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From: Yonghong Song <yhs-b10kYP2dOMg@public.gmane.org>
To: Arnaldo Carvalho de Melo
	<acme-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Lorenz Bauer <lmb-lDpJ742SOEtZroRs9YW3xA@public.gmane.org>
Cc: bpf <bpf-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	dwarves-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	kernel-team <kernel-team-lDpJ742SOEtZroRs9YW3xA@public.gmane.org>
Subject: Re: pahole generates invalid BTF for code compiled with recent clang
Date: Wed, 24 Jun 2020 10:05:41 -0700
Message-ID: <dc0a8fc6-d0be-45ee-be19-9feb1f0d21e8@fb.com> (raw)
In-Reply-To: <20200624160659.GA20203-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>



On 6/24/20 9:06 AM, Arnaldo Carvalho de Melo wrote:
> Em Wed, Jun 24, 2020 at 12:05:50PM +0100, Lorenz Bauer escreveu:
>> Hi,
>>
>> If pahole -J is used on an ELF that has BTF info from clang, it
>> produces an invalid
>> output. This is because pahole rewrites the .BTF section (which
>> includes a new string
>> table) but it doesn't touch .BTF.ext at all.
>   
>> To demonstrate, on a recent check out of bpf-next:
>>      $ cp connect4_prog.o connect4_pahole.o
>>      $ pahole -J connect4_pahole.o
>>      $ llvm-objcopy-10 --dump-section .BTF=pahole-btf.bin
>> --dump-section .BTF.ext=pahole-btf-ext.bin connect4_pahole.o
>>      $ llvm-objcopy-10 --dump-section .BTF=btf.bin --dump-section
>> .BTF.ext=btf-ext.bin connect4_prog.o
>>      $ sha1sum *.bin
>>      1b5c7407dd9fd13f969931d32f6b864849e66a68  btf.bin
>>      4c43efcc86d3cd908ddc77c15fc4a35af38d842b  btf-ext.bin
>>      2a60767a3a037de66a8d963110601769fa0f198e  pahole-btf.bin
>>      4c43efcc86d3cd908ddc77c15fc4a35af38d842b  pahole-btf-ext.bin
>>
>> This problem crops up when compiling old kernels like 4.19 which have
>> an extra pahole
>> build step with clang-10.
> 
>   
>> I think a possible fix is to strip .BTF.ext if .BTF is rewritten.
> 
> Agreed.
> 
> Longer term pahole needs to generate the .BTF.ext from DWARF, but then,
> if clang is generating it already, why use pahole -J?
> 
> Does clang do deduplication for multi-object binaries?

No. It does not. clang did not do static linking for bpf objects 
currently but we may do it in the future...

> 
> Also its nice to see that the BTF generated ends up with the same
> sha1sum, cool :-)
>   
>> Best
>> Lorenz
>>
>> -- 
>> Lorenz Bauer  |  Systems Engineer
>> 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK
>>
>> https://urldefense.proofpoint.com/v2/url?u=http-3A__www.cloudflare.com&d=DwIBAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=VBET_4Cak79EDDrdr9dfzHXXwBqdhb1fmqug2a7lpPc&s=9Wa1gFDt7uR6HK2w9FgzzLXJ2pMq8Ep0i9IXaaqUnb0&e=
> 

  parent reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-24 11:05 Lorenz Bauer
     [not found] ` <CACAyw9-cinpz=U+8tjV-GMWuth71jrOYLQ05Q7_c34TCeMJxMg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-06-24 16:06   ` Arnaldo Carvalho de Melo
     [not found]     ` <20200624160659.GA20203-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2020-06-24 16:22       ` Lorenz Bauer
     [not found]         ` <CACAyw9-zLLDJ4vXo7jGS_XoYsiiv4c5NmUCjCnAf0eZBXU3dVA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-06-24 17:23           ` Arnaldo Carvalho de Melo
2020-06-24 17:05       ` Yonghong Song [this message]
2020-06-24 17:41   ` Andrii Nakryiko
     [not found]     ` <CAEf4BzbSc-wykq1_62CQwtszO+76rkudz_B=GkzE6ZheMUAusw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-06-24 17:57       ` Arnaldo Carvalho de Melo
     [not found]         ` <20200624175754.GD20203-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2020-06-24 18:40           ` Andrii Nakryiko
     [not found]             ` <CAEf4BzY8b71tE5B4rw5sfy=xajtgqUGHVaoHNf_YzVtQ9aLCBg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-06-24 18:57               ` Arnaldo Carvalho de Melo
     [not found]                 ` <20200624185737.GA25807-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2020-06-24 19:06                   ` Andrii Nakryiko
     [not found]                     ` <CAEf4Bza=ZT1yZvoJNMK72pYm6VGwGp22detc7kgC_24OBt4-FA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-06-24 19:11                       ` Arnaldo Carvalho de Melo
     [not found]                         ` <20200624191159.GB25807-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2020-06-24 19:25                           ` Andrii Nakryiko
2020-06-25  9:25       ` Lorenz Bauer
     [not found]         ` <CACAyw98ojwGjQm+Xk+_-B8Ah-hEt-Tgv_LQ1BdH4yBLYgVwpiA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-06-25 16:56           ` Andrii Nakryiko
     [not found]             ` <CAEf4BzYjge6fijFadwuuHf-vr2VUqneT5b0k-GgQSkLMTj=UAA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2020-06-30 15:52               ` Lorenz Bauer

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