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

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Message-ID: <01279bfe-95a3-dbdb-3785-bd2d92dc4f95@gmail.com>
Date:   Wed, 6 May 2020 09:03:46 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>
Subject: Re: Proper use for linking foo.o_shipped after 69ea912fda74 ("kbuild:
 remove unneeded link_multi_deps")?



On 5/6/2020 7:37 AM, Masahiro Yamada wrote:
> On Wed, May 6, 2020 at 1:45 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>>
>> 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.
> 
> 
> 
> Why don't you do like this?
> 
> obj-m   := hello.o
> hello-y := test.o

I tried it in the original environment where it failed, not my contrived
test case, and this did not work, as we really need test.o and
test.o_shipped to be separate objects, doing what you suggest results in
a circular dependency.

To me this is a regression, as it used to work and now it does not, thus
we should be fixing it, any idea about how we go about it without doing
a plain revert?

Thank you!
-- 
Florian