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

[Kees Cook <kees@kernel.org>]

On Thu, Sep 17, 2026 at 05:06:21PM +0100, Lorenzo Stoakes (ARM) wrote:
> Each kernel make invocation begins with ~30 compiler and linker runs each
> of which performs duplicate probe for a number of compiler and linker
> options.
> 
> This is useless work - the compiler and its version is known, so use these
> to determine which options are available, once.
> 
> A convention already exists for this - CC_HAS_xxx, LD_HAS_xxx in Kconfig
> files (for example, CC_HAS_COUNTED_BY), so convert these probes to Kconfig
> options where appropriate.

Yeah, I agree about the rationale here.

It does, however, now drive a long-time annoyance of mine to the top of
mind: the repetition of the compiler command-line options in two places:
the Kconfig and the Makefile. I dislike that pattern so much that I really
really worked hard to use cc-option instead where ever I possibly could
(though it continued to add to my growing concern about the repetition
of running those checks all the time, so I'm motivated to see something
like what you have here actually land).

But I would really like to find a way to avoid the duplication. It's
fragile and it's weird and it's split across 2 files that don't always
have an obvious relationship. I really don't like it. And with it being
used for things that are "detected" (i.e. not part of always required
builds), that fragility means typos may go unnoticed, etc.

We've had a need for some kind of kconfig "append to a list" logic that
we've been working around in places, e.g. include/linux/lsm_count.h for
how "count the list of enabled LSMs" got dealt with. If we could have
had:

config LSM_LIST
	list
	separator " "

config SECURITY_SELINUX
	...
	append_to LSM_LIST
	...

We could just parse CONFIG_LSM_LIST directly. And I think we can do the
same with this:

config CC_OPTION_LIST
	list
	separator " "

config CC_OPTION_ZERO_INIT_PADDING_BITS
	string
	default "$(cc-option-bit,-fzero-init-padding-bits=all)"
	append_to CC_OPTION_LIST

And the dump all of it into the Makefile in one via CONFIG_CC_OPTION_LIST

(And we'd need to implement ld-option-bit. Though really I think
cc-option-bit should be renamed to cc-option-str or something)

But even without the new "list" Kconfig type, it'd be nicer to use the
cc-option-bit string default method and dump all the newly created
CC_OPTION_... strings into the makefile manually. The "append_to" idea
could be a follow-up.

-Kees

-- 
Kees Cook