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

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Message-ID: <CAEf4Bza31J_3Puwf7gnq0jDYRH2_JkRM9L+PO8dFrCzp+==8Lg@mail.gmail.com>
Date:   Tue, 3 Mar 2020 20:29:39 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Kees Cook <keescook@...omium.org>
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 3, 2020 at 6:11 PM Kees Cook <keescook@...omium.org> wrote:
>
> 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. ;)

Ah, I see, got it!

>
> > > 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