linux-kernel - Re: question about all*config and COMPILE

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Message-ID: <CAK8P3a1wsa1makDonP8xdbMt5Tc4rU2a_4LDfXLpSp9+uFd73w@mail.gmail.com>
Date:   Thu, 9 Dec 2021 08:45:30 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: question about all*config and COMPILE_TEST

On Thu, Dec 9, 2021 at 6:07 AM Kees Cook <keescook@...omium.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