On Wed, Jul 02, 2014 at 06:00:19PM -0700, Andi Kleen wrote:
> > The problem is that if you make kbuild hard-fail when selecting this missing
> > compiler option, you can never switch it back because "make menuconfig" will
> > refuse to build since the compiler option would be missing. Being silent
> > about the missing option (and/or falling back to other options) means that
> > you could get two different kernel features selection with the same CONFIG_*
> > set, depending on the kernel, which is extremely bad ("I selected
> > stack-protector-strong but it built without it?!").
>
> The assumption that every flag in a .config has been consciouscly
> selected by a human is a quite dubious one ...
Sure; it was actually via automated config updates that I uncovered this
limitation of kbuild in the first place.
> LTO just turns itself off if the toolchain doesn't support it.
For this situation, it is more surprising to have this option fall back,
since it is a security feature selection. The build must fail in some way if
compiler doesn't match CONFIG.
> > So, the middle ground was to warn about it during the kbuild logic so
> > you could find the source of the problem, but ultimately fail the build
> > when the compiler doesn't support it so there weren't any silent failure
> > modes.
>
> Longer term it would be of course best to move all the cc-options
> probing into Kconfig. I bet that would speed up builds too.
Absolutely. I presently lack the deep knowledge to figure this out. :)
-Kees
--
Kees Cook @outflux.net
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