linux-kernel - Proper use for linking foo.o_shipped after 69ea912fda74 ("kbuild: remove unneeded link_multi

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Message-ID: <cd20b888-7f3d-e7ff-8096-2cc8305a5107@gmail.com>
Date:   Tue, 5 May 2020 21:45:08 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     masahiroy@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-kbuild@...r.kernel.org,
        Michal Marek <michal.lkml@...kovi.net>
Subject: Proper use for linking foo.o_shipped after 69ea912fda74 ("kbuild:
 remove unneeded link_multi_deps")?

Hi Masahiro, Michal,

While updating our systems from 4.9 to 5.4, we noticed that one of the
kernel modules that we build, which is done by linking an object that we
pre-compile out of Kbuild stopped working.

I bisected it down to:

commit 69ea912fda74a673d330d23595385e5b73e3a2b9 (refs/bisect/bad)
Author: Masahiro Yamada <yamada.masahiro@...ionext.com>
Date:   Thu Oct 4 13:25:19 2018 +0900

    kbuild: remove unneeded link_multi_deps

    Since commit c8589d1e9e01 ("kbuild: handle multi-objs dependency
    appropriately"), $^ really represents all the prerequisite of the
    composite object being built.

    Hence, $(filter %.o,$^) contains all the objects to link together,
    which is much simpler than link_multi_deps calculation.

    Please note $(filter-out FORCE,$^) does not work here. When a single
    object module is turned into a multi object module, $^ will contain
    header files that were previously included for building the single
    object, and recorded in the .*.cmd file. To filter out such headers,
    $(filter %.o,$^) should be used here.

    Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>

and the linker now fails with the following:

mkdir -p /home/florian/dev/lkm/.tmp_versions ; rm -f
/home/florian/dev/lkm/.tmp_versions/*

  WARNING: Symbol version dump ./Module.symvers
           is missing; modules will have no dependencies and modversions.

make -f ./scripts/Makefile.build obj=/home/florian/dev/lkm
(cat /dev/null;   echo kernel//home/florian/dev/lkm/hello.ko;) >
/home/florian/dev/lkm/modules.order
  ld -m elf_x86_64  -z max-page-size=0x200000    -r -o
/home/florian/dev/lkm/hello.o
ld: no input files
make[1]: *** [scripts/Makefile.build:492: /home/florian/dev/lkm/hello.o]
Error 1
make: *** [Makefile:1530: _module_/home/florian/dev/lkm] Error 2

and here are some steps to reproduce this:

Kbuild:
obj-m   := hello.o
hello-y := test.o_shipped

test.c can be a simple hello world, and you can compile it using a
standard Kbuild file first, and then move test.o as test.o_shipped.

I am afraid I do not speak Kbuild fluently enough to recommend a fix for
that.

Thanks!
-- 
Florian