netdev - Re: [PATCH bpf-next 01/10] libbpf: add .BTF.ext offset relocation section loading

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Message-ID: <CAEf4BzYgyAiPt0wVESrWSJ_tLheq0BRWLgrqMfLZsnp11+F77Q@mail.gmail.com>
Date:   Fri, 26 Jul 2019 22:11:12 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>, Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 01/10] libbpf: add .BTF.ext offset relocation
 section loading

On Wed, Jul 24, 2019 at 10:20 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Jul 24, 2019, at 5:37 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> >
> > On Wed, Jul 24, 2019 at 5:00 PM Song Liu <songliubraving@...com> wrote:
> >>
> >>
> >>
> >>> On Jul 24, 2019, at 12:27 PM, Andrii Nakryiko <andriin@...com> wrote:
> >>>
> >>> Add support for BPF CO-RE offset relocations. Add section/record
> >>> iteration macros for .BTF.ext. These macro are useful for iterating over
> >>> each .BTF.ext record, either for dumping out contents or later for BPF
> >>> CO-RE relocation handling.
> >>>
> >>> To enable other parts of libbpf to work with .BTF.ext contents, moved
> >>> a bunch of type definitions into libbpf_internal.h.
> >>>
> >>> Signed-off-by: Andrii Nakryiko <andriin@...com>
> >>> ---
> >>> tools/lib/bpf/btf.c             | 64 +++++++++--------------
> >>> tools/lib/bpf/btf.h             |  4 ++
> >>> tools/lib/bpf/libbpf_internal.h | 91 +++++++++++++++++++++++++++++++++
> >>> 3 files changed, 118 insertions(+), 41 deletions(-)
> >>>
> >
> > [...]
> >
> >>> +
> >>> static int btf_ext_parse_hdr(__u8 *data, __u32 data_size)
> >>> {
> >>>      const struct btf_ext_header *hdr = (struct btf_ext_header *)data;
> >>> @@ -1004,6 +979,13 @@ struct btf_ext *btf_ext__new(__u8 *data, __u32 size)
> >>>      if (err)
> >>>              goto done;
> >>>
> >>> +     /* check if there is offset_reloc_off/offset_reloc_len fields */
> >>> +     if (btf_ext->hdr->hdr_len < sizeof(struct btf_ext_header))
> >>
> >> This check will break when we add more optional sections to btf_ext_header.
> >> Maybe use offsetof() instead?
> >
> > I didn't do it, because there are no fields after offset_reloc_len.
> > But now I though that maybe it would be ok to add zero-sized marker
> > field, kind of like marking off various versions of btf_ext header?
> >
> > Alternatively, I can add offsetofend() macro somewhere in libbpf_internal.h.
> >
> > Do you have any preference?
>
> We only need a stable number to compare against. offsetofend() works.
> Or we can simply have something like
>
>     if (btf_ext->hdr->hdr_len <= offsetof(struct btf_ext_header, offset_reloc_off))
>           goto done;
> or
>     if (btf_ext->hdr->hdr_len < offsetof(struct btf_ext_header, offset_reloc_len))
>           goto done;
>
> Does this make sense?

I think offsetofend() is the cleanest solution, I'll do just that.

>
> Thanks,
> Song