On Fri, Feb 9, 2018 at 4:46 AM, Ulf Magnusson <ulfali...@gmail.com> wrote:
> One thing that makes Kconfig confusing (though it works well enough in
> practice) is that .config files both record user selections (the saved
> configuration) and serve as a configuration output format for make.
> It becomes easier to think about .config files once you realize that
> assignments to promptless symbols never have an effect on Kconfig
> itself: They're just configuration output, intermixed with the saved
> user selections.
> Assume 'option env' symbols got written out for example:
>         - For a non-user-assignable symbol, the entry in the .config
>           file is just configuration output and ignored by Kconfig,
>           which will fetch the value from the environment instead.
>         - For an assignable 'option env' symbol, the entry in the
>           .config file is a saved user selection (as well as
>           configuration output), and will be respected by Kconfig.

In the stack-protector case, this becomes quite important, since the
goal is to record the user's selection regardless of compiler
capability. For example, if someone selects _REGULAR, it shouldn't
"upgrade" to _STRONG. (Similarly for _NONE.) Having _AUTO provides a
way to pick "best possible for this compiler", though. If a user had
previously selected _STRONG but they're doing builds with an older
compiler (or a misconfigured newer compiler) without support, the goal
is to _fail_ to build, not silently select _REGULAR.

So, in this case, what's gained is the logic for _AUTO, and the logic
to not show, say, _STRONG when it's not available in the compiler. But
we must still fail to build if _STRONG was in the .config. It can't
silently rewrite it to _REGULAR because the compiler support for
_STRONG regressed.


Kees Cook
Pixel Security

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