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

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Date:   Thu, 20 Aug 2020 08:51:19 -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 3:18 AM, Mark Wielaard wrote:
> Hi,
> 
> On Wed, 2020-08-19 at 20:23 -0700, Yonghong Song wrote:
>> On 8/19/20 7:27 PM, Fāng-ruì Sòng wrote:
>>>>>>
>>> I think this is resolve_btfids's bug. GNU ld and LLD are innocent.
>>> These .debug_* sections work fine if their sh_addralign is 1.
>>> When the section flag SHF_COMPRESSED is set, the meaningful
>>> alignment
>>> is Elf64_Chdr::ch_addralign, after the header is uncompressed.
>>>
>>> On Wed, Aug 19, 2020 at 2:30 PM Yonghong Song <yhs@...com> wrote:
>> Since Fangrui mentioned this is not a ld/lld bug, then changing
>> alighment from 1 to 4 might have some adverse effect for the binary,
>> I guess.
> 
> The bug isn't about a wrong ch_addralign, which seems to have been set
> correctly. But it is a bug about incorrectly setting the sh_addralign
> of the section. The sh_addralign indicates the alignment of the data in
> the section, which is the Elf32/64_Chdr plus compressed data, not the
> alignment of the uncompressed data. It helps the consumer make sure
> they lay out the data so that the ELF data structures can be read
> through their natural alignment.
> 
> In practice it often isn't a real issue, because consumers, including
> libelf, will correct the data alignment before usage anyway. But that
> doesn't mean it isn't a bug to set it wrongly.
> 
>> 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?

> 
> Cheers,
> 
> Mark
>