On Thu, Dec 09, 2021 at 08:45:30AM +0100, Arnd Bergmann wrote:

On Thu, Dec 9, 2021 at 6:07 AM Kees Cook [off-list ref] wrote:

quoted

tl;dr: is there a way to force a config default to "off" under
all*config builds, but still leave it configurable? (i.e. not "depends
on !COMPILE_TEST")

I'm trying to understand a Kconfig behavior with regard to
COMPILE_TEST. I'm able to use an "all*config" target, followed by specific
additional config changes (e.g. turning off KCOV), but I can't enable
things like DEBUG_INFO because of their "depends on !COMPILE_TEST".
Whenever I want to examine debug info from all*config build I need to
patch lib/Kconfig.debug to remove the depends. I was hoping I could,
instead do:

This would be a minor hassle for my randconfig testing because I really
want to have DEBUG_INFO disabled when building randconfigs in order
to keep down compile times. I could however just force DEBUG_INFO=n
the same way as forcing COMPILE_TEST=y at the moment.

Right, yes, I want the default for DEBUG_INFO to be off for the
COMPILE_TEST=y case (for savings on speed, storage, etc), but I want to
be _able_ to turn it on when I'm doing whole-build binary comparisons or
pahole archaeology. :)

quoted

I then thought I could use:

        default !COMPILE_TEST

since this works:

config WERROR
        bool "Compile the kernel with warnings as errors"
        default COMPILE_TEST

but I think the above is a no-op: it's the same as not having
"default COMPILE_TEST" when doing an all*config build: it'll be enabled
not because of COMPILE_TEST but because of the all*config pass.

Correct. One trick that works here is to use a 'choice' statement, as those
still honor the 'default' value even for allmodconfig -- Kconfig has no
idea which one of them is the 'all' version.

quoted

How can I make DEBUG_INFO configurable, but default off under
all*config?

I'd try generalizing the "DWARF version" choice to offer 'none' as a
default, like

choice
       prompt "Debug information"
       default DEBUG_INFO_NONE  if COMPILE_TEST
       default DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT if DEBUG_KERNEL

config DEBUG_INFO_NONE
       bool "Turn off all debug information"

config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
       bool "Rely on the toolchain's implicit default DWARF version"

config DEBUG_INFO_DWARF4
       bool "Generate DWARF Version 4 debuginfo"

config DEBUG_INFO_DWARF5
        bool "Generate DWARF Version 5 debuginfo"
        depends on !CC_IS_CLANG || (CC_IS_CLANG && (AS_IS_LLVM ||
(AS_IS_GNU && AS_VERSION >= 23502)))
        depends on !DEBUG_INFO_BTF

endchoice

Ooooh! Yes, that's excellent. I will give that a try. Thanks!

-Kees

-- 
Kees Cook