From: Andrii Nakryiko <email@example.com>
To: Vamsi Kodavanty <firstname.lastname@example.org>
Cc: bpf <email@example.com>
Subject: Re: [BPF CO-RE clarification] Use CO-RE on older kernel versions.
Date: Wed, 6 Jan 2021 15:55:25 -0800 [thread overview]
Message-ID: <CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com> (raw)
On Wed, Jan 6, 2021 at 10:04 AM Vamsi Kodavanty
> Had a few questions on CO-RE dependencies and usage. From what I read
> CO-RE needs a supported kernel version and be compiled with
> I also understand there are three pieces to enable CO-RE
> functionality. (1) The BTF format. For efficient/compressed kernel
> symbol table. (2) clang changes to emit the BTF relocations. (3)
BTF is not really a symbol table, rather a type information. Like
simpler and more compact DWARF.
> `libbpf` changes to locate a BTF file and fix-up relocations. Once
> these 3 steps are done the resulting byte code is no different from
> non-CO-RE byte code.
> Given this I am hoping the knowledgeable folks on this mailer correct
> and guide me if I am stating something incorrectly.
> (1) Is the kernel support requirement ONLY for the purposes of
> generating and exposing the BTF file information on
> `/sys/kernel/btf/vmlinux`? So that the eBPF CO-RE applications
> `libbpf` can find the BTF information at a standard location?.
/sys/kernel/btf/vmlinux is a standardized place, but libbpf will also
try to search for vmlinux image (and BTF info within it) in a few
standard locations, see . Early versions of in-kernel BTF didn't
even expose /sys/kernel/btf/vmlinux.
> (2) If the answer to the above question is YES. Could the below
> mechanism be used so that it works on all kernels whether they support
> the `CONFIG_DEBUG_INFO_BTF` flag or not?.
> (a) Extract BTF generation process outside of the kernel build.
> Use this to generate the equivalent BTF file for it.
Yes, CONFIG_DEBUG_INFO_BTF=y is the most convenient way to add BTF
info, but it's also possible to just embed BTF manually with a direct
invocation of pahole -J, see  on how it's done for
CONFIG_DEBUG_INFO_BTF. You can do that for *any* kernel image, no
matter the version, and it will work with CO-RE relocations.
> (b) Make changes to `libbpf` to look for BTF not only at the
> standard locations but also at a user specified location. The BTF file
> generated in (a) can be presented here.
You can already do that, actually, though it's not very obvious. You
can specify (or override) kernel BTF location by using
bpf_object__load_xattr() and passing target_btf_path pointing to your
BTF location (see ). I've been meaning to add it instead to a
bpf_object_open_opts, btw, to make its use possible with a BPF
skeleton. Also keep in mind that currently libbpf expects that custom
BTF to be an ELF file with .BTF section, not just a raw BTF data. But
we can improve that, of course.
> This should provide us a way to enable CO-RE functionality on older
> kernel versions as well. I tried to make the above changes and tried
> against a 4.14 kernel and it did not work. Either I am not doing
> something right or my assumptions are wrong.
> Thanks in advance for your time. And I hope someone here can guide me
> in the right direction.
next prev parent reply other threads:[~2021-01-06 23:56 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 [this message]
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
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
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).