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

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Song Liu <songliubraving@fb.com>
Cc: Andrii Nakryiko <andriin@fb.com>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Yonghong Song <yhs@fb.com>, Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH bpf-next 01/10] libbpf: add .BTF.ext offset relocation section loading
Date: Wed, 24 Jul 2019 17:37:16 -0700	[thread overview]
Message-ID: <CAEf4BzZsU8qXa08neQ=nrFFTXpSWsxrZuZz=kVjS2BXNUoofUw@mail.gmail.com> (raw)
In-Reply-To: <B5E772A5-C0D9-4697-ADE2-2A94C4AD37B5@fb.com>

On Wed, Jul 24, 2019 at 5:00 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Jul 24, 2019, at 12:27 PM, Andrii Nakryiko <andriin@fb.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@fb.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?

>
> > +             goto done;
> > +     err = btf_ext_setup_offset_reloc(btf_ext);
> > +     if (err)
> > +             goto done;
> > +
> > done:

[...]