linux-kernel - Re: [PATCH] kbuild: add extra-y to targets-for-modules

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Message-ID: <CAK7LNATmfzt0kTF0BWkSRaZV3p01mvN4OSGPZsCp+KHghCWf0Q@mail.gmail.com>
Date:   Thu, 17 Dec 2020 17:26:07 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Artem Savkov <artem.savkov@...il.com>,
        WANG Chao <chao.wang@...oud.cn>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH] kbuild: add extra-y to targets-for-modules

On Thu, Dec 17, 2020 at 8:04 AM Joe Lawrence <joe.lawrence@...hat.com> wrote:
>
> On 12/16/20 1:14 AM, Masahiro Yamada wrote:
> > On Tue, Dec 8, 2020 at 11:31 PM Artem Savkov <artem.savkov@...il.com> wrote:
> >>
> >> On Tue, Dec 08, 2020 at 05:20:35PM +0800, WANG Chao wrote:
> >>> Sorry for the late reply.
> >>>
> >>> On 11/25/20 at 10:42P, Masahiro Yamada wrote:
> >>>> On Tue, Nov 24, 2020 at 12:05 AM WANG Chao <chao.wang@...oud.cn> wrote:
> >>>>>
> >>>>> On 11/23/20 at 02:23P, Masahiro Yamada wrote:
> >>>>>> On Tue, Nov 3, 2020 at 3:23 PM WANG Chao <chao.wang@...oud.cn> wrote:
> >>>>>>>
> >>>>>>> extra-y target doesn't build for 'make M=...' since commit 6212804f2d78
> >>>>>>> ("kbuild: do not create built-in objects for external module builds").
> >>>>>>>
> >>>>>>> This especially breaks kpatch, which is using 'extra-y := kpatch.lds'
> >>>>>>> and 'make M=...' to build livepatch patch module.
> >>>>>>>
> >>>>>>> Add extra-y to targets-for-modules so that such kind of build works
> >>>>>>> properly.
> >>>>>>>
> >>>>>>> Signed-off-by: WANG Chao <chao.wang@...oud.cn>
> >>>>>>> ---
> >>>>>>>   scripts/Makefile.build | 2 +-
> >>>>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>>>>
> >>>>>>> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> >>>>>>> index ae647379b579..0113a042d643 100644
> >>>>>>> --- a/scripts/Makefile.build
> >>>>>>> +++ b/scripts/Makefile.build
> >>>>>>> @@ -86,7 +86,7 @@ ifdef need-builtin
> >>>>>>>   targets-for-builtin += $(obj)/built-in.a
> >>>>>>>   endif
> >>>>>>>
> >>>>>>> -targets-for-modules := $(patsubst %.o, %.mod, $(filter %.o, $(obj-m)))
> >>>>>>> +targets-for-modules := $(extra-y) $(patsubst %.o, %.mod, $(filter %.o, $(obj-m)))
> >>>>>>>
> >>>>>>>   ifdef need-modorder
> >>>>>>>   targets-for-modules += $(obj)/modules.order
> >>>>>>> --
> >>>>>>> 2.29.1
> >>>>>>>
> >>>>>>
> >>>>>> NACK.
> >>>>>>
> >>>>>> Please fix your Makefile.
> >>>>>>
> >>>>>> Hint:
> >>>>>> https://patchwork.kernel.org/project/linux-kbuild/patch/20201123045403.63402-6-masahiroy@kernel.org/
> >>>>>>
> >>>>>>
> >>>>>> Probably what you should use is 'targets'.
> >>>>>
> >>>>> I tried with 'targets' and 'always-y'. Both doesn't work for me.
> >>>>>
> >>>>> I narraw it down to the following example:
> >>>>>
> >>>>> cat > Makefile << _EOF_
> >>>>> obj-m += foo.o
> >>>>>
> >>>>> ldflags-y += -T $(src)/kpatch.lds
> >>>>> always-y += kpatch.lds
> >>>>>
> >>>>> foo-objs += bar.o
> >>>>>
> >>>>> all:
> >>>>>          make -C /lib/modules/$(shell uname -r)/build M=$(PWD)
> >>>>> clean:
> >>>>>          make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
> >>>>> _EOF_
> >>>>>
> >>>>> Take a look into scripts/Makefile.build:488:
> >>>>>
> >>>>> __build: $(if $(KBUILD_BUILTIN), $(targets-for-builtin)) \
> >>>>>           $(if $(KBUILD_MODULES), $(targets-for-modules)) \
> >>>>>           $(subdir-ym) $(always-y)
> >>>>>          @:
> >>>>>
> >>>>> 'always-y' is built after 'targets-for-modules'. This makes
> >>>>> 'targets-for-modules' fails because kpatch.lds isn't there.
> >>>>
> >>>>
> >>>> Heh, you rely on the targets built from left to right,
> >>>> and you have never thought Make supports the parallel option -j.
> >>>
> >>> You're right. I missed that.
> >>>
> >>>>
> >>>>
> >>>> You need to specify the dependency if you expect objects
> >>>> are built in the particular order.
> >>>>
> >>>> However, in this case, using ldflags-y looks wrong
> >>>> in the first place.
> >>>>
> >>>> The linker script is used when combining the object
> >>>> as well as the final link of *.ko
> >>
> >> We want linker script to be used on both those steps, otherwise modpost
> >> fails.
> >
> >
> > In that case, does the following work?
> > (untested)
> >
> >
> >
> > diff --git a/kmod/patch/Makefile b/kmod/patch/Makefile
> > index e017b17..02d4c66 100644
> > --- a/kmod/patch/Makefile
> > +++ b/kmod/patch/Makefile
> > @@ -12,7 +12,9 @@ endif
> >
> >   obj-m += $(KPATCH_NAME).o
> >   ldflags-y += -T $(src)/kpatch.lds
> > -extra-y := kpatch.lds
> > +targets += kpatch.lds
> > +
> > +$(obj)/$(KPATCH_NAME).o: $(obj)/kpatch.lds
> >
> >   $(KPATCH_NAME)-objs += patch-hook.o output.o
> >
>
> Hi Masahiro,
>
> Yeah this is more or less what Artem came up with:
> https://github.com/dynup/kpatch/pull/1149
>
> though we hadn't added kpatch.lds to targets.  Is there documentation
> somewhere on what effect "targets" has for out-of-tree builds?
>
> Thanks,
>
> -- Joe
>


Please try the rebuild without changing any source code.

If kpatch.lds is needlessly rebuilt, you need to add
it to 'targets'.


In linux-next, this is documented.

Documentation/kbuild/makefile.rst:

        Any target that utilizes if_changed must be listed in $(targets),
        otherwise the command line check will fail, and the target will
        always be built.







-- 
Best Regards
Masahiro Yamada