linux-kernel - Re: [PATCH v3 2/3] kconfig: Add transitional symbol attribute for migration support

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Message-ID: <20250924163827.GA3322486@ax162>
Date: Wed, 24 Sep 2025 09:38:27 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Vegard Nossum <vegard.nossum@...cle.com>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Jonathan Corbet <corbet@....net>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Randy Dunlap <rdunlap@...radead.org>, Arnd Bergmann <arnd@...db.de>,
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
	linux-kbuild@...r.kernel.org, linux-doc@...r.kernel.org,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Puranjay Mohan <puranjay@...nel.org>, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3 2/3] kconfig: Add transitional symbol attribute for
 migration support

On Tue, Sep 23, 2025 at 02:34:18PM -0700, Kees Cook wrote:
> During kernel option migrations (e.g. CONFIG_CFI_CLANG to CONFIG_CFI),
> existing .config files need to maintain backward compatibility while
> preventing deprecated options from appearing in newly generated
> configurations. This is challenging with existing Kconfig mechanisms
> because:
> 
> 1. Simply removing old options breaks existing .config files.
> 2. Manually listing an option as "deprecated" leaves it needlessly
>    visible and still writes them to new .config files.
> 3. Using any method to remove visibility (.e.g no 'prompt', 'if n',
>    etc) prevents the option from being processed at all.
> 
> Add a "transitional" attribute that creates symbols which are:
> - Processed during configuration (can influence other symbols' defaults)
> - Hidden from user menus (no prompts appear)
> - Omitted from newly written .config files (gets migrated)
> - Restricted to only having help sections (no defaults, selects, etc)
>   making it truly just a "prior value pass-through" option.
> 
> The transitional syntax requires a type argument and prevents type
> redefinition:
> 
>     config NEW_OPTION
>         bool "New option"
>         default OLD_OPTION
> 
>     config OLD_OPTION
>         bool
>         transitional
>         help
>           Transitional config for OLD_OPTION migration.
> 
> This allows seamless migration: olddefconfig processes existing
> CONFIG_OLD_OPTION=y settings to enable CONFIG_NEW_OPTION=y, while
> CONFIG_OLD_OPTION is omitted from newly generated .config files.
> 
> Added positive and negative testing via "testconfig" make target.
> 
> Co-developed-by: Vegard Nossum <vegard.nossum@...cle.com>
> Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>
> Signed-off-by: Kees Cook <kees@...nel.org>

This seems reasonable to me but I am by no means a Kconfig expert.

Reviewed-by: Nathan Chancellor <nathan@...nel.org>
Tested-by: Nathan Chancellor <nathan@...nel.org>