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

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Date:   Wed, 3 Jun 2020 10:45:11 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Masahiro Yamada <masahiroy@...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, 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