of userspace applications
> that omit frame pointers.
> 
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---
>   include/uapi/linux/bpf.h | 25 +++++++++++++++++++++++-
>   kernel/trace/bpf_trace.c | 41 ++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 65 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f1d74a2bd234..332aa433d045 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/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 buf_size, u64 flags)

Small nit: s/buf_size/size/, so that it matches with your BPF_CALL below.

  +BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, 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
> + *		*buf_size* bytes.
> + *
> + *		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.
> + *	Return
> + *		On success, number of bytes written to *buf*. On error, a
> + *		negative value.

Maybe pull the 2nd paragraph from above in here so that it reflects the description
of the return value when flag is used also for this case in the 'Return' description.

> + *		**-EINVAL** if arguments invalid or **buf_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,
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 19e793aa441a..efd119de95b8 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1028,6 +1028,45 @@ static const struct bpf_func_proto bpf_perf_prog_read_value_proto = {
>            .arg3_type      = ARG_CONST_SIZE,
>   };
>   
> +BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
> +	   void *, buf, u32, size, u64, flags)
> +{
> +#ifndef CONFIG_X86
> +	return -ENOENT;
> +#else
> +	struct perf_branch_stack *br_stack = ctx->data->br_stack;
> +	u32 br_entry_size = sizeof(struct perf_branch_entry);

'static const u32 br_entry_size' if we use it as such below.

> +	u32 to_copy;
> +
> +	if (unlikely(flags & ~BPF_F_GET_BRANCH_RECORDS_SIZE))
> +		return -EINVAL;
> +
> +	if (unlikely(!br_stack))
> +		return -EINVAL;

Why the ifdef X86? In previous thread I meant to change it into since it's
implicit:

         if (unlikely(!br_stack))
                 return -ENOENT;

Or is there any other additional rationale?

> +	if (flags & BPF_F_GET_BRANCH_RECORDS_SIZE)
> +		return br_stack->nr * br_entry_size;
> +
> +	if (!buf || (size % br_entry_size != 0))
> +		return -EINVAL;
> +
> +	to_copy = min_t(u32, br_stack->nr * br_entry_size, size);
> +	memcpy(buf, br_stack->entries, to_copy);
> +
> +	return to_copy;
> +#endif
> +}
> +
> +static const struct bpf_func_proto bpf_read_branch_records_proto = {
> +	.func           = bpf_read_branch_records,
> +	.gpl_only       = true,
> +	.ret_type       = RET_INTEGER,
> +	.arg1_type      = ARG_PTR_TO_CTX,
> +	.arg2_type      = ARG_PTR_TO_MEM_OR_NULL,
> +	.arg3_type      = ARG_CONST_SIZE_OR_ZERO,
> +	.arg4_type      = ARG_ANYTHING,
> +};
> +
>   static const struct bpf_func_proto *
>   pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   {
> @@ -1040,6 +1079,8 @@ pe_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>   		return &bpf_get_stack_proto_tp;
>   	case BPF_FUNC_perf_prog_read_value:
>   		return &bpf_perf_prog_read_value_proto;
> +	case BPF_FUNC_read_branch_records:
> +		return &bpf_read_branch_records_proto;
>   	default:
>   		return tracing_func_proto(func_id, prog);
>   	}
> 

Re: [PATCH v6 bpf-next 1/2] bpf: Add bpf_read_branch_records() helper

[Daniel Xu]

On Mon Jan 27, 2020 at 1:26 PM, Daniel Borkmann wrote:
[...]
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index f1d74a2bd234..332aa433d045 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/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 buf_size, u64 flags)
>
> 
> Small nit: s/buf_size/size/, so that it matches with your BPF_CALL
> below.

Ok

> 
> +BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *,
> 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
> > + *		*buf_size* bytes.
> > + *
> > + *		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.
> > + *	Return
> > + *		On success, number of bytes written to *buf*. On error, a
> > + *		negative value.
>
> 
> Maybe pull the 2nd paragraph from above in here so that it reflects the
> description
> of the return value when flag is used also for this case in the 'Return'
> description.

Ok.

[...]
> >   
> > +BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
> > +	   void *, buf, u32, size, u64, flags)
> > +{
> > +#ifndef CONFIG_X86
> > +	return -ENOENT;
> > +#else
> > +	struct perf_branch_stack *br_stack = ctx->data->br_stack;
> > +	u32 br_entry_size = sizeof(struct perf_branch_entry);
>
> 
> 'static const u32 br_entry_size' if we use it as such below.

Ok

>
> 
> > +	u32 to_copy;
> > +
> > +	if (unlikely(flags & ~BPF_F_GET_BRANCH_RECORDS_SIZE))
> > +		return -EINVAL;
> > +
> > +	if (unlikely(!br_stack))
> > +		return -EINVAL;
>
> 
> Why the ifdef X86? In previous thread I meant to change it into since
> it's
> implicit:
>
> 
> if (unlikely(!br_stack))
> return -ENOENT;
>
> 
> Or is there any other additional rationale?

Yeah, so br_stack can be null if the perf_event is misconfigured (branch
record not enabled). So we need to differentiate that from arch not
supporting it.

[...]


Thanks,
Daniel