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

[Song Liu <songliubraving@fb.com>]

> On Jul 24, 2019, at 5:37 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> 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?

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?

Thanks,
Song