On Fri, 29 Aug 2025, Kees Cook <kees@xxxxxxxxxx> 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. How long do you think we'll need to keep the transitional config options around? Forever? BR, Jani. > 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@xxxxxxxxxx> > --- > With help from Claude Code to show me how to navigate the kconfig parser. > > v2: fixed human-introduced errors > v1: https://lore.kernel.org/all/20250830014438.work.682-kees@xxxxxxxxxx/ > > Cc: Nathan Chancellor <nathan@xxxxxxxxxx> > Cc: Nicolas Schier <nicolas.schier@xxxxxxxxx> > Cc: Jonathan Corbet <corbet@xxxxxxx> > Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx> > Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> > Cc: Arnd Bergmann <arnd@xxxxxxxx> > Cc: Krzysztof Kozlowski <krzysztof.kozlowski@xxxxxxxxxx> > Cc: <linux-kbuild@xxxxxxxxxxxxxxx> > Cc: <linux-doc@xxxxxxxxxxxxxxx> > --- > scripts/kconfig/expr.h | 15 +++++++ > scripts/kconfig/lexer.l | 1 + > scripts/kconfig/parser.y | 51 +++++++++++++++++++++++ > scripts/kconfig/symbol.c | 11 +++-- > Documentation/kbuild/kconfig-language.rst | 31 ++++++++++++++ > 5 files changed, 106 insertions(+), 3 deletions(-) > > diff --git a/scripts/kconfig/expr.h b/scripts/kconfig/expr.h > index fe2231e0e6a4..be51574d6c77 100644 > --- a/scripts/kconfig/expr.h > +++ b/scripts/kconfig/expr.h > @@ -127,6 +127,21 @@ struct symbol { > /* SYMBOL_* flags */ > int flags; > > + /* > + * Transitional symbol - processed during configuration but hidden from > + * user in menus and omitted from newly written .config files. Used for > + * backward compatibility during config option migrations (e.g., > + * CFI_CLANG → CFI). Transitional symbols can still influence default > + * expressions of other symbols. > + */ > + bool transitional:1; > + > + /* > + * Symbol usability - calculated as (visible != no || transitional). > + * Determines if symbol can be used in expressions. > + */ > + bool usable:1; > + > /* List of properties. See prop_type. */ > struct property *prop; > > diff --git a/scripts/kconfig/lexer.l b/scripts/kconfig/lexer.l > index 9c2cdfc33c6f..6d2c92c6095d 100644 > --- a/scripts/kconfig/lexer.l > +++ b/scripts/kconfig/lexer.l > @@ -126,6 +126,7 @@ n [A-Za-z0-9_-] > "select" return T_SELECT; > "source" return T_SOURCE; > "string" return T_STRING; > +"transitional" return T_TRANSITIONAL; > "tristate" return T_TRISTATE; > "visible" return T_VISIBLE; > "||" return T_OR; > diff --git a/scripts/kconfig/parser.y b/scripts/kconfig/parser.y > index e9c3c664e925..01d2d0f720ce 100644 > --- a/scripts/kconfig/parser.y > +++ b/scripts/kconfig/parser.y > @@ -75,6 +75,7 @@ struct menu *current_menu, *current_entry, *current_choice; > %token T_SELECT > %token T_SOURCE > %token T_STRING > +%token T_TRANSITIONAL > %token T_TRISTATE > %token T_VISIBLE > %token T_EOL > @@ -205,6 +206,16 @@ config_option: T_PROMPT T_WORD_QUOTE if_expr T_EOL > printd(DEBUG_PARSE, "%s:%d:prompt\n", cur_filename, cur_lineno); > }; > > +config_option: T_TRANSITIONAL type T_EOL > +{ > + if (current_entry->sym->type != S_UNKNOWN) > + yyerror("transitional type cannot be set after symbol type is already defined"); > + menu_set_type($2); > + current_entry->sym->transitional = true; > + printd(DEBUG_PARSE, "%s:%d:transitional(%u)\n", cur_filename, cur_lineno, > + $2); > +}; > + > config_option: default expr if_expr T_EOL > { > menu_add_expr(P_DEFAULT, $2, $3); > @@ -482,6 +493,43 @@ assign_val: > > %% > > +/** > + * transitional_check_sanity - check transitional symbols have no other > + * properties > + * > + * @menu: menu of the potentially transitional symbol > + * > + * Return: -1 if an error is found, 0 otherwise. > + */ > +static int transitional_check_sanity(const struct menu *menu) > +{ > + struct property *prop; > + > + if (!menu->sym || !menu->sym->transitional) > + return 0; > + > + /* Check for depends and visible conditions. */ > + if ((menu->dep && !expr_is_yes(menu->dep)) || > + (menu->visibility && !expr_is_yes(menu->visibility))) { > + fprintf(stderr, "%s:%d: error: %s", > + menu->filename, menu->lineno, > + "transitional symbols can only have help sections\n"); > + return -1; > + } > + > + /* Check for any property other than "help". */ > + for (prop = menu->sym->prop; prop; prop = prop->next) { > + if (prop->type != P_COMMENT) { > + fprintf(stderr, "%s:%d: error: %s", > + prop->filename, prop->lineno, > + "transitional symbols can only have help sections\n"); > + return -1; > + } > + } > + > + return 0; > +} > + > /** > * choice_check_sanity - check sanity of a choice member > * > @@ -558,6 +606,9 @@ void conf_parse(const char *name) > if (menu->sym && sym_check_deps(menu->sym)) > yynerrs++; > > + if (transitional_check_sanity(menu)) > + yynerrs++; > + > if (menu->sym && sym_is_choice(menu->sym)) { > menu_for_each_sub_entry(child, menu) > if (child->sym && choice_check_sanity(child)) > diff --git a/scripts/kconfig/symbol.c b/scripts/kconfig/symbol.c > index 26ab10c0fd76..b822c0c897e5 100644 > --- a/scripts/kconfig/symbol.c > +++ b/scripts/kconfig/symbol.c > @@ -447,6 +447,9 @@ void sym_calc_value(struct symbol *sym) > if (sym->visible != no) > sym->flags |= SYMBOL_WRITE; > > + /* Calculate usable flag */ > + sym->usable = (sym->visible != no || sym->transitional); > + > /* set default if recursively called */ > sym->curr = newval; > > @@ -459,13 +462,15 @@ void sym_calc_value(struct symbol *sym) > sym_calc_choice(choice_menu); > newval.tri = sym->curr.tri; > } else { > - if (sym->visible != no) { > + if (sym->usable) { > /* if the symbol is visible use the user value > * if available, otherwise try the default value > */ > if (sym_has_value(sym)) { > + tristate value = sym->transitional ? > + sym->def[S_DEF_USER].tri : sym->visible; > newval.tri = EXPR_AND(sym->def[S_DEF_USER].tri, > - sym->visible); > + value); > goto calc_newval; > } > } > @@ -497,7 +502,7 @@ void sym_calc_value(struct symbol *sym) > case S_STRING: > case S_HEX: > case S_INT: > - if (sym->visible != no && sym_has_value(sym)) { > + if (sym->usable && sym_has_value(sym)) { > newval.val = sym->def[S_DEF_USER].val; > break; > } > diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst > index a91abb8f6840..345c334ce680 100644 > --- a/Documentation/kbuild/kconfig-language.rst > +++ b/Documentation/kbuild/kconfig-language.rst > @@ -232,6 +232,37 @@ applicable everywhere (see syntax). > enables the third modular state for all config symbols. > At most one symbol may have the "modules" option set. > > +- transitional attribute: "transitional" > + This declares the symbol as transitional, meaning it should be processed > + during configuration but omitted from newly written .config files. > + Transitional symbols are useful for backward compatibility during config > + option migrations - they allow olddefconfig to process existing .config > + files while ensuring the old option doesn't appear in new configurations. > + > + A transitional symbol: > + - Has no prompt (is not visible to users in menus) > + - Is processed normally during configuration (values are read and used) > + - Can be referenced in default expressions of other symbols > + - Is not written to new .config files > + - Cannot have any other properties (it is a pass-through option) > + > + Example migration from OLD_NAME to NEW_NAME:: > + > + config NEW_NAME > + bool "New option name" > + default OLD_NAME > + help > + This replaces the old CONFIG_OLD_NAME option. > + > + config OLD_NAME > + transitional bool > + help > + Transitional config for OLD_NAME to NEW_NAME migration. > + > + With this setup, existing .config files with "CONFIG_OLD_NAME=y" will > + result in "CONFIG_NEW_NAME=y" being set, while CONFIG_OLD_NAME will be > + omitted from newly written .config files. > + > Menu dependencies > ----------------- -- Jani Nikula, Intel
