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From: Yonghong Song <yhs@fb.com>
To: "Mark Wielaard" <mark@klomp.org>,
	"Fāng-ruì Sòng" <maskray@google.com>,
	"Jiri Olsa" <jolsa@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Nick Clifton <nickc@redhat.com>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Network Development <netdev@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>, Martin KaFai Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>,
	Andrii Nakryiko <andriin@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH bpf-next] tools/resolve_btfids: Fix sections with wrong alignment
Date: Thu, 20 Aug 2020 08:51:19 -0700	[thread overview]
Message-ID: <7029ff8f-77d3-584b-2e7e-388c001cd648@fb.com> (raw)
In-Reply-To: <5ef90a283aa2f68018763258999fa66cd34cb3bb.camel@klomp.org>



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@fb.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
> 

  reply	other threads:[~2020-08-20 15:51 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-19  9:23 Jiri Olsa
2020-08-19 15:31 ` Yonghong Song
2020-08-19 17:36   ` Jiri Olsa
2020-08-19 18:16     ` Nick Desaulniers
2020-08-19 21:30       ` Yonghong Song
2020-08-20  2:27         ` Fāng-ruì Sòng
2020-08-20  3:23           ` Yonghong Song
2020-08-20 10:18             ` Jiri Olsa
2020-08-20 10:18             ` Mark Wielaard
2020-08-20 15:51               ` Yonghong Song [this message]
2020-08-20 17:36                 ` Mark Wielaard
2020-08-20 17:54                   ` Yonghong Song
2020-08-20 21:24                     ` Alexei Starovoitov
2020-08-19 17:02 ` Jesper Dangaard Brouer
2020-08-19 21:32 ` Yonghong Song

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