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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Rafael David Tinoco <rafaeldtinoco@ubuntu.com>,
	Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Vamsi Kodavanty <vamsi@araalinetworks.com>,
	rafaeldtinoco@gmail.com, bpf <bpf@vger.kernel.org>
Subject: Re: [BPF CO-RE clarification] Use CO-RE on older kernel versions.
Date: Wed, 3 Mar 2021 23:05:44 -0800	[thread overview]
Message-ID: <CAEf4BzZE_Ss7-cNdVpKJbC57mr2V_-OMcC9fvHw7XTntn3K2jA@mail.gmail.com> (raw)
In-Reply-To: <20210303181457.172434-1-rafaeldtinoco@ubuntu.com>

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?

>
> 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

  reply	other threads:[~2021-03-04  7:07 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-06 18:02 [BPF CO-RE clarification] Use CO-RE on older kernel versions Vamsi Kodavanty
2021-01-06 23:55 ` Andrii Nakryiko
2021-01-07 18:12   ` Vamsi Kodavanty
2021-01-07 18:52     ` Andrii Nakryiko
2021-01-07 22:45       ` Vamsi Kodavanty
2021-01-07 23:32         ` Andrii Nakryiko
2021-01-08  0:16           ` Vamsi Kodavanty
2021-01-08  1:31             ` Vamsi Kodavanty
2021-03-03 18:14               ` Rafael David Tinoco
2021-03-04  7:05                 ` Andrii Nakryiko [this message]
2021-03-04 13:10                   ` Arnaldo Carvalho de Melo
2021-03-05  6:32                   ` Rafael David Tinoco
     [not found]                     ` <67E3C788-2835-4793-8A9C-51C5D807C294@ubuntu.com>
2021-03-10  6:00                       ` Fwd: " Rafael David Tinoco
2021-03-10 19:19                       ` Andrii Nakryiko
2021-03-10 22:45                         ` Rafael David Tinoco
2021-03-12 18:36                           ` Andrii Nakryiko
2021-03-17  4:39                             ` Rafael David Tinoco
2021-03-17 14:31                               ` Rafael David Tinoco
2021-03-19  4:36                                 ` Andrii Nakryiko
2021-03-19  4:42                                   ` Rafael David Tinoco

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