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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Mon, 12 Feb 2018 23:21:20 +0900
Message-ID: <CAK7LNAQPGht_wBPr_66shL-59mvaXRzDeuSNKicf+zu5Y1CmCA@mail.gmail.com>
Subject: Re: [RFC PATCH 4/7] kconfig: support new special property shell=
To:     Ulf Magnusson <ulfalizer@gmail.com>
Cc:     Kees Cook <keescook@chromium.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
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2018-02-12 21:54 GMT+09:00 Ulf Magnusson <ulfalizer@gmail.com>:
> 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.

Right.  This is a historical misnomer.

My plan is, as already posted below, to rename 'silentoldconfig' to 'synconfig'
https://lkml.org/lkml/2018/1/17/1359


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 It's probably outdated.

Good catch.


>
> 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.


No.  NACK.

silentoldconfig touches include/generated/autoconf.h
so, files that depend on it will be re-compiled, unnecessarily.


silentoldconfig ( 'syncconfig' in a more proper name)
should be run only when necessary.


-- 
Best Regards
Masahiro Yamada