question about all*config and COMPILE_TEST

question about all*config and COMPILE_TEST

[Kees Cook]

Hi,

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:

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 0e2de4b375f3..e8533ffc92c3 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -212,7 +212,8 @@ menu "Compile-time checks and compiler options"
 
 config DEBUG_INFO
 	bool "Compile the kernel with debug info"
-	depends on DEBUG_KERNEL && !COMPILE_TEST
+	depends on DEBUG_KERNEL
+	default n if COMPILE_TEST
 	help
 	  If you say Y here the resulting kernel image will include
 	  debugging info resulting in a larger kernel image.

Which would turn this off when COMPILE_TEST was enabled, but I assume it
doesn't work because an all*config target turns everything on first, and
therefore this "default" gets ignored since DEBUG_INFO already has a
value set.

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.

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

Thanks!

-- 
Kees Cook

Re: question about all*config and COMPILE_TEST

[Arnd Bergmann]

On Thu, Dec 9, 2021 at 6:07 AM Kees Cook <keescook@chromium.org> wrote:
>
> 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.


> 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.

> 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

        Arnd

Re: question about all*config and COMPILE_TEST

[Kees Cook]

On Thu, Dec 09, 2021 at 08:45:30AM +0100, Arnd Bergmann wrote:
> On Thu, Dec 9, 2021 at 6:07 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > 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. :)

> 
> > 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.
> 
> > 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