Date:   Fri, 28 Jan 2022 13:58:19 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Peter Zijlstra <peterz@...radead.org>,
        Isabella Basso <isabbasso@...eup.net>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] Kconfig.debug: Make DEBUG_INFO selectable from a
 choice

Hi Kees,

On Mon, Jan 24, 2022 at 11:51:26PM -0800, Kees Cook 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 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 they were enabling features _above_
> it, making it weird to navigate menuconfig.
> 
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Reviewed-by: Arnd Bergmann <arnd@...db.de>
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> v1: https://lore.kernel.org/lkml/20211210000910.3597609-1-keescook@chromium.org
> v2: https://lore.kernel.org/lkml/20220121001204.4023842-1-keescook@chromium.org
> v3: - rebase to v5.17-rc1
> ---
>  lib/Kconfig.debug | 140 +++++++++++++++++++++++++---------------------
>  1 file changed, 75 insertions(+), 65 deletions(-)
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 14b89aa37c5c..1290e0906236 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -208,20 +208,88 @@ 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

I apologize for not bringing this up sooner in review (especially with
my tag) but I wanted to make sure this change is intentional.

Prior to this commit, we would have to explicitly enable DEBUG_INFO
after turning on DEBUG_KERNEL (i.e. DEBUG_KERNEL would not select
DEBUG_INFO).

Now, DEBUG_INFO automatically gets turned on with DEBUG_KERNEL because
of the depends plus the default for the choice. Is this intentional?

For example, with x86_64_defconfig:

mainline:

$ make mrproper defconfig && rg DEBUG_INFO .config
4571:# CONFIG_DEBUG_INFO is not set

next-20220128:

$ make mrproper defconfig && rg DEBUG_INFO .config
4573:CONFIG_DEBUG_INFO=y
4574:CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y
4575:# CONFIG_DEBUG_INFO_DWARF4 is not set
4576:# CONFIG_DEBUG_INFO_DWARF5 is not set
4577:# CONFIG_DEBUG_INFO_NONE is not set
4578:# CONFIG_DEBUG_INFO_REDUCED is not set
4579:# CONFIG_DEBUG_INFO_COMPRESSED is not set
4580:# CONFIG_DEBUG_INFO_SPLIT is not set

As far as I can tell, this will have a big impact on everyone's binary
and modules size that either enables DEBUG_KERNEL or EXPERT (which
selects DEBUG_KERNEL), which is a lot of configs.

$ for file in $(rg -l "CONFIG_(DEBUG_KERNEL|EXPERT)" arch/*/configs); do rg DEBUG_INFO $file &>/dev/null || echo $file; done |& wc -l
170

If this change is intentional, then it probably warrants more of an
explicit callout in the commit message. I only noticed this because the
vmlinux for the multi_v7_defconfig kernel I build for my Raspberry Pi 3
went from 32.44 MB to 262.75 MB.

Cheers,
Nathan

> +	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_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.
> +
> +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.
> +
> +endchoice # "Debug information"
>  
>  if DEBUG_INFO
>  
> @@ -267,56 +335,12 @@ 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 BPF_SYSCALL
> +	depends on !DEBUG_INFO_DWARF5
>  	help
>  	  Generate deduplicated BTF type information from DWARF debug info.
>  	  Turning this on expects presence of pahole tool, which will convert
> @@ -585,20 +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 "Networking Debugging"
>  
>  source "net/Kconfig.debug"
> -- 
> 2.30.2
> 
> 

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