netdev - Re: [PATCH bpf-next v4 2/3] bpf: selftests: remove percpu macros from bpf

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Message-ID: <CAEf4Bzau9AZrJ0zKAsVptwLtJsSY_n7DbcKD9GmZ-cyv2RNpYg@mail.gmail.com>
Date:   Tue, 20 Apr 2021 09:42:20 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Pedro Tammela <pctammela@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Shuah Khan <shuah@...nel.org>,
        Pedro Tammela <pctammela@...atatu.com>,
        David Verbeiren <david.verbeiren@...sares.net>,
        Matthieu Baerts <matthieu.baerts@...sares.net>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v4 2/3] bpf: selftests: remove percpu macros from bpf_util.h

On Tue, Apr 20, 2021 at 8:58 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 4/20/21 3:17 AM, Alexei Starovoitov wrote:
> > On Thu, Apr 15, 2021 at 10:47 AM Pedro Tammela <pctammela@...il.com> wrote:
> >>
> >> Andrii suggested to remove this abstraction layer and have the percpu
> >> handling more explicit[1].
> >>
> >> This patch also updates the tests that relied on the macros.
> >>
> >> [1] https://lore.kernel.org/bpf/CAEf4BzYmj_ZPDq8Zi4dbntboJKRPU2TVopysBNrdd9foHTfLZw@mail.gmail.com/
> >>
> >> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> >> Signed-off-by: Pedro Tammela <pctammela@...atatu.com>
> >> ---
> >>   tools/testing/selftests/bpf/bpf_util.h        |  7 --
> >>   .../bpf/map_tests/htab_map_batch_ops.c        | 87 +++++++++----------
> >>   .../selftests/bpf/prog_tests/map_init.c       |  9 +-
> >>   tools/testing/selftests/bpf/test_maps.c       | 84 +++++++++++-------
> >>   4 files changed, 96 insertions(+), 91 deletions(-)
> >>
> >> diff --git a/tools/testing/selftests/bpf/bpf_util.h b/tools/testing/selftests/bpf/bpf_util.h
> >> index a3352a64c067..105db3120ab4 100644
> >> --- a/tools/testing/selftests/bpf/bpf_util.h
> >> +++ b/tools/testing/selftests/bpf/bpf_util.h
> >> @@ -20,13 +20,6 @@ static inline unsigned int bpf_num_possible_cpus(void)
> >>          return possible_cpus;
> >>   }
> >>
> >> -#define __bpf_percpu_val_align __attribute__((__aligned__(8)))
> >> -
> >> -#define BPF_DECLARE_PERCPU(type, name)                         \
> >> -       struct { type v; /* padding */ } __bpf_percpu_val_align \
> >> -               name[bpf_num_possible_cpus()]
> >> -#define bpf_percpu(name, cpu) name[(cpu)].v
> >> -
> >
> > Hmm. I wonder what Daniel has to say about it, since he
> > introduced it in commit f3515b5d0b71 ("bpf: provide a generic macro
> > for percpu values for selftests")
> > to address a class of bugs.
>
> I would probably even move those into libbpf instead. ;-) The problem is that this can
> be missed easily and innocent changes would lead to corruption of the applications
> memory if there's a map lookup. Having this at least in selftest code or even in libbpf
> would document code-wise that care needs to be taken on per cpu maps. Even if we'd put
> a note under Documentation/bpf/ or such, this might get missed easily and finding such
> bugs is like looking for a needle in a haystack.. so I don't think this should be removed.
>

See [0] for previous discussion. I don't mind leaving bpf_percpu() in
selftests. I'm not sure I ever suggested removing it from selftests,
but I don't think it's a good idea to add it to libbpf. I think it's
better to have an extra paragraph in bpf_lookup_map_elem() in
uapi/linux/bpf.h mentioning how per-CPU values should be read/updated.
I think we should just recommend to use u64 for primitive values (or
otherwise users can embed their int in custom aligned(8) struct, if
they insist on <u64) and __attribute__((aligned(8))) for structs.

  [0] https://lore.kernel.org/bpf/CAEf4BzaLKm_fy4oO4Rdp76q2KoC6yC1WcJLuehoZUu9JobG-Cw@mail.gmail.com/


> Thanks,
> Daniel