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

[Ulf Magnusson]

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