linux-kernel - Re: [PATCH] efi/libstub: refactor Makefile to not use lib-y syntax

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Message-ID: <CAK7LNARLUeuiu3Y1cFqT7550MaF8fnBmMTZxKEG0Cy3vpxVkMQ@mail.gmail.com>
Date:   Wed, 3 Jun 2020 17:59:00 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Atish Patra <atish.patra@....com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] efi/libstub: refactor Makefile to not use lib-y syntax

On Wed, Jun 3, 2020 at 5:45 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Wed, 3 Jun 2020 at 10:36, Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > On Wed, Jun 3, 2020 at 3:45 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> > >
> > > On Wed, 3 Jun 2020 at 07:34, Masahiro Yamada <masahiroy@...nel.org> wrote:
> > > >
> > > > Documentation/kbuild/makefiles.rst says:
> > > >
> > > >   Use of lib-y is normally restricted to `lib/` and `arch/*/lib`.
> > > >
> > > > I want to disallow lib-y outside of them.
> > > >
> > >
> > > Why?
> >
> >
> > Because I plan to remove lib-y entirely at some point.
> >
> > lib-y is not so useful to shrink the image size because:
> >
> >   - An object in lib.a can be omitted only when no symbol
> >     in that object is referenced.  This rarely happens.
> >
> >   -  lib-y objects are often exported by nature
> >      because lib-y is a collection of utility functions.
> >      Even if no in-tree user, we always need to keep them
> >      because EXPORT_SYMBOL() is the interface to modules.
> >
> >
> > When I worked on commit 7273ad2b08f8ac9563579d16a3cf528857b26f49,
> > I made some research.
> >
> > The benefit of lib-y is just 362 byte for x86_64_defconfig.
> > ( Before: 26578002, After: 26578364)
> >
> > My hope is lib-y will be replaced by dead-code elimination or
> > ultimately by LTO.
> >
> > drivers/firmware/efi/libstub/Makefile
> > is the only Makefile that breaks the rule:
> > "Use of lib-y is normally restricted to `lib/` and `arch/*/lib`"
> >
> >
> >
> >
> > >
> > > > Add a custom rule to build lib.a, which is linked to the decompressor
> > > > for ARCH=x86, ARCH=arm.
> > > >
> > > > For ARCH=arm64, use obj-y to link objects to vmlinux in the ordinary
> > > > way.
> > > >
> > >
> > > The code works perfectly fine as is, and I don't see what is
> > > fundamentally wrong with using static libraries outside of lib/ and
> > > arch/*/lib.
> >
> > The intended usage of lib-y is to hook lib.a
> > to scripts/vmlinux.sh via KBUILD_VMLINUX_LIBS.
> >
> > This Makefile is just what you found to work.
> >
> >
> > >
> > > Also, I would like this code to still be incorporated as a static
> > > library into arm64 as well, so that only pieces that are actually
> > > needed are incorporated into the final image.
> >
> > No.
> > It is not working like that because you set
> > lib.a to core-y.
> >
> > All objects in core-y are always linked to vmlinux.
> >
>
> The lib.a file is passed to the linker as a static library, so it will
> only grab what it needs.
>
> For instance, if you build arm64 from mainline today, the
> efi_relocate_kernel will not be in the final image, even though it is
> built as part of libstub


I built today's mainline kernel
(d6f9469a03d832dcd17041ed67774ffb5f3e73b3).


I see it in vmlinux.


$ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-  defconfig
$ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-  -j24
    ...
$ aarch64-linux-gnu-nm  -n  vmlinux | grep  efi_relocate_kernel
ffff8000114afb90 t __efistub_efi_relocate_kernel



-- 
Best Regards
Masahiro Yamada