Re: Problem with endianess of pahole BTF output for vmlinux

[Tony Ambardar <tony.ambardar@gmail.com>]

On Wed, 9 Sep 2020 at 07:27, Arnaldo Carvalho de Melo
<arnaldo.melo@gmail.com> wrote:
>
> Em Wed, Sep 09, 2020 at 11:02:24AM +0200, Ilya Leoshkevich escreveu:
> > On Tue, 2020-09-08 at 13:18 -0700, Andrii Nakryiko wrote:
> > > On Mon, Sep 7, 2020 at 9:02 AM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
> > > > On Sat, 2020-09-05 at 21:16 -0700, Tony Ambardar wrote:
>
[...]
> > > > > Is this expected? Is DEBUG_INFO_BTF supported in general when
> > > > > cross-compiling? How does one generate BTF encoded for the
> > > > > target endianness with pahole?
>
> The BTF loader has support for endianness, its just the encoder that
> doesn't :-\
>
> I.e. pahole can grok a big endian BTF payload on a little endian machine
> and vice-versa, just can't cross-build BTF payloads ATM.
>
> > > Yes, it's expected, unfortunately. Right now cross-compiling to a
> > > different endianness isn't supported. You can cross-compile only if
> > > target endianness matches host endianness.
>
> I agree that having this in libbpf is better, it should be done as part
> of producing the result of the deduplication phase.
>
Thanks for confirming this wasn't a case of operator error. My platforms for
learning/experimenting with BPF have been small embedded ones, including
cross-compiling to different archs, word-size and endianness, which have
"helped" me run into multiple problems till now. This one is the first I
couldn't work around however...

[...]
> > > I'm working on extending BTF APIs in libbpf at the moment. Switching
> > > endianness would be rather easy once all that is done. With these new
> > > APIs it will be possible to switch pahole to use libbpf APIs to
> > > produce BTF output and support arbitrary endianness as well. Right
> > > now, I'd rather avoid implementing this in pahole, libbpf is a much
> > > better place for this (and will require ongoing updates if/when we
> > > introduce new types and fields to BTF).
>
> Right, we could do it right after btf_dedup() and before
> btf_elf__write(), doing the same process as in btf_loader.c, i.e.
> checking if the ELF target arch is different in endianness and doing the
> reverse of the loader.
>
> > > Hope this plan works for you guys.
> >
> > That sounds really good to me, thanks!
>
Andrii and Arnaldo, I really appreciate your working on a proper endianness fix.
If you have a WIP or staging branch and could use some help please let me know.

Best regards,
Tony