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

[Song Liu <songliubraving@fb.com>]

> On Jul 26, 2019, at 10:11 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Wed, Jul 24, 2019 at 10:20 PM Song Liu <songliubraving@fb.com> wrote:
>> 
>> 
>> 
>>> 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?
> 
> I think offsetofend() is the cleanest solution, I'll do just that.

Agreed that offsetofend() is the best. 

Song