2018-02-12 23:21 GMT+09:00 Masahiro Yamada <yamada.masah...@socionext.com>:
> 2018-02-12 21:54 GMT+09:00 Ulf Magnusson <ulfali...@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 <ulfali...@gmail.com> wrote:
>>> > On Sun, Feb 11, 2018 at 6:56 PM, Kees Cook <keesc...@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).
>> 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
>> # 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
>> # we execute the config step to be sure to catch updated Kconfig
>> 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
>> 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
> No. NACK.
> silentoldconfig touches include/generated/autoconf.h
> so, files that depend on it will be re-compiled, unnecessarily.
Sorry, I take back this comment.
fixdep takes care of this.
So, autoconf.h is removed from dependency.
> silentoldconfig ( 'syncconfig' in a more proper name)
> should be run only when necessary.
> Best Regards
> Masahiro Yamada