netdev - Re: [PATCH bpf-next] tools/resolve_btfids: Fix sections with wrong alignment

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Message-ID: <a6f1d7be73ca5d9f767a746927e7872ddcf18244.camel@klomp.org>
Date:   Thu, 20 Aug 2020 19:36:37 +0200
From:   Mark Wielaard <mark@...mp.org>
To:     Yonghong Song <yhs@...com>,
        Fāng-ruì Sòng <maskray@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Nick Clifton <nickc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH bpf-next] tools/resolve_btfids: Fix sections with wrong
 alignment

Hi

On Thu, 2020-08-20 at 08:51 -0700, Yonghong Song wrote:
> > > Do you think we could skip these .debug_* sections somehow in elf
> > > parsing in resolve_btfids? resolve_btfids does not need to read
> > > these sections. This way, no need to change their alignment
> > > either.
> > 
> > The issue is that elfutils libelf will not allow writing out the
> > section when it notices the sh_addralign field is setup wrongly.
> 
> Maybe resolve_btfids can temporarily change sh_addralign to 4/8
> before elf manipulation (elf_write) to make libelf happy.
> After all elf_write is done, change back to whatever the
> original value (1). Does this work?

Unfortunately no, because there is no elf_write, elf_update is how you
write out the ELF image to disc.

Since the code is using ELF_F_LAYOUT this will not change the actual
layout of the ELF image if that is what you are worried about.

And the workaround to set sh_addralign correctly before calling
elf_update is precisely what the fix in elfutils libelf will do itself
in the next release. Also binutils ld has been fixed to setup
sh_addralign to 4/8 as appropriate now (in git).

Cheers,

Mark