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

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Date:   Tue, 8 Dec 2020 15:31:17 +0100
From:   Artem Savkov <artem.savkov@...il.com>
To:     WANG Chao <chao.wang@...oud.cn>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        joe.lawrence@...hat.com
Subject: Re: [PATCH] kbuild: add extra-y to targets-for-modules

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.

It looks like the right thing to do here is leave ldflags-y in, get rid
of always-y/extra-y altogether and specify our linker script as a
dependency for the object.

> I don't have a clean fix to kpatch right now.
> 
> I'm looping kpatch forks in. They're also looking at this right now:
> 
> https://github.com/dynup/kpatch/pull/1149
> 
> Thanks
> WANG Chao
> 
> > 
> > 
> > > For 'targets', in case of OOT, does not seem to be useful.
> > >
> > > What change do you suggest to make to fix this kind of Makefile?
> > >
> > > Thanks,
> > > WANG Chao
> > 
> > 
> > 
> > -- 
> > Best Regards
> > Masahiro Yamada
> > 
> 

-- 
Regards,
  Artem Savkov