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From: Andrii Nakryiko <andrii.nakryiko-Re5JQEeQqe8AvxtiuMwx3w@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 10:41:10 -0700
Message-ID: <CAEf4BzbSc-wykq1_62CQwtszO+76rkudz_B=GkzE6ZheMUAusw@mail.gmail.com> (raw)
In-Reply-To: <CACAyw9-cinpz=U+8tjV-GMWuth71jrOYLQ05Q7_c34TCeMJxMg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

On Wed, Jun 24, 2020 at 4:07 AM Lorenz Bauer <lmb-lDpJ742SOEtZroRs9YW3xA@public.gmane.org> wrote:
>
> 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.

Why do you run `pahole -J` on BPF .o file? Clang already generates
.BTF (and .BTF.ext, of course) for you.

pahole -J is supposed to be used for vmlinux, not for clang-compiled
-target BPF object files.

>
> 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 was under impression that clang generates .BTF and .BTF.ext only for
-target BPF. In this case, kernel is compiled for "real" target arch,
so there shouldn't be .BTF.ext in the first place? If that's not the
case, then I guess it's a bug in Clang.

>
> I think a possible fix is to strip .BTF.ext if .BTF is rewritten.
>
> Best
> Lorenz
>
> --
> Lorenz Bauer  |  Systems Engineer
> 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK
>
> www.cloudflare.com

  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
2020-06-24 17:41   ` Andrii Nakryiko [this message]
     [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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