Re: [RFC PATCH 4/7] kconfig: support new special property shell=

[Ulf Magnusson]

On Sun, Feb 11, 2018 at 9:29 PM, Ulf Magnusson <ulfalizer@xxxxxxxxx> wrote:
> On Sun, Feb 11, 2018 at 6:56 PM, Kees Cook <keescook@xxxxxxxxxxxx> 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