Re: [PATCH v3 11/20] kbuild: avoid re-running compiler and linker probes

On Thu, Sep 17, 2026 at 09:44:24PM -0700, Kees Cook wrote:
> On Thu, Sep 17, 2026 at 06:21:19PM -0700, Nathan Chancellor wrote:
> > While we could certainly try to cook something like this up, we could
> > also avoid the split across two files by just defining the flags in
> > Kconfig directly and using them in the Makefile through their symbol,
> > like we already do for -Wimplicit-fallthrough. For example, instead of
> > 
> >   config CC_HAS_STRICT_FLEX_ARRAYS
> >       def_bool $(cc-option,-fstrict-flex-arrays=3)
> > 
> >   KBUILD_CFLAGS += $(if $(CONFIG_CC_HAS_STRICT_FLEX_ARRAYS),-fstrict-flex-arrays=3)
> > 
> > We would just do
> > 
> >   config CC_STRICT_FLEX_ARRAYS
> >       string
> >       default "-fstrict-flex-arrays=3" if $(cc-option,-fstrict-flex-arrays=3)
> > 
> >   KBUILD_CFLAGS += $(CONFIG_CC_STRICT_FLEX_ARRAYS)
> > 
> > While we still get the duplication (and we could look at getting rid of
> > it with your cc-option-str idea or whatever), it is at least contained
> > to the same location, so out of sync issues should be much rarer. I do
> > diff Kconfigs so this might make certain issues with checks a little bit
> > more obvious if something changes on the compiler side.
> 
> Yeah, and note that cc-option-bit already exists. It's just a weird name
> is all and there isn't an ld-option-bit.

Ah, I see. I worry that the current cc-option-bit is not sufficient for
all options that we may want to test because it uses '-E' to stop after
preprocessing (whereas some options may need to get to code generation
to be validated properly) and it does not use $(CLANG_FLAGS) to properly
set the target (so we might not get consistent results). I wonder if we
would be better off refactoring cc-option-bit into cc-option-str that
shares most of the machinery of cc-option. Then we could just add
ld-option-str.

> As for naming conventions, I think "CC_OPT_" would be a good prefix. And
> this works today with no change:
> 
> config CC_OPT_STRICT_FLEX_ARRAYS
> 	string
> 	default "$(cc-option-bit,-fstrict-flex-arrays=3)"

This does look pretty nice.

> (do we need "def_string" now?)

def_string would certainly make these even nicer.

-- 
Cheers,
Nathan