linux-kernel - Re: [PATCH] kbuild: Remove debug info from kallsyms linking

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Message-ID: <202003031758.AE8FEB7@keescook>
Date:   Tue, 3 Mar 2020 18:11:16 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kbuild: Remove debug info from kallsyms linking

On Tue, Mar 03, 2020 at 01:50:52PM -0800, Andrii Nakryiko wrote:
> On Tue, Mar 3, 2020 at 1:06 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Mon, Mar 02, 2020 at 10:55:04PM -0800, Andrii Nakryiko wrote:
> > > On Mon, Feb 24, 2020 at 9:17 PM Kees Cook <keescook@...omium.org> wrote:
> > > > [...]
> > > > @@ -118,8 +128,9 @@ gen_btf()
> > > >                 return 1
> > > >         fi
> > > >
> > > > -       info "BTF" ${2}
> > > >         vmlinux_link ${1}
> > > > +
> > > > +       info "BTF" ${2}
> > >
> > > Any reason to exclude linking from "BTF" step? It's still a part of
> > > BTF generation, so seems fair to have BTF encompass both vmlinux
> > > linking and BTF generation/deduplication?
> >
> > I'm not sure I'm following what you're saying here. If you're asking why
> > BTF linking is separate from the final vmlinux link, it's because of how
> > kallsyms is generated. Currently it's using a rather brute-force
> 
> No, I meant that you moved `info "BTF"` to after `vmlinux_link` call,
> which will make it appear (from make output) as if BTF generation
> phase is shorter than it is. No big deal, was just wondering if it was
> done on purpose.

Oh! Yes. I changed the reporting in commit 8959e39272d6 ("kbuild:
Parameterize kallsyms generation and correct reporting") so that
vmlinux_link reports the "info LD ..." line instead of each of the callers.

This current patch adjusts it so "info BTF ..." is reported for the BTF
generation stage (right now there's no delay between "info BTF ..." and
"info LD ...", and it looks like the "info LD" stages takes way too
long. ;)

> > approach to figure out exactly where everything is going to be in the
> > final link, and for that it need to have both the BTF symbols present
> > and the kallysms symbols present. So, unfortunately, each needs to be a
> > separate step. I spent some time trying to merge BTF and kallsyms phase
> > 1, but I didn't find a viable solution. I'm *sure* there is a better way
> > to handle kallsyms, but I haven't had the time to really investigate it.
> > I think it would require some close coordination with linker behavior
> > changes...
> >
> > >
> > > >         LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
> > > >
> > > >         # dump .BTF section into raw binary file to link with final vmlinux
> >
> > BTW, in looking at BTF generation, why is this cut up into three steps:
> > pahole, objcopy, objcopy... shouldn't pahole just gross an output method
> > to dump the final .o file? That would be MUCH nicer. Especially since
> > the first step ends up rewriting (?!) the original ELF. This is a lot of
> > needless IO...
> 
> Just mostly historical reasons, that was the interface pahole already
> supported. I agree that it's a good idea to teach pahole to just emit
> a binary BTF section dump.

/me adds it to giant TODO list ;)

-- 
Kees Cook