dwarves.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Cc: Yonghong Song <yhs@fb.com>, bpf <bpf@vger.kernel.org>,
	dwarves@vger.kernel.org, siudin@fb.com
Subject: Re: latest pahole breaks libbpf CI and let's talk about staging
Date: Thu, 17 Jun 2021 14:52:57 -0700	[thread overview]
Message-ID: <CAEf4BzZWq21zP+1C4=qqWGQ3WUXK-pvt+rWpcsh_971qAw4Wzw@mail.gmail.com> (raw)
In-Reply-To: <YMuzAfK6vwbeN3XX@kernel.org>

On Thu, Jun 17, 2021 at 1:39 PM Arnaldo Carvalho de Melo
<arnaldo.melo@gmail.com> wrote:
>
> Em Thu, Jun 17, 2021 at 01:00:11PM -0700, Andrii Nakryiko escreveu:
> > On Thu, Jun 17, 2021 at 7:48 AM Arnaldo Carvalho de Melo
> > <arnaldo.melo@gmail.com> wrote:
> > >
> > > Em Wed, Jun 16, 2021 at 03:36:54PM -0700, Andrii Nakryiko escreveu:
> > > > On Wed, Jun 16, 2021 at 1:41 PM Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com> wrote:
> > > > > And if I use pahole's BTF loader I find the info about that function:
> > > > >
> > > > > [acme@seventh linux]$ strace -e openat -o /tmp/bla pfunct -F btf tcp_cong_avoid_ai  ; grep vmlinux /tmp/bla
> > > > > void tcp_cong_avoid_ai(struct tcp_sock * tp, u32 w, u32 acked);
> > > > > openat(AT_FDCWD, "/sys/kernel/btf/vmlinux", O_RDONLY) = 3
> > > > >
> > > > > So this should be unrelated to the breakage you noticed in the CI.
> > > > >
> > > > > I'm trying to to reproduce the CI breakage by building the kernel and
> > > > > running selftests after a reboot.
> > > > >
> > > > > I suspect I'm missing something, can you see what it is?
> > > >
> > > > Oh, I didn't realize initially what it is. This is not kernel-related,
> > > > you are right. You just need newer Clang. Can you please use nightly
> > > > version or build from sources? Basically, your Clang is too old and it
> > > > doesn't generate BTF information for extern functions in BPF code.
> > >
> > > Oh well, I thought that that clang was new enough, the system being
> > > Fedora rawhide:
> > >
> > > [acme@seventh ~]$ clang -v |& head -1
> > > clang version 12.0.0 (https://github.com/llvm/llvm-project 87369c626114ae17f4c637635c119e6de0856a9a)
> > >
> > > I'm now building the single-repo main...
> > >
> > > Would you consider a patch for libbpf that would turn this:
> > >
> > > > > > libbpf: failed to find BTF for extern 'tcp_cong_avoid_ai' [27] section: -2
> > > > > > Error: failed to open BPF object file: No such file or directory
> > > > > > make: *** [Makefile:460: /mnt/linux/tools/testing/selftests/bpf/bpf_cubic.skel.h] Error 255
> > > > > > make: *** Deleting file '/mnt/linux/tools/testing/selftests/bpf/bpf_cubic.skel.h'
> > > > > > make: Leaving directory '/mnt/linux/tools/testing/selftests/bpf'
> > >
> > > Into:
> > >
> > > libbpf: failed to find BTF for extern 'tcp_cong_avoid_ai' [27] section: -2
> > > HINT: Please update your clang/llvm toolchain to at least cset abcdef123456
> > > HINT: That is where clang started generating BTF information for extern functions in BPF code.
> > >
> > > ?
> > >
> > > :-)
> >
> > I'd rather not :)
>
> Not even a "please update clang?"
>

It could be old clang, it could also be because BPF program wasn't
built with BTF (i.e., you didn't specify -g during clang invocation),
it could probably be due to some other problems as well.

I don't want libbpf to turn into a library that's constantly trying to
guess possible problems. It will become a complete mess to maintain.
And when it will still be wrong sometimes, causing more harm than
being helpful. Especially for relatively uncommon problems like this.

Those people who are trying to use features like BPF unstable helpers
(calling whitelisted kernel functions), should know that they need
Clang of some version and build with BTF. We have that also mentioned
in selftest/bpf/README.rst. I'd rather not duplicate all that in
libbpf code as well.

> "-2" and "Error 255" doesn't seem that helpful :-\

But "failed to find BTF for extern 'tcp_cong_avoid_ai'" is pretty
helpful. -2 is for more involved debugging, if necessary.

>
> - Arnaldo

  reply	other threads:[~2021-06-17 21:53 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-15 23:30 Andrii Nakryiko
2021-06-16 14:56 ` Arnaldo Carvalho de Melo
2021-06-16 16:40   ` Arnaldo Carvalho de Melo
2021-06-16 20:41     ` Arnaldo Carvalho de Melo
2021-06-16 22:36       ` Andrii Nakryiko
2021-06-17 14:48         ` Arnaldo Carvalho de Melo
2021-06-17 19:53           ` Arnaldo Carvalho de Melo
2021-06-17 20:03             ` Andrii Nakryiko
2021-06-17 20:41               ` Arnaldo Carvalho de Melo
2021-06-17 21:56                 ` Andrii Nakryiko
2021-06-17 20:00           ` Andrii Nakryiko
2021-06-17 20:39             ` Arnaldo Carvalho de Melo
2021-06-17 21:52               ` Andrii Nakryiko [this message]
2021-06-18 17:21                 ` Arnaldo Carvalho de Melo
2021-06-16 17:40   ` Andrii Nakryiko
2021-06-16 18:25     ` Arnaldo Carvalho de Melo
2021-06-16 22:38       ` Andrii Nakryiko
2021-06-17  0:16         ` Arnaldo Carvalho de Melo
2021-06-17  4:26           ` Andrii Nakryiko

Reply instructions:

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:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAEf4BzZWq21zP+1C4=qqWGQ3WUXK-pvt+rWpcsh_971qAw4Wzw@mail.gmail.com' \
    --to=andrii.nakryiko@gmail.com \
    --cc=arnaldo.melo@gmail.com \
    --cc=bpf@vger.kernel.org \
    --cc=dwarves@vger.kernel.org \
    --cc=siudin@fb.com \
    --cc=yhs@fb.com \
    --subject='Re: latest pahole breaks libbpf CI and let'\''s talk about staging' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

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).