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From: Arnaldo Carvalho de Melo <acme-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
To: 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 14:23:50 -0300	[thread overview]
Message-ID: <20200624172350.GB20203@kernel.org> (raw)
In-Reply-To: <CACAyw9-zLLDJ4vXo7jGS_XoYsiiv4c5NmUCjCnAf0eZBXU3dVA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

Em Wed, Jun 24, 2020 at 05:22:59PM +0100, Lorenz Bauer escreveu:
> On Wed, 24 Jun 2020 at 17:07, Arnaldo Carvalho de Melo <acme-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> 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?
 
> Beats me, but then sometimes you don't have control over the workflow, see
> my v4.19 kernel example.
 
> > Does clang do deduplication for multi-object binaries?

> > Also its nice to see that the BTF generated ends up with the same
> > sha1sum, cool :-)
 
> Unfortunately it's the .BTF.ext section that has the same sha1, because
> pahole doesn't touch it ;(

My bad... I guess I saw what I wanted to see... ;-\

- Arnaldo

  parent reply	other threads:[~2020-06-24 17:23 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-24 11:05 pahole generates invalid BTF for code compiled with recent clang 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 [this message]
2020-06-24 17:05       ` Yonghong Song
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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