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

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Date:   Thu, 7 May 2020 01:24:29 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Florian Fainelli <f.fainelli@...il.com>
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 Thu, May 7, 2020 at 1:03 AM Florian Fainelli <f.fainelli@...il.com> wrote:
>
>
>
> 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.


I do not understand why you are trying to link *.o_shipped
directly to the module.


Did you notice the following code in scripts/Makefile.lib ?

quiet_cmd_shipped = SHIPPED $@
cmd_shipped = cat $< > $@

$(obj)/%: $(src)/%_shipped
        $(call cmd,shipped)




*_shipped should be copied to *
before the use.



> 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?


In fact, a patch exists.

https://patchwork.kernel.org/patch/11318691/

It allows you to link whatever file name to a module,
but please read the comments from Christoph and Greg.


In this case, your Makefile is doing strange.

I'd recommend to fix your Makefile.



>
> Thank you!
> --
> Florian



-- 
Best Regards
Masahiro Yamada