netdev - Re: [PATCH v4 bpf-next 7/9] selftests/bpf: switch test to new attach_perf

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Date:   Mon, 1 Jul 2019 17:16:00 +0000
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andriin@...com>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "sdf@...ichev.me" <sdf@...ichev.me>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH v4 bpf-next 7/9] selftests/bpf: switch test to new
 attach_perf_event API



On 6/28/19 8:49 PM, Andrii Nakryiko wrote:
> Use new bpf_program__attach_perf_event() in test previously relying on
> direct ioctl manipulations.
> 
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> Reviewed-by: Stanislav Fomichev <sdf@...gle.com>
> Acked-by: Song Liu <songliubraving@...com>
> ---
>   .../bpf/prog_tests/stacktrace_build_id_nmi.c  | 31 +++++++++----------
>   1 file changed, 15 insertions(+), 16 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/stacktrace_build_id_nmi.c b/tools/testing/selftests/bpf/prog_tests/stacktrace_build_id_nmi.c
> index 1c1a2f75f3d8..9557b7dfb782 100644
> --- a/tools/testing/selftests/bpf/prog_tests/stacktrace_build_id_nmi.c
> +++ b/tools/testing/selftests/bpf/prog_tests/stacktrace_build_id_nmi.c
> @@ -17,6 +17,7 @@ static __u64 read_perf_max_sample_freq(void)
>   void test_stacktrace_build_id_nmi(void)
>   {
>   	int control_map_fd, stackid_hmap_fd, stackmap_fd, stack_amap_fd;
> +	const char *prog_name = "tracepoint/random/urandom_read";
>   	const char *file = "./test_stacktrace_build_id.o";
>   	int err, pmu_fd, prog_fd;
>   	struct perf_event_attr attr = {
> @@ -25,7 +26,9 @@ void test_stacktrace_build_id_nmi(void)
>   		.config = PERF_COUNT_HW_CPU_CYCLES,
>   	};
>   	__u32 key, previous_key, val, duration = 0;
> +	struct bpf_program *prog;
>   	struct bpf_object *obj;
> +	struct bpf_link *link;
>   	char buf[256];
>   	int i, j;
>   	struct bpf_stack_build_id id_offs[PERF_MAX_STACK_DEPTH];
> @@ -39,6 +42,10 @@ void test_stacktrace_build_id_nmi(void)
>   	if (CHECK(err, "prog_load", "err %d errno %d\n", err, errno))
>   		return;
>   
> +	prog = bpf_object__find_program_by_title(obj, prog_name);
> +	if (CHECK(!prog, "find_prog", "prog '%s' not found\n", prog_name))
> +		goto close_prog;
> +
>   	pmu_fd = syscall(__NR_perf_event_open, &attr, -1 /* pid */,
>   			 0 /* cpu 0 */, -1 /* group id */,
>   			 0 /* flags */);
> @@ -47,15 +54,12 @@ void test_stacktrace_build_id_nmi(void)
>   		  pmu_fd, errno))
>   		goto close_prog;
>   
> -	err = ioctl(pmu_fd, PERF_EVENT_IOC_ENABLE, 0);
> -	if (CHECK(err, "perf_event_ioc_enable", "err %d errno %d\n",
> -		  err, errno))
> -		goto close_pmu;
> -
> -	err = ioctl(pmu_fd, PERF_EVENT_IOC_SET_BPF, prog_fd);
> -	if (CHECK(err, "perf_event_ioc_set_bpf", "err %d errno %d\n",
> -		  err, errno))
> -		goto disable_pmu;
> +	link = bpf_program__attach_perf_event(prog, pmu_fd);
> +	if (CHECK(IS_ERR(link), "attach_perf_event",
> +		  "err %ld\n", PTR_ERR(link))) {
> +		close(pmu_fd);
> +		goto close_prog;
> +	}
>   
>   	/* find map fds */
>   	control_map_fd = bpf_find_map(__func__, obj, "control_map");
> @@ -134,8 +138,7 @@ void test_stacktrace_build_id_nmi(void)
>   	 * try it one more time.
>   	 */
>   	if (build_id_matches < 1 && retry--) {
> -		ioctl(pmu_fd, PERF_EVENT_IOC_DISABLE);
> -		close(pmu_fd);
> +		bpf_link__destroy(link);
>   		bpf_object__close(obj);
>   		printf("%s:WARN:Didn't find expected build ID from the map, retrying\n",
>   		       __func__);
> @@ -154,11 +157,7 @@ void test_stacktrace_build_id_nmi(void)
>   	 */
>   
>   disable_pmu:
> -	ioctl(pmu_fd, PERF_EVENT_IOC_DISABLE);
> -
> -close_pmu:
> -	close(pmu_fd);
> -
> +	bpf_link__destroy(link);

There is a problem in bpf_link__destroy(link).
The "link = bpf_program__attach_perf_event(prog, pmu_fd)"
may be an error pointer (IS_ERR(link) is true), in which
case, link should be reset to NULL and then call 
bpf_link__destroy(link). Otherwise, the program may
segfault or function incorrectly.

>   close_prog:
>   	bpf_object__close(obj);
>   }
>