linux-kernel - Re: [PATCH v2] kconfig: Add transitional symbol attribute for migration support

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Message-ID: <a123ab8b-a335-48a1-9ac3-e3b348d78cd1@infradead.org>
Date: Mon, 1 Sep 2025 09:39:46 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Kees Cook <kees@...nel.org>, Nathan Chancellor <nathan@...nel.org>
Cc: Nicolas Schier <nicolas.schier@...ux.dev>,
 Jonathan Corbet <corbet@....net>, Masahiro Yamada <masahiroy@...nel.org>,
 Arnd Bergmann <arnd@...db.de>,
 Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
 linux-kbuild@...r.kernel.org, linux-doc@...r.kernel.org,
 Miguel Ojeda <ojeda@...nel.org>,
 Stephen Brennan <stephen.s.brennan@...cle.com>,
 Marco Bonelli <marco@...eim.net>, Petr Vorel <pvorel@...e.cz>,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] kconfig: Add transitional symbol attribute for
 migration support

Hi Kees,

On 8/29/25 7:01 PM, 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 OLD_OPTION
>         transitional bool
>         help
>           Transitional config for OLD_OPTION migration.
> 
>     config NEW_OPTION
>         bool "New option"
>         default OLD_OPTION
> 
> 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.
> 
> Implementation details:
> - Parser validates transitional symbols can only have help sections
> - Symbol visibility logic updated: usable = (visible != no || transitional)
> - Transitional symbols preserve user values during configuration
> - Type safety enforced to prevent redefinition after transitional declaration
> - Used distinct struct members instead of new flags for readability
> - Documentation added to show the usage
> 
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> With help from Claude Code to show me how to navigate the kconfig parser.

Are you (implicitly?) saying that all previous attempts at transitional
kconfig symbols have failed?  If so, I just wasn't aware of that.

Or is there some new prime directive that requires this?

Thanks.
-- 
~Randy