Re: [BPF CO-RE clarification] Use CO-RE on older kernel versions.

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

Em Wed, Mar 03, 2021 at 11:05:44PM -0800, Andrii Nakryiko escreveu:
> On Wed, Mar 3, 2021 at 10:15 AM Rafael David Tinoco
> <rafaeldtinoco@ubuntu.com> wrote:
> >
> > > > From:   Vamsi Kodavanty <vamsi@araalinetworks.com>
> > > > Date:   Thu, 7 Jan 2021 17:31:11 -0800
> > > > To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > > > Cc:     bpf <bpf@vger.kernel.org>
> > > >
> > > >
> > > > Right. Libbpf only supports a newer and safer way to attach to
> > > > kprobes. For your experiments, try to stick to tracepoints and you'll
> > > > have a better time.
> > > >
> > > > But it's another thing I've been meaning to add to libbpf for
> > > > supporting older kernels. I even have code written to do legacy kprobe
> > > > attachment, just need to find time to send a patch to add it as a
> > > > fallback for kernels that don't support new kprobe interface.
> >
> > Initially I'd like to thank you *a lot* for this thread, it helped me
> > creating:
> >
> > https://github.com/rafaeldtinoco/portablebpf
> >
> > showing up exactly what was discussed here AND I could run the same
> > binary in v4.15 and v.5.8 kernels as long as BTF was generated with:
> >
> > https://github.com/rafaeldtinoco/portablebpf/blob/master/patches/link-vmlinux.sh.patch
> 
> I was wondering if it might be useful to have a script that would use
> pahole to do DWARF to BTF conversion for existing vmlinux image (e.g.,
> from /boot/vmlinux-$(uname -r)), assuming DWARF is in that vmlinux (or
> could be found somewhere nearby), and then would spit out only .BTF
> contents as a binary file, which can be passed to libbpf on
> bpf_object__open(). That seems useful and there have been at least a
> few cases where people tried to use CO-RE on old kernels
> pre-CONFIG_DEBUG_INFO_BTF, but were always confused by how to get that
> BTF data.
> 
> [cc Arnaldo]
> It would also simplify things a bunch if pahole had an option to emit
> .BTF into a separate non-ELF file, instead of modifying vmlinux
> in-place. WDYT?

Sure, that is a nice addition, makes it more flexible to cover this
usecase.

- Arnaldo
 
> >
> > Specially the attach_kprobe_legacy() function:
> >
> > https://github.com/rafaeldtinoco/portablebpf/blob/master/mine.c#L31
> >
> > I wanted to reply here in case others also face this.
> 
> Great, glad it worked out. It would be great if you could contribute
> legacy kprobe support for libbpf as a proper patch, since it probably
> would be useful for a bunch of other people stuck with old kernels.
> 
> >
> > Only bad thing was kernel v4.15 missed global data support as showed in:
> >
> > https://github.com/iovisor/bcc/blob/master/docs/kernel-versions.md
> >
> > But using perf event was good enough for an example.
> >
> > - rafaeldtinoco