Re: Problem with endianess of pahole BTF output for vmlinux

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Sep 21, 2020 at 11:19 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Sat, Sep 19, 2020 at 12:58 AM Tony Ambardar <tony.ambardar@gmail.com> wrote:
> >
> > 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.
> >
>
> I have a bunch of code changes locally. I'll clean that up, partition
> libbpf and pahole patches, and post them for review this week. To
> address endianness support, those are the prerequisites. Once those
> changes land, I'll be able to solve endianness issues you are having.
> So just a bit longer till all that is done, sorry for the wait!
>

Question to folks that are working with 32-bit and/or big-endian
architectures. Do you guys have an VM image that you'd be able to
share with me, such that I can use it with qemu to test patches like
this. My normal setup is all 64-bit/little-endian, so testing changes
like this (and a few more I'm planning to do to address mixed 32-bit
on the host vs 64-bit in BPF cases) is a bit problematic. And it's
hard to get superpumped about spending lots of time setting up a new
Linux image (never goes easy or fast for me).

So, if you do have something like this, please share. Thank you!

> > Best regards,
> > Tony