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

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Message-ID: <35b05eda-f76a-a071-d69e-9ba8c6f48382@fb.com>
Date:   Thu, 20 Aug 2020 10:54:29 -0700
From:   Yonghong Song <yhs@...com>
To:     Mark Wielaard <mark@...mp.org>,
        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



On 8/20/20 10:36 AM, Mark Wielaard wrote:
> 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).

Sounds good then.
Thanks for fixing the issue in upstream, both libelf and binutils!

> 
> Cheers,
> 
> Mark
>