Re: [PATCH dwarves v2 1/1] btf_encoder: handle .BTF_ids section endianness

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Tue, Nov 26, 2024 at 09:32:28AM -0800, Eduard Zingerman wrote:
> On Tue, 2024-11-26 at 13:25 -0300, Arnaldo Carvalho de Melo wrote:
 
> [...]
 
> Hi Arnaldo,
 
> > I think I saw instructions in one of the messages in this thread to get
> > hold of a vmlinux for s390 and test it. Right?
 
> Yes, in cover letter. Full vmlinux is not needed, a vmlinux binary for
> s390 would be sufficient for testing. Repeating the recipe for convenience:
 
>   To reproduce the bug:
>   - follow the instructions in [0] to build an s390 vmlinux;
>   - generate BTF requesting declaration tags for kfuncs:
>     $ pahole --btf_features_strict=decl_tag_kfuncs,decl_tag \
>              --btf_encode_detached=test.btf vmlinux
>   - observe that no kfuncs are generated:
>     $ bpftool btf dump file test.btf format c | grep __ksym
 
> [0] https://docs.kernel.org/bpf/s390.html

Thanks.
 
> > One extra question: this solves the BTF encoder case, the loader already
> > supported loading BTF from a different endianness, right? Lemme
> > check.
 
> > cus__load_btf()
> >   cu->little_endian = btf__endianness(btf) == BTF_LITTLE_ENDIAN;

> > enum btf_endianness btf__endianness(const struct btf *btf)
> > {
> >         if (is_host_big_endian())
> >                 return btf->swapped_endian ? BTF_LITTLE_ENDIAN : BTF_BIG_ENDIAN;
> >         else
> >                 return btf->swapped_endian ? BTF_BIG_ENDIAN : BTF_LITTLE_ENDIAN;
> > }
 
> I can switch to is_host_big_endian() instead of `BYTE_ORDER` macro
> if you think that's better.

No need for that, I think, is_host_big_endian() is a static function
inlib/bpf/src/btf.c.

I was just looking at how endianness was being handled and noticed
libbpf does it, but, as you say below...
 
> > So we have parts of BTF byte swapping happening in libbpf and with this
> > patch, parts of it done in pahole, have you tought about doing this in
> > libbpf instead?
 
> BTF id lists handling is currently not a part of libbpf.

So we should do it in pahole, as you did, so all is clarified now, I'm
now testing it with the provided instructions.

Thanks,

- Arnaldo