Re: [PATCH] Kconfig.debug: Make DEBUG_INFO selectable from a choice

[Nick Desaulniers]

On Thu, Dec 9, 2021 at 4:09 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> Currently it's not possible to enable DEBUG_INFO for an all*config
> build, since it is marked as "depends on !COMPILE_TEST". This generally

Indeed.

> makes sense because a debug build of an all*config target ends up taking
> much longer and the output is much larger. Having this be "default off"
> makes sense. However, there are cases where enabling DEBUG_INFO for such
> builds is useful for doing treewide A/B comparisons of build options,
> etc.
>
> Make DEBUG_INFO selectable from any of the DWARF version choice options,
> with DEBUG_INFO_NONE being the default for COMPILE_TEST. The mutually
> exclusive relationship between DWARF5 and BTF must be inverted, but the
> result remains the same. Additionally moves DEBUG_KERNEL and DEBUG_MISC
> up to the top of the menu because was enabling features _above_ it,

s/was/they were/

Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

> making it weird to navigate menuconfig.
>
> Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  lib/Kconfig.debug | 142 +++++++++++++++++++++++++---------------------
>  1 file changed, 76 insertions(+), 66 deletions(-)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 5c12bde10996..70ef47e92681 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -208,20 +208,89 @@ config DEBUG_BUGVERBOSE
>
>  endmenu # "printk and dmesg options"
>
> +config DEBUG_KERNEL
> +       bool "Kernel debugging"
> +       help
> +         Say Y here if you are developing drivers or trying to debug and
> +         identify kernel problems.
> +
> +config DEBUG_MISC
> +       bool "Miscellaneous debug code"
> +       default DEBUG_KERNEL
> +       depends on DEBUG_KERNEL
> +       help
> +         Say Y here if you need to enable miscellaneous debug code that should
> +         be under a more specific debug option but isn't.
> +
>  menu "Compile-time checks and compiler options"
>
>  config DEBUG_INFO
> -       bool "Compile the kernel with debug info"
> -       depends on DEBUG_KERNEL && !COMPILE_TEST
> +       bool
>         help
> -         If you say Y here the resulting kernel image will include
> -         debugging info resulting in a larger kernel image.
> +         A kernel debug info option other than "None" has been selected
> +         in the "Debug information" choice below, indicating that debug
> +         information will be generated for build targets.
> +
> +choice
> +       prompt "Debug information"
> +       depends on DEBUG_KERNEL
> +       default DEBUG_INFO_NONE if COMPILE_TEST
> +       default DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT if DEBUG_KERNEL
> +       help
> +         Selecting something other than "None" results in a kernel image
> +         that will include debugging info resulting in a larger kernel image.
>           This adds debug symbols to the kernel and modules (gcc -g), and
>           is needed if you intend to use kernel crashdump or binary object
>           tools like crash, kgdb, LKCD, gdb, etc on the kernel.
> -         Say Y here only if you plan to debug the kernel.
>
> -         If unsure, say N.
> +         Choose which version of DWARF debug info to emit. If unsure,
> +         select "Toolchain default".
> +
> +config DEBUG_INFO_NONE
> +       bool "Disable debug information"
> +       help
> +         Do not build the kernel with debugging information, which will
> +         result in a faster and smaller build.
> +
> +config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> +       bool "Rely on the toolchain's implicit default DWARF version"
> +       select DEBUG_INFO
> +       help
> +         The implicit default version of DWARF debug info produced by a
> +         toolchain changes over time.
> +
> +         This can break consumers of the debug info that haven't upgraded to
> +         support newer revisions, and prevent testing newer versions, but
> +         those should be less common scenarios.
> +
> +config DEBUG_INFO_DWARF4
> +       bool "Generate DWARF Version 4 debuginfo"
> +       select DEBUG_INFO
> +       help
> +         Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
> +
> +         If you have consumers of DWARF debug info that are not ready for
> +         newer revisions of DWARF, you may wish to choose this or have your
> +         config select this.
> +
> +config DEBUG_INFO_DWARF5
> +       bool "Generate DWARF Version 5 debuginfo"
> +       select DEBUG_INFO
> +       depends on !CC_IS_CLANG || (CC_IS_CLANG && (AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)))
> +       help
> +         Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc 5.0+ (gcc
> +         5.0+ accepts the -gdwarf-5 flag but only had partial support for some
> +         draft features until 7.0), and gdb 8.0+.
> +
> +         Changes to the structure of debug info in Version 5 allow for around
> +         15-18% savings in resulting image and debug info section sizes as
> +         compared to DWARF Version 4. DWARF Version 5 standardizes previous
> +         extensions such as accelerators for symbol indexing and the format
> +         for fission (.dwo/.dwp) files. Users may not want to select this
> +         config if they rely on tooling that has not yet been updated to
> +         support DWARF Version 5.
> +
> +endchoice # "Debug information"
>
>  if DEBUG_INFO
>
> @@ -267,55 +336,11 @@ config DEBUG_INFO_SPLIT
>           to know about the .dwo files and include them.
>           Incompatible with older versions of ccache.
>
> -choice
> -       prompt "DWARF version"
> -       help
> -         Which version of DWARF debug info to emit.
> -
> -config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> -       bool "Rely on the toolchain's implicit default DWARF version"
> -       help
> -         The implicit default version of DWARF debug info produced by a
> -         toolchain changes over time.
> -
> -         This can break consumers of the debug info that haven't upgraded to
> -         support newer revisions, and prevent testing newer versions, but
> -         those should be less common scenarios.
> -
> -         If unsure, say Y.
> -
> -config DEBUG_INFO_DWARF4
> -       bool "Generate DWARF Version 4 debuginfo"
> -       help
> -         Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
> -
> -         If you have consumers of DWARF debug info that are not ready for
> -         newer revisions of DWARF, you may wish to choose this or have your
> -         config select this.
> -
> -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
> -       help
> -         Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc 5.0+ (gcc
> -         5.0+ accepts the -gdwarf-5 flag but only had partial support for some
> -         draft features until 7.0), and gdb 8.0+.
> -
> -         Changes to the structure of debug info in Version 5 allow for around
> -         15-18% savings in resulting image and debug info section sizes as
> -         compared to DWARF Version 4. DWARF Version 5 standardizes previous
> -         extensions such as accelerators for symbol indexing and the format
> -         for fission (.dwo/.dwp) files. Users may not want to select this
> -         config if they rely on tooling that has not yet been updated to
> -         support DWARF Version 5.
> -
> -endchoice # "DWARF version"
> -
>  config DEBUG_INFO_BTF
>         bool "Generate BTF typeinfo"
>         depends on !DEBUG_INFO_SPLIT && !DEBUG_INFO_REDUCED
>         depends on !GCC_PLUGIN_RANDSTRUCT || COMPILE_TEST
> +       depends on !DEBUG_INFO_DWARF5

I did hit a trivial conflict applying this to mainline/next-20211208 against
d9847eb8be3d8 ("bpf: Make CONFIG_DEBUG_INFO_BTF depend upon CONFIG_BPF_SYSCALL")

>         help
>           Generate deduplicated BTF type information from DWARF debug info.
>           Turning this on expects presence of pahole tool, which will convert
> @@ -584,21 +609,6 @@ source "lib/Kconfig.kcsan"
>
>  endmenu
>
> -config DEBUG_KERNEL
> -       bool "Kernel debugging"
> -       help
> -         Say Y here if you are developing drivers or trying to debug and
> -         identify kernel problems.
> -
> -config DEBUG_MISC
> -       bool "Miscellaneous debug code"
> -       default DEBUG_KERNEL
> -       depends on DEBUG_KERNEL
> -       help
> -         Say Y here if you need to enable miscellaneous debug code that should
> -         be under a more specific debug option but isn't.
> -
> -
>  menu "Memory Debugging"
>
>  source "mm/Kconfig.debug"
> --
> 2.30.2
>


-- 
Thanks,
~Nick Desaulniers