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From: Arnaldo Carvalho de Melo <acme-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
To: Andrii Nakryiko
	<andrii.nakryiko-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Arnaldo Carvalho de Melo
	<arnaldo.melo-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Lorenz Bauer <lmb-lDpJ742SOEtZroRs9YW3xA@public.gmane.org>,
	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 16:11:59 -0300
Message-ID: <20200624191159.GB25807@kernel.org> (raw)
In-Reply-To: <CAEf4Bza=ZT1yZvoJNMK72pYm6VGwGp22detc7kgC_24OBt4-FA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

Em Wed, Jun 24, 2020 at 12:06:24PM -0700, Andrii Nakryiko escreveu:
> On Wed, Jun 24, 2020 at 11:57 AM Arnaldo Carvalho de Melo
> <arnaldo.melo-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >
> > Em Wed, Jun 24, 2020 at 11:40:21AM -0700, Andrii Nakryiko escreveu:
> > > On Wed, Jun 24, 2020 at 10:57 AM Arnaldo Carvalho de Melo
> > > <arnaldo.melo-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > > >
> > > > Em Wed, Jun 24, 2020 at 10:41:10AM -0700, Andrii Nakryiko escreveu:
> > > > > 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.
> > > >
> > > > yeah, I was thinking this was for a vmlinux generated by clang, which,
> > > > from the commands below (the suffix _prog.o) should have told me this is
> > > > a target BPF object file.
> > > >
> > > > But then, if one insists for some reason in generating BTF from the
> > > > DWARF in a BPF target object file, stripping .BTF.ext, if present, is
> > > > the right thing to do at this point.
> > >
> > > I disagree. Those who insist probably have some wrong conceptual
> > > understanding and it's better to fix that (understanding), rather than
> > > lose focus and bend tool to do what it's not supposed to do and
> > > ultimately cause more confusion.
> >
> > So we can instead notice the presence of .BTF.ext when the user calls
> > 'pahole -J' on a target BPF object file and bail out, only allowing it
> > to convert from DWARF to BTF and thus encode the .BTF elf section when
> > .BTF.ext isn't present, as we can't easily figure out if the present of
> > just .BTF section was done by clang or pahole on a BTF target object
> > file built without -g.
> 
> Can't we check ELF's target machine and reject if it's a BPF one?

I think there is value in allowing pahole to convert DWARF to BTF even
for a BPF target object file, say in some case people may think clang is
not generating correct BTF so one may want to see what pahole generates
and compare.

> Someday we might also support "cross-compilation" to be able to dedup
> arm ELF from x86 machine. It's sort of ok today for little-endian
> ARMs, so maybe not outright reject if architecture is not the same as
> the local one?

I think outright reject if arch is not t he same it not necessary.

We may warn the user that using -g in clang is the preferred method for
generating BTF, wdyt?

- Arnaldo
 
> >
> > - Arnaldo
> >
> > > pahole's BTF conversion is really driven towards kernel use-case
> > > (e.g., with global variables, etc). I wouldn't distract ourselves with
> > > supporting de-duplicating BPF object files. Single .o's BTF is already
> > > deduplicated as produced by Clang. Once we add static linking of
> > > multiple BPF .o's (which I hope to start working on very soon), that
> > > de-duplication will be handled automatically by libbpf (and hopefully
> > > integrated into lld as well), among many other things that need to
> > > happen to make static linking work.
> > >
> > > >
> > > > - Arnaldo
> > > >
> > > > > >
> > > > > > 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
> > > >
> > > > --
> > > >
> > > > - Arnaldo
> >
> > --
> >
> > - Arnaldo

-- 

- 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
     [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 [this message]
     [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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