linux-kernel - Re: [PATCH rebased] kbuild: rpm-pkg: Fix build with non-default MODLIB

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Date:   Tue, 10 Oct 2023 12:15:52 +0200
From:   Michal Suchánek <msuchanek@...e.de>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nicolas Schier <nicolas@...sle.eu>,
        linux-modules@...r.kernel.org, Takashi Iwai <tiwai@...e.com>,
        Lucas De Marchi <lucas.de.marchi@...il.com>,
        Michal Koutný <mkoutny@...e.com>,
        Jiri Slaby <jslaby@...e.com>, Jan Engelhardt <jengelh@...i.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH rebased] kbuild: rpm-pkg: Fix build with non-default
 MODLIB

On Tue, Oct 10, 2023 at 12:14:01AM +0900, Masahiro Yamada wrote:
> On Mon, Oct 9, 2023 at 11:07 PM Michal Suchánek <msuchanek@...e.de> wrote:
> >
> > On Mon, Oct 09, 2023 at 09:34:10PM +0900, Masahiro Yamada wrote:
> > > On Mon, Oct 9, 2023 at 5:52 PM Michal Suchánek <msuchanek@...e.de> wrote:
> > > >
> > > > Hello,
> > > >
> > > > On Mon, Oct 09, 2023 at 05:31:02PM +0900, Masahiro Yamada wrote:
> > > > > On Fri, Oct 6, 2023 at 12:49 AM Michal Suchanek <msuchanek@...e.de> wrote:
> > > > > >
> > > > > > The default MODLIB value is composed of two variables and the hardcoded
> > > > > > string '/lib/modules/'.
> > > > > >
> > > > > > MODLIB = $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE)
> > > > > >
> > > > > > Defining this middle part as a variable was rejected on the basis that
> > > > > > users can pass the whole MODLIB to make, such as
> > > > >
> > > > >
> > > > > In other words, do you want to say
> > > > >
> > > > > "If defining this middle part as a variable had been accepted,
> > > > > this patch would have been unneeded." ?
> > > >
> > > > If it were accepted I would not have to guess what the middle part is,
> > > > and could use the variable that unambiguosly defines it instead.
> > >
> > >
> > > How?
> > >
> > > scripts/package/kernel.spec hardcodes 'lib/modules'
> > > in a couple of places.
> > >
> > > I am asking how to derive the module path.
> >
> > Not sure what you are asking here. The path is hardcoded, everywhere.
> >
> > The current Makefile has
> >
> > MODLIB  = $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE)
> >
> > and there is no reliable way to learn what the middle part was after the
> > fact - $(INSTALL_MOD_PATH) can be non-empty.
> >
> > The rejected patch was changing this to a variable, and also default to
> > adjusting the content to what kmod exports in pkgconfig after applying a
> > proposed patch to make this hardcoded part configurable:
> >
> > export KERNEL_MODULE_DIRECTORY := $(shell pkg-config --print-variables kmod 2>/dev/null | grep '^module_directory$$' >/dev/null && pkg-config --variable=module_directory kmod || echo /lib/modules)
> >
> > MODLIB  = $(INSTALL_MOD_PATH)$(KERNEL_MODULE_DIRECTORY)/$(KERNELRELEASE)
> >
> > It would be completely posible to only define the middle part as a
> > variable that could then be used in rpm-pkg:
> >
> > export KERNEL_MODULE_DIRECTORY := /lib/modules
> >
> > MODLIB  = $(INSTALL_MOD_PATH)$(KERNEL_MODULE_DIRECTORY)/$(KERNELRELEASE)
> >
> > Thanks
> >
> > Michal
> >
> >
> 
> 
> Let me add more context to my question.
> 
> 
> I am interested in the timing when
> 'pkg-config --print-variables kmod | grep module_directory'
> is executed.
> 
> 
> 
> 1.  Build a SRPM on machine A
> 
> 2.  Copy the SRPM from machine A to machine B
> 
> 3.  Run rpmbuild on machine B to build the SRPM into a RPM
> 
> 4.  Copy the RPM from machine B to machine C
> 
> 5.  Install the RPM to machine C

As far as I am aware the typical use case is two step:

1. run make rpm-pkg on machine A
2. install the binary rpm on machine C that might not have build tools
   or powerful enough CPU

While it's theoretically possible to use the srpm to rebuild the binary
rpm independently of the kernel git tree I am not aware of people
commonly doing this.

If rebuilding the source rpm on a different machine from where the git
tree is located, and possibly on a different distribution is desirable
then the detection of the KERNEL_MODULE_DIRECTORY should be added in the
rpm spec file as well.

> Of course, we are most interested in the module path
> of machine C, but it is difficult/impossible to
> guess it at the time of building.
> 
> We can assume machine B == machine C.
> 
> We are the second most interested in the module
> path on machine B.
> 
> The module path of machine A is not important.
> 
> So, I am asking where you would inject
> 'pkg-config --print-variables kmod | grep module_directory'.

I don't. I don't think there will be a separate machine B.

And I can't really either - so far any attempt at adding support for
this has been rejected.

Technically the KERNEL_MODULE_DIRECTORY could be set in two steps - one
giving the script to run, and one running it, and then it could be run
independently in the SRPM as well.

Thanks

Michal