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

[Nathan Chancellor <nathan@kernel.org>]

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@oracle.com>
> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
> Signed-off-by: Kees Cook <kees@kernel.org>

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

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