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 15:32:34 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...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 Mon, Jul 1, 2019 at 10:16 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> 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.

Not really, if bpf_program__attach_perf_event fails and IS_ERR(link)
is true, we'll close pmu_fd explicitly and `goto close_prog` bypassing
bpf_link__destroy. `goto disable_pmu` is done only after we
successfully established attached link.

So unless I still miss something, I think this will work reliably.

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