linux-kernel - Re: [PATCH kmod v4 4/4] libkmod, depmod, modprobe: Make directory for kernel modules configurable

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Date:   Tue, 18 Jul 2023 12:29:59 +0200
From:   Michal Suchánek <msuchanek@...e.de>
To:     Jan Engelhardt <jengelh@...i.de>
Cc:     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>,
        Masahiro Yamada <masahiroy@...nel.org>,
        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 kmod v4 4/4] libkmod, depmod, modprobe: Make directory
 for kernel modules configurable

On Tue, Jul 18, 2023 at 11:41:32AM +0200, Jan Engelhardt wrote:
> 
> On Tuesday 2023-07-18 10:43, Michal Suchánek wrote:
> >> >With this distributions that do not want to ship files in /lib can also
> >> >move kernel modules to /usr while others can keep them in /lib.
> >> 
> >> This patch breaks kernel builds/installation / bisecting [when the
> >> system has a kmod ./configure'd --with-module-directory=/usr/lib/modules]
> >
> >It might be nice to provide backwads compatibility with earlier
> >configurations.
> >
> >However, if it comes at the cost of making the implementation more
> >complex and future maintenance more difficult it might not be such a
> >great idea. So far I have not seen a proposal how to do it nicely.
> 
> 
> 
> diff --git a/configure.ac b/configure.ac
> index a195c8e..7fde927 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -84,11 +84,7 @@ AC_ARG_WITH([rootlibdir],
>          [], [with_rootlibdir=$libdir])
>  AC_SUBST([rootlibdir], [$with_rootlibdir])
>  
> -# Ideally this would be $prefix/lib/modules but default to /lib/modules for compatibility with earlier versions
> -AC_ARG_WITH([module_directory],
> -        AS_HELP_STRING([--with-module-directory=DIR], [directory in which to look for kernel modules - typically '/lib/modules' or '${prefix}/lib/modules']),
> -        [], [with_module_directory=/lib/modules])
> -AC_SUBST([module_directory], [$with_module_directory])
> +AC_SUBST([module_directory], [/lib/modules])
>  
>  AC_ARG_WITH([zstd],
>  	AS_HELP_STRING([--with-zstd], [handle Zstandard-compressed modules @<:@default=disabled@:>@]),
> diff --git a/libkmod/libkmod-internal.h b/libkmod/libkmod-internal.h
> index 4a4af58..e2f9c95 100644
> --- a/libkmod/libkmod-internal.h
> +++ b/libkmod/libkmod-internal.h
> @@ -199,3 +199,5 @@ void kmod_module_signature_info_free(struct kmod_signature_info *sig_info) __att
>  
>  /* libkmod-builtin.c */
>  ssize_t kmod_builtin_get_modinfo(struct kmod_ctx *ctx, const char *modname, char ***modinfo) __attribute__((nonnull(1, 2, 3)));
> +
> +extern const char *dirname_default_prefix;
> diff --git a/libkmod/libkmod.c b/libkmod/libkmod.c
> index d2ed874..b426cde 100644
> --- a/libkmod/libkmod.c
> +++ b/libkmod/libkmod.c
> @@ -209,7 +209,7 @@ static int log_priority(const char *priority)
>  	return 0;
>  }
>  
> -static const char *dirname_default_prefix = MODULE_DIRECTORY;
> +const char *dirname_default_prefix = MODULE_DIRECTORY;
>  
>  static char *get_kernel_release(const char *dirname)
>  {
> diff --git a/tools/depmod.c b/tools/depmod.c
> index 22bc1d8..929060f 100644
> --- a/tools/depmod.c
> +++ b/tools/depmod.c
> @@ -65,6 +65,7 @@ static const struct option cmdopts[] = {
>  	{ "quick", no_argument, 0, 'A' },
>  	{ "basedir", required_argument, 0, 'b' },
>  	{ "outdir", required_argument, 0, 'o' },
> +	{ "modulesdir", required_argument, 0, 'M' },
>  	{ "config", required_argument, 0, 'C' },
>  	{ "symvers", required_argument, 0, 'E' },
>  	{ "filesyms", required_argument, 0, 'F' },
> @@ -2943,6 +2944,9 @@ static int do_depmod(int argc, char *argv[])
>  				free(out_root);
>  			out_root = path_make_absolute_cwd(optarg);
>  			break;
> +		case 'M':
> +			dirname_default_prefix = optarg;
> +			break;
>  		case 'C': {
>  			size_t bytes = sizeof(char *) * (n_config_paths + 2);
>  			void *tmp = realloc(config_paths, bytes);

That breaks kmod for the usrmerged distributions, though.

It might be fine to provide an option to override the build-time
default. Still the build-time default has to match where the modules are
placed in the distribution for things to work correctly out of the box.

Note: it will work either way at the times the module directory can be
accessed through the lib -> usr/lib compatibility symlink. The default
needs to be correct for the cases when the symlink is not provided.

So you could do

make DEPMOD='depmod -M /lib/modules'

to build an old kernel but you could equally do

make DEPMOD='/path/to/special/depmod'

If you needed

make DEPMOD='depmod -M /usr/lib/modules'

to build an usrmerged kernel on usrmerged distribution then this whole
exercise is pointless. kmod cannot find the usrmerged modules then.

Thanks

Michal