Re: [PATCH bpf-next v2 6/6] bpf/selftests: Test for bpf_per_cpu_ptr() and bpf_this_cpu_ptr()

[Hao Luo]

Thanks for taking a look!

On Fri, Sep 4, 2020 at 1:15 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Sep 3, 2020 at 3:35 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
> > Test bpf_per_cpu_ptr() and bpf_this_cpu_ptr(). Test two paths in the
> > kernel. If the base pointer points to a struct, the returned reg is
> > of type PTR_TO_BTF_ID. Direct pointer dereference can be applied on
> > the returned variable. If the base pointer isn't a struct, the
> > returned reg is of type PTR_TO_MEM, which also supports direct pointer
> > dereference.
> >
> > Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> > Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> > ---
> >  .../selftests/bpf/prog_tests/ksyms_btf.c      | 10 +++++++
> >  .../selftests/bpf/progs/test_ksyms_btf.c      | 26 +++++++++++++++++++
> >  2 files changed, 36 insertions(+)
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c b/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c
> > index 7b6846342449..22cc642dbc0e 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/ksyms_btf.c
> > @@ -58,6 +58,16 @@ void test_ksyms_btf(void)
> >         CHECK(data->out__bpf_prog_active != bpf_prog_active_addr, "bpf_prog_active",
> >               "got %llu, exp %llu\n", data->out__bpf_prog_active, bpf_prog_active_addr);
> >
> > +       CHECK(data->out__rq_cpu == -1, "rq_cpu",
> > +             "got %u, exp != -1\n", data->out__rq_cpu);
> > +       CHECK(data->out__percpu_bpf_prog_active == -1, "percpu_bpf_prog_active",
> > +             "got %d, exp != -1\n", data->out__percpu_bpf_prog_active);
> > +
> > +       CHECK(data->out__this_rq_cpu == -1, "this_rq_cpu",
> > +             "got %u, exp != -1\n", data->out__this_rq_cpu);
> > +       CHECK(data->out__this_bpf_prog_active == -1, "this_bpf_prog_active",
> > +             "got %d, exp != -1\n", data->out__this_bpf_prog_active);
>
> see below for few suggestions to make these test more specific
>
> out__this_bpf_prog_active it should always be > 0, no?
>

I could be wrong, but I remember raw_trace_point is not tracked by
bpf_prog_active. So I used bpf_prog_active >= 0 to be safe.

> > +
> >  cleanup:
> >         test_ksyms_btf__destroy(skel);
> >  }
> > diff --git a/tools/testing/selftests/bpf/progs/test_ksyms_btf.c b/tools/testing/selftests/bpf/progs/test_ksyms_btf.c
> > index e04e31117f84..02d564349892 100644
> > --- a/tools/testing/selftests/bpf/progs/test_ksyms_btf.c
> > +++ b/tools/testing/selftests/bpf/progs/test_ksyms_btf.c
> > @@ -8,15 +8,41 @@
> >  __u64 out__runqueues = -1;
> >  __u64 out__bpf_prog_active = -1;
> >
> > +__u32 out__rq_cpu = -1; /* percpu struct fields */
> > +int out__percpu_bpf_prog_active = -1; /* percpu int */
> > +
> > +__u32 out__this_rq_cpu = -1;
> > +int out__this_bpf_prog_active = -1;
> > +
> >  extern const struct rq runqueues __ksym; /* struct type global var. */
> >  extern const int bpf_prog_active __ksym; /* int type global var. */
> >
> >  SEC("raw_tp/sys_enter")
> >  int handler(const void *ctx)
> >  {
> > +       struct rq *rq;
> > +       int *active;
> > +       __u32 cpu;
> > +
> >         out__runqueues = (__u64)&runqueues;
> >         out__bpf_prog_active = (__u64)&bpf_prog_active;
> >
> > +       cpu = bpf_get_smp_processor_id();
> > +
> > +       /* test bpf_per_cpu_ptr() */
> > +       rq = (struct rq *)bpf_per_cpu_ptr(&runqueues, cpu);
> > +       if (rq)
> > +               out__rq_cpu = rq->cpu;
> > +       active = (int *)bpf_per_cpu_ptr(&bpf_prog_active, cpu);
> > +       if (active)
> > +               out__percpu_bpf_prog_active = *active;
>
> this is equivalent to using bpf_this_cpu_ptr(), so:
>
> 1. you can compare value with out__this_xxx in user-space
>
> 2. it's interesting to also test that you can read value from some
> other CPU. Can you add another variable and get value from CPU #0
> always? E.g., for out__cpu_0_rq_cpu it should always be zero, right?
>

Ack. That makes sense. You are right, out__cpu_0_rq_cpu is always zero.

> > +
> > +       /* test bpf_this_cpu_ptr */
> > +       rq = (struct rq *)bpf_this_cpu_ptr(&runqueues);
> > +       out__this_rq_cpu = rq->cpu;
> > +       active = (int *)bpf_this_cpu_ptr(&bpf_prog_active);
> > +       out__this_bpf_prog_active = *active;
> > +
> >         return 0;
> >  }
> >
> > --
> > 2.28.0.526.ge36021eeef-goog
> >