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

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Message-ID: <20231017151050.GJ6241@kitsune.suse.cz>
Date:   Tue, 17 Oct 2023 17:10:50 +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 17, 2023 at 11:46:45PM +0900, Masahiro Yamada wrote:
> On Tue, Oct 17, 2023 at 9:27 PM Michal Suchánek <msuchanek@...e.de> wrote:
> >
> > On Tue, Oct 17, 2023 at 09:05:29PM +0900, Masahiro Yamada wrote:
> > > On Tue, Oct 17, 2023 at 7:44 PM Michal Suchánek <msuchanek@...e.de> wrote:
> > > >
> > > > On Tue, Oct 17, 2023 at 07:15:50PM +0900, Masahiro Yamada wrote:

> > > > > If MOD_PREFIX is given from an env variable or from the command line,
> > > > > it is respected.
> > > > >
> > > > > If "pkg-config --variable=module_prefix libkmod" works,
> > > > > that configuration is applied.
> > > > >
> > > > > Otherwise, MOD_PREFIX is empty, i.e. fall back to the current behavior.
> > > > >
> > > > >
> > > > > I prefer 'MOD_PREFIX' to 'KERNEL_MODULE_DIRECTORY' in your patch [1]
> > > > > because "|| echo /lib/modules" can be omitted.
> > > > >
> > > > > I do not think we will have such a crazy distro that
> > > > > installs modules under /opt/ directory.
> > > >
> > > > However, I can easily imagine a distribution that would want to put
> > > > modules in /usr/lib-amd64-linux/modules.
> > >
> > >
> > > Sorry, it is not easy for me.
> > >
> > > What is the background of your thought?
> >
> > That's where every other library and module would go on distributions
> > that care about ability to install packages for multiple architectures
> > at the same time. AFAIK the workaround is to inclclude the CPU
> > architecture in extraversion for the kernel to fit.
> 
> 
> In my system (Ubuntu), I see the directory paths
> 
> /usr/aarch64-linux-gnu/lib/
> /usr/i686-linux-gnu/lib/
> /usr/x86_64-linux-gnu/lib/
> 
> If there were such a crazy distro that supports multiple kernel arches
> within a single image, modules might be installed:
> /usr/x86_64-linux-gnu/lib/module/<version>/

For me it's /usr/lib/i386-linux-gnu/.

Did they change the scheme at some point?

> > > >
> > > > > I could not understand why you inserted
> > > > > "--print-variables kmod 2>/dev/null | grep '^module_directory$$' >/dev/null"
> > > > > but I guess the reason is the same.
> > > > > "pkg-config --variable=module_directory kmod" always succeeds,
> > > > > so "|| echo /lib/modules" is never processed.
> > > >
> > > > Yes, that's the semantics of the tool. The jq version was slightly less
> > > > convoluted but required additional tool for building the kernel.
> > >
> > >
> > > It IS convoluted.
> >
> > That's unfortunate result of how the pkgconfig tool works. By now it is
> > even too late to complain to the tool author because it's been like that
> > forever, best bet is to to use it as is or pick a different tool for
> > configuration.
> 
> "pkg-config --variable=<name>" returns its value.
> It is pretty simple, and I do not think it is a big problem.
> 
> Your code is long, but the reason is that you implemented
> it in that way.
> 
> 
> If you go with KERNEL_MODULE_DIRECTORY for max flexibility,
> 
>   KERNEL_MODULE_DIRECTORY := $(or $(shell pkg-config
> --variable=module_directory kmod 2>/dev/null),/lib/modules)
> 
> should work with less characters and less process forks.

And assumes that the module_directory cannot be empty.

Which may or may not be a reasonable assumption, the script as proposed
in the patch does not rely on it.

> But, now I started to prefer confining the long code
> into the shell script, "scripts/modinst-dir",
> and calling it where needed.

That's also an option.

Thanks

Michal

> > > > > [1] https://lore.kernel.org/linux-kbuild/20230718120348.383-1-msuchanek@suse.de/
> > > > > [2] https://github.com/kmod-project/kmod/blob/v31/configure.ac#L295