linux-kernel - Re: Segfault in pahole 1.18 when building kernel 5.9.1 for arm64

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Date:   Tue, 20 Oct 2020 10:18:45 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Érico Rolim <erico.erc@...il.com>,
        dwarves@...r.kernel.org, open list <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Andrii Nakryiko <andriin@...com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>
Subject: Re: Segfault in pahole 1.18 when building kernel 5.9.1 for arm64

On Tue, Oct 20, 2020 at 10:10 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Tue, Oct 20, 2020 at 10:05 AM Hao Luo <haoluo@...gle.com> wrote:
> >
> > Thanks for reporting this and cc'ing me. I forgot to update the error
> > messages when renaming the flags. I will send a patch to fix the error
> > message.
> >
> > The commit
> >
> > commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> > Author:     Hao Luo <haoluo@...gle.com>
> > AuthorDate: Wed Jul 8 13:44:10 2020 -0700
> >
> >      btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.
> >
> > encodes kernel global variables into BTF so that bpf programs can
> > directly access them. If there is no need to access kernel global
> > variables, it's perfectly fine to use '--btf_encode_force' to skip
> > encoding bad symbols into BTF, or '--skip_encoding_btf_vars' to skip
> > encoding all global vars all together. I will add these info into the
> > updated error message.
> >
> > Also cc bpf folks for attention of this bug.
>
> I've already fixed the message as part of
> 2e719cca6672 ("btf_encoder: revamp how per-CPU variables are encoded")
>

Ah, that's awesome! Thanks for fixing this, Andrii. I haven't got time
to take a look at your patches last week, I will try to look at it
ASAP.

> It's currently still in the tmp.libbtf_encoder branch in pahole repo.
>
> >
> > Hao
> >
> > On Tue, Oct 20, 2020 at 5:20 AM Arnaldo Carvalho de Melo
> > <acme@...nel.org> wrote:
> > >
> > > Em Tue, Oct 20, 2020 at 11:01:39AM +0200, Jiri Slaby escreveu:
> > > > Hi,
> > > >
> > > > On 19. 10. 20, 1:18, Érico Rolim wrote:
> > > > > I'm trying to build kernel 5.9.1 for arm64, and my dotconfig has
> > > > > `CONFIG_DEBUG_INFO_BTF=y`, which requires pahole for building. However, pahole
> > > > > version 1.18 segfaults during the build, as can be seen below:
> > > > >
> > > > > PAHOLE: Error: Found symbol of zero size when encoding btf (sym:
> > > > > '__kvm_nvhe_arm64_ssbd_callback_required', cu:
> > > > > 'arch/arm64/kernel/cpu_errata.c').
> > > >
> > > > The symbol is an alias coming from arch/arm64/kernel/vmlinux.lds:
> > > > __kvm_nvhe_arm64_ssbd_callback_required = arm64_ssbd_callback_required;;
> > > >
> > > > > PAHOLE: Error: Use '-j' or '--force' to ignore such symbols and force
> > > > > emit the btf.
> > > > > scripts/link-vmlinux.sh: line 141: 43837 Segmentation fault
> > > > > LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > > > >    LD      .tmp_vmlinux.kallsyms1
> > > > >    KSYM    .tmp_vmlinux.kallsyms1.o
> > > > >    LD      .tmp_vmlinux.kallsyms2
> > > > >    KSYM    .tmp_vmlinux.kallsyms2.o
> > > > >    LD      vmlinux
> > > > >    BTFIDS  vmlinux
> > > > > FAILED: load BTF from vmlinux: Unknown error -2make: ***
> > > > > [Makefile:1162: vmlinux] Error 255
> > > > >
> > > > > It is possible to force the build to continue if
> > > > >
> > > > >    LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > > > >
> > > > > in scripts/link-vmlinux.sh is changed to
> > > > >
> > > > >    LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J --btf_encode_force ${1}
> > > > >
> > > > > The suggested `-j` or `--force` flags don't exist, since they were removed in
> > > > > [1]. I believe `--btf_encode_force` should be suggested instead.
> > > >
> > > > Agreed, '--btf_encode_force' makes pahole to proceed without crashes.
> > > >
> > > > > It should be noted that the same build, but with pahole version 1.17, works
> > > > > without issue, so I think this is either a regression in pahole or the script
> > > > > will need to be changed for newer versions of pahole.
> > > >
> > > > Yeah, I observe the very same. I reported it at:
> > > > https://bugzilla.suse.com/show_bug.cgi?id=1177921
> > >
> > > Would it be possible to try with
> > > https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?h=tmp.libbtf_encoder
> > > ?
> > >
> > > This switches to using libbpf for the BTF encoder and may have fixed
> > > this problem.
> > >
> > > - Arnaldo
> > >
>
> [...]
>
> > > >
> > > >
> > > > I suspect:
> > > > commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> > > > Author:     Hao Luo <haoluo@...gle.com>
> > > > AuthorDate: Wed Jul 8 13:44:10 2020 -0700
> > > >
> > > >     btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.
> > > >
> > > >
> > > > Which added this machinery (btf_elf__add_datasec_type in particular).
> > > >
> > > > > - [1] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/pahole.c?h=v1.18&id=1abc001417b579b86a9b27ff88c9095d8f498a46
> > > > >
> > > > > Thanks,
> > > > > Érico
> > > > >
> > > >
> > > >
> > > > --
> > > > js
> > > > suse labs
> > >
> > > --
> > >
> > > - Arnaldo