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Date:   Mon, 12 Feb 2018 13:54:58 +0100
From:   Ulf Magnusson <ulfalizer@gmail.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nicolas Pitre <nicolas.pitre@linaro.org>,
        "Luis R . Rodriguez" <mcgrof@suse.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Pavel Machek <pavel@ucw.cz>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Jiri Kosina <jkosina@suse.cz>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Tejun Heo <tj@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [RFC PATCH 4/7] kconfig: support new special property shell=
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On Sun, Feb 11, 2018 at 09:42:09PM +0100, Ulf Magnusson wrote:
> On Sun, Feb 11, 2018 at 9:29 PM, Ulf Magnusson <ulfalizer@gmail.com> wrote:
> > On Sun, Feb 11, 2018 at 6:56 PM, Kees Cook <keescook@chromium.org> wrote:
> >> Another case I mentioned before that I just want to make sure we don't
> >> reintroduce the problem of getting "stuck" with a bad .config file.
> >> While adding _STRONG support, I discovered the two-phase Kconfig
> >> resolution that happens during the build. If you selected _STRONG with
> >> a strong-capable compiler, everything was fine. If you then tried to
> >> build with an older compiler, you'd get stuck since _STRONG wasn't
> >> support (as detected during the first Kconfig phase) so the
> >> generated/autoconf.h would never get updated with the newly selected
> >> _REGULAR). I moved the Makefile analysis of available stack-protector
> >> options into the second phase (i.e. after all the Kconfig runs), and
> >> that worked to both unstick such configs and provide a clear message
> >> early in the build about what wasn't available.
> >>
> >> If all this detection is getting moved up into Kconfig, I'm worried
> >> we'll end up in this state again. If the answer is "you have to delete
> >> autoconf.h if you change compilers", then that's fine, but it sure
> >> seems unfriendly. :)
> >
> > Did you mean include/config/auto.conf? That's the one that gets
> > included by the Makefiles.
> >
> > If the feature detection is moved into Kconfig, you should only need
> > to rerun the configuration (make menuconfig/oldconfig/olddefconfig) if
> > you change the compiler. That will update .config while taking the new
> > features into account, and then the second phase during 'make' will
> > update include/config/auto.conf from .config.
> >
> > That second Kconfig phase generates include/generated/autoconf.h and
> > include/config/. The include/config/ directory implements dependencies
> > between source files and Kconfig symbols by turning the symbols into
> > (empty) files. When building (during the "second phase"), Kconfig
> > compares .config with include/config/auto.conf to see what changed,
> > and signals the changes to 'make' by touch'ing the files corresponding
> > to the changed symbols. The idea is to avoid having to do a full
> > rebuild whenever the configuration is changed.
> >
> > Check out scripts/basic/fixdep.c as well if you want to understand how it works.
> >
> > Cheers,
> > Ulf
> 
> By the way:
> 
> That second phase is also a "normal" Kconfig run in the sense that it
> does all the usual dependency checking stuff. Even if .config doesn't
> respect dependencies, include/config/auto.conf will. So I think you
> might not even need to rerun the configuration (though .config will be
> out-of-date until you do).
> 
> Cheers,
> Ulf

Seems you'd have to rerun the configuration, because
include/config/auto.conf is only regenerated if it's older than .config.

Here's the bit in the root Makefile that does it (KCONFIG_CONFIG is
.config).

	# If .config is newer than include/config/auto.conf, someone tinkered
	# with it and forgot to run make oldconfig.
	# if auto.conf.cmd is missing then we are probably in a cleaned tree so
	# we execute the config step to be sure to catch updated Kconfig files
	include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
		$(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig

silentoldconfig is a terrible name. What it actually does is run that
"second phase" stuff.

Pretty sure that comment lies by the way. 'make oldconfig' doesn't
update include/config/auto.conf. It's probably outdated.


I wonder if it would be simpler to just always run silentoldconfig when
building. It's not that slow on my system:

	$ export ARCH=x86 SRCARCH=x86 KERNELVERSION=`make kernelversion`
	$ time scripts/kconfig/conf --silentoldconfig Kconfig

	real	0m0.167s
	user	0m0.162s
	sys	0m0.004s

That'd both simplify the Makefiles, and make sure that the latest
features are always used if you do feature testing in Kconfig.

I don't know how strongly people feel about a few tenths of a second
though.

Cheers,
Ulf