Dwarves Archive on lore.kernel.org
 help / color / Atom feed
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 13:06:59 -0300
Message-ID: <20200624160659.GA20203@kernel.org> (raw)
In-Reply-To: <CACAyw9-cinpz=U+8tjV-GMWuth71jrOYLQ05Q7_c34TCeMJxMg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

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?

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
> 
> www.cloudflare.com

-- 

- Arnaldo

  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 [this message]
     [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
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200624160659.GA20203@kernel.org \
    --to=acme-dgejt+ai2ygdnm+yrofe0a@public.gmane.org \
    --cc=bpf-u79uwXL29TY76Z2rM5mHXA@public.gmane.org \
    --cc=dwarves-u79uwXL29TY76Z2rM5mHXA@public.gmane.org \
    --cc=kernel-team-lDpJ742SOEtZroRs9YW3xA@public.gmane.org \
    --cc=lmb-lDpJ742SOEtZroRs9YW3xA@public.gmane.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Dwarves Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/dwarves/0 dwarves/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 dwarves dwarves/ https://lore.kernel.org/dwarves \
		dwarves@vger.kernel.org
	public-inbox-index dwarves

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.dwarves


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git