linux-kernel - Re: [PATCH] modules: CONFIG_MODULE_COMPRESS: add hint that userspace support may easily be missing.

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Date:	Thu, 4 Jun 2015 21:53:34 +0200
From:	Andreas Mohr <andi@...as.de>
To:	Lucas De Marchi <lucas.de.marchi@...il.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Andreas Mohr <andi@...as.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bertrand Jacquin <beber@...eeweb.net>,
	Marco d'Itri <md@...ux.it>,
	linux-modules <linux-modules@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>, Jon Masters <jcm@...hat.com>
Subject: Re: [PATCH] modules: CONFIG_MODULE_COMPRESS: add hint that userspace
 support may easily be missing.

On Wed, Jun 03, 2015 at 11:31:20PM -0300, Lucas De Marchi wrote:
> On Wed, Jun 3, 2015 at 10:30 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 968a001790af..5422c44be5f0 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1948,26 +1948,22 @@ config MODULE_COMPRESS
> >         bool "Compress modules on installation"
> >         depends on MODULES
> >         help
> > -         This option compresses the kernel modules when 'make
> > -         modules_install' is run.
> >
> > -         The modules will be compressed either using gzip or xz depend on the
> > -         choice made in "Compression algorithm".
> > +         Compresses kernel modules when 'make modules_install' is run; gzip or
> > +         xz depending on "Compression algorithm" below.
> >
> > -         module-init-tools has support for gzip format while kmod handle gzip
> > -         and xz compressed modules.
> > +         module-init-tools supports gzip, and kmod can be configured to handle
> > +         gzip and xz (but doesn't by default, at least as of version 18!).
> 
> This is pretty much misleading. There's no such a default option. It's
> like saying kernel doesn't support feature X, Y or Z by default
> because make defconfig doesn't turn them on.
> 
> The option is there since the beginning for the distros to adapt to their needs.

Hmm? Perhaps I'm misunderstanding something in this discussion,
but Debian kmod-20 (kmod_20.orig.tar.xz - BTW side note:
noted the .xz file rather than .gz? ;))
configure.ac contains:

AC_ARG_WITH([xz],
        AS_HELP_STRING([--with-xz], [handle Xz-compressed modules
@<:@default=disabled@:>@]),
        [], [with_xz=no])
AS_IF([test "x$with_xz" != "xno"], [
        PKG_CHECK_MODULES([liblzma], [liblzma >= 4.99])
        AC_DEFINE([ENABLE_XZ], [1], [Enable Xz for modules.])
], [
        AC_MSG_NOTICE([Xz support not requested])
])

AC_ARG_WITH([zlib],
        AS_HELP_STRING([--with-zlib], [handle gzipped modules
@<:@default=disabled@:>@]),
        [], [with_zlib=no])

So it clearly seems there *is* a specific
(hard-coded, as opposed to system introspection) default
(i.e., the usual way of getting defaults, namely to simply not specify
--with-something configure options),
and that is (drumroll...) "disabled".

Andreas
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