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

[Andrii Nakryiko]

On Tue, Mar 3, 2020 at 1:06 PM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> wrote:
> > >
> > > When CONFIG_DEBUG_INFO is enabled, the two kallsyms linking steps spend
> > > time collecting and writing the dwarf sections to the temporary output
> > > files. kallsyms does not need this information, and leaving it off
> > > halves their linking time. This is especially noticeable without
> > > CONFIG_DEBUG_INFO_REDUCED. The BTF linking stage, however, does still
> > > need those details.
> > >
> > > Refactor the BTF and kallsyms generation stages slightly for more
> > > regularized temporary names. Skip debug during kallsyms links.
> > >
> > > For a full debug info build with BTF, my link time goes from 1m06s to
> > > 0m54s, saving about 12 seconds, or 18%.
> > >
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> >
> > I've tested locally, seems to be generating BTF properly (I haven't
> > timed anything, though). See nit below, but otherwise:
> >
> > Acked-by: Andrii Nakryiko <andriin@xxxxxx>
>
> Thanks!
>
> >
> > >  scripts/link-vmlinux.sh | 28 +++++++++++++++++++---------
> > >  1 file changed, 19 insertions(+), 9 deletions(-)
> > >
> >
> > [...]
> >
> > > @@ -106,6 +114,8 @@ gen_btf()
> > >  {
> > >         local pahole_ver
> > >         local bin_arch
> > > +       local bin_format
> > > +       local bin_file
> > >
> > >         if ! [ -x "$(command -v ${PAHOLE})" ]; then
> > >                 echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
> > > @@ -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.

> 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.

>
> --
> Kees Cook