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From:   Quentin Monnet <quentin@isovalent.com>
Date:   Thu, 4 May 2023 00:43:52 +0100
Message-ID: <CACdoK4+KdM-sQKMO9WXk7kTL-x=Renjd0MuvSRT3JKbtzByyHQ@mail.gmail.com>
Subject: Re: [PATCH v2 bpf 02/11] bpftool: define a local bpf_perf_link to fix
 accessing its fields
To:     =?UTF-8?Q?Michal_Such=C3=A1nek?= <msuchanek@suse.de>
Cc:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Alexander Lobakin <aleksander.lobakin@intel.com>,
        Alexander Lobakin <alobakin@mailbox.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Shung-Hsi Yu <shung-hsi.yu@suse.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Song Liu <songliubraving@fb.com>,
        Kumar Kartikeya Dwivedi <memxor@gmail.com>,
        bpf@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
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On Fri, 21 Apr 2023 at 08:39, Michal Such=C3=A1nek <msuchanek@suse.de> wrot=
e:
>
> On Thu, Apr 20, 2023 at 04:07:38PM -0700, Andrii Nakryiko wrote:
> > On Fri, Apr 14, 2023 at 9:28=E2=80=AFAM Michal Such=C3=A1nek <msuchanek=
@suse.de> wrote:
> > >
> > > On Fri, Apr 14, 2023 at 05:18:27PM +0200, Alexander Lobakin wrote:
> > > > From: Michal Such=C3=A1nek <msuchanek@suse.de>
> > > > Date: Fri, 14 Apr 2023 11:54:57 +0200
> > > >
> > > > > Hello,
> > > >
> > > > Hey-hey,
> > > >
> > > > >
> > > > > On Thu, Apr 21, 2022 at 12:38:58AM +0000, Alexander Lobakin wrote=
:
> > > > >> When building bpftool with !CONFIG_PERF_EVENTS:
> > > > >>
> > > > >> skeleton/pid_iter.bpf.c:47:14: error: incomplete definition of t=
ype 'struct bpf_perf_link'
> > > > >>         perf_link =3D container_of(link, struct bpf_perf_link, l=
ink);
> > > > >>                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~=
~~
> > > > >> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:74:=
22: note: expanded from macro 'container_of'
> > > > >>                 ((type *)(__mptr - offsetof(type, member)));    =
\
> > > > >>                                    ^~~~~~~~~~~~~~~~~~~~~~
> > > > >> tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:68:=
60: note: expanded from macro 'offsetof'
> > > > >>  #define offsetof(TYPE, MEMBER)  ((unsigned long)&((TYPE *)0)->M=
EMBER)
> > > > >>                                                   ~~~~~~~~~~~^
> > > > >> skeleton/pid_iter.bpf.c:44:9: note: forward declaration of 'stru=
ct bpf_perf_link'
> > > > >>         struct bpf_perf_link *perf_link;
> > > > >>                ^
> > > > >>
> > > > >> &bpf_perf_link is being defined and used only under the ifdef.
> > > > >> Define struct bpf_perf_link___local with the `preserve_access_in=
dex`
> > > > >> attribute inside the pid_iter BPF prog to allow compiling on any
> > > > >> configs. CO-RE will substitute it with the real struct bpf_perf_=
link
> > > > >> accesses later on.
> > > > >> container_of() is not CO-REd, but it is a noop for
> > > > >> bpf_perf_link <-> bpf_link and the local copy is a full mirror o=
f
> > > > >> the original structure.
> > > > >>
> > > > >> Fixes: cbdaf71f7e65 ("bpftool: Add bpf_cookie to link output")
> > > > >
> > > > > This does not solve the problem completely. Kernels that don't ha=
ve
> > > > > CONFIG_PERF_EVENTS in the first place are also missing the enum v=
alue
> > > > > BPF_LINK_TYPE_PERF_EVENT which is used as the condition for handl=
ing the
> > > > > cookie.
> > > >
> > > > Sorry, I haven't been working with my home/private stuff for more t=
han a
> > > > year already. I may get back to it some day when I'm tired of Lua (=
curse
> > > > words, sorry :D), but for now the series is "a bit" abandoned.
> > >
> > > This part still appllies and works for me with the caveat that
> > > BPF_LINK_TYPE_PERF_EVENT also needs to be defined.
> > >
> > > > I think there was alternative solution proposed there, which promis=
ed to
> > > > be more flexible. But IIRC it also doesn't touch the enum (was it a=
dded
> > > > recently? Because it was building just fine a year ago on config wi=
thout
> > > > perf events).
> > >
> > > It was added in 5.15. Not sure there is a kernel.org LTS kernel usabl=
e
> > > for CO-RE that does not have it, technically 5.4 would work if it was
> > > built monolithic, it does not have module BTF, only kernel IIRC.
> > >
> > > Nonetheless, the approach to handling features completely missing in =
the
> > > running kernel should be figured out one way or another. I would be
> > > surprised if this was the last feature to be added that bpftool needs=
 to
> > > know about.
> >
> > Are we talking about bpftool built from kernel sources or from Github?
> > Kernel source version should have access to latest UAPI headers and so
> > BPF_LINK_TYPE_PERF_EVENT should be available. Github version, if it
> > doesn't do that already, can use UAPI headers distributed (and used
> > for building) with libbpf through submodule.
>
> It does have a copy of the uapi headers but apparently does not use
> them. Using them directly might cause conflict with vmlinux.h, though.

Indeed, using the UAPI header here conflicts with vmlinux.h.

Looking again at some code I started last year but never finalised, I
used the following approach, redefining BPF_LINK_TYPE_PERF_EVENT with
CO-RE:

    enum bpf_link_type___local {
        BPF_LINK_TYPE_PERF_EVENT___local =3D 7,
    };

Then guarding accordingly in iter():

    [...]
    if (obj_type =3D=3D BPF_OBJ_LINK &&
        bpf_core_enum_value_exists(enum bpf_link_type___local,
                       BPF_LINK_TYPE_PERF_EVENT___local)) {
        struct bpf_link *link =3D (struct bpf_link *) file->private_data;

        if (link->type =3D=3D bpf_core_enum_value(enum bpf_link_type___loca=
l,
                  BPF_LINK_TYPE_PERF_EVENT___local)) {
            e.has_bpf_cookie =3D true;
            e.bpf_cookie =3D get_bpf_cookie(link);
        }
    }
    [...]

Would that approach make sense? I had a VM around with kernel 5.8, and
bpftool compiles there with that change. If I remember correctly, some
older kernel versions required yet more CO-RE work in pid_iter.bpf.c,
and at some point I was struggling, which is why I never submitted
this set.

If this approach looks correct to you Andrii, I can resubmit these
patches with my addition so we can at least fix the build on 5.8
onwards.

Thanks,
Quentin