linux-kernel - Re: [PATCH 0/2] kmod /usr support

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Date: Tue, 19 Dec 2023 17:37:31 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Lucas De Marchi <lucas.demarchi@...el.com>
Cc: Michal Suchanek <msuchanek@...e.de>, 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>, Nicolas Schier <nicolas@...sle.eu>, 
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] kmod /usr support

On Thu, Dec 7, 2023 at 3:37 AM Lucas De Marchi <lucas.demarchi@...el.com> wrote:
>
> On Fri, Nov 10, 2023 at 01:13:53PM +0100, Michal Suchanek wrote:
> >Hello,
> >
> >This is resend of the last patch in the series that adds prefix support
> >to kernel module location together with additional patch for validating
> >the user supplied input to options that are interpreted as directories.
> >
> >Thanks
>
> applied, thanks
>
> Lucas De Marchi

If I understood this correctly, MODULE_DIRECTORY is determined
by "configure --with-module-directory=...", and there is no
way to change it after that.

If so, how to work with cross-building?

Cross-building is typical when building embedded Linux systems.

Consider this scenario:

- Your build machine adopts
    MODULE_DIRECTORY=/usr/lib/modules
- The target embedded system adopts
    MODULE_DIRECTORY=/lib/modules

(or vice a versa)

depmod is used also for cross-building because
it is executed as a part of "make module_install".

The counterpart patch set for Kbuild provides
KERNEL_MODULE_DIRECTORY, which only changes
the destination directory to which *.ko are copied.

You cannot change the directory where the
depmod searches for modules, as it is fixed
at the compile-time of kmod.

In this case, what we can do is to build another
instance of kmod configured for the target system,
and use it for modules_install:

1. In the kmod source directory
    ./configure --with=module-directory=/lib/modules
    make

2. make modules_install INSTALL_MOD_PATH=<staging-dir>
     KERNEL_MODULE_DIRECTORY=/lib/modules
     DEPMOD=<new-depmod-you-has-just-built>

If you use OpenEmbedded etc., this is what you do
because host tools are built from sources.

But, should it be required all the time?
Even when the target embedded system uses
busybox-based modprobe instead of kmod?

depmod provides --basedir option, which changes
the prefix part, but there is no way to override
the stem part, MODULE_DIRECTRY.

In the review of the counter patch set,
I am suggesting an option to override MODULE_DIRECTRY
(let's say --moduledir) at least for depmod.

(Perhaps modinfo too, as it also supports --basedir)

Then, we can change scripts/depmod.sh so that
Kbuild can propagate KERNEL_MODULE_DIRECTORY
to depmod.

if  <depmod supports --moduledir>; then
    set -- "$@"  --moduledir "${KERNEL_MODULE_DIRECTORY}"
fi

Does it make sense?

-- 
Best Regards
Masahiro Yamada