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

[Andrii Nakryiko]

On Tue, Oct 20, 2020 at 3:51 AM Jiri Slaby <jirislaby@xxxxxxxxxx> wrote:
>
> 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;;

What's readelf's output for that symbol? If it's legal for SST_OBJECT
to have size zero, then we should just skip those in pahole. But it
shouldn't crash in either case, of course. But as Arnaldo mentioned,
that code changed significantly recently, so please check with latest
pahole from tmp.libbtf_encoder branch.

>
> > 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
>
> The backtrace:

[...]

>
>
> I suspect:
> commit f3d9054ba8ff1df0fc44e507e3a01c0964cabd42
> Author:     Hao Luo <haoluo@xxxxxxxxxx>
> 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