kconfig: Add transitional symbol attribute for migration support

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>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250923213422.1105654-2-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Author: kees

Documentation/kbuild/kconfig-language.rst

@@ -232,6 +232,38 @@ 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
+	bool
+	transitional
+	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
 -----------------
 

scripts/kconfig/expr.h

@@ -145,6 +145,7 @@ struct symbol {
 #define SYMBOL_CONST      0x0001  /* symbol is const */
 #define SYMBOL_CHECK      0x0008  /* used during dependency checking */
 #define SYMBOL_VALID      0x0080  /* set when symbol.curr is calculated */
+#define SYMBOL_TRANS      0x0100  /* symbol is transitional only (not visible)*/
 #define SYMBOL_WRITE      0x0200  /* write symbol to file (KCONFIG_CONFIG) */
 #define SYMBOL_WRITTEN    0x0800  /* track info to avoid double-write to .config */
 #define SYMBOL_CHECKED    0x2000  /* used during dependency checking */

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;

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 +206,12 @@ 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 T_EOL
+{
+	current_entry->sym->flags |= SYMBOL_TRANS;
+	printd(DEBUG_PARSE, "%s:%d:transitional\n", cur_filename, cur_lineno);
+};
+
 config_option: default expr if_expr T_EOL
 {
 	menu_add_expr(P_DEFAULT, $2, $3);
@@ -482,6 +489,43 @@ 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->flags & SYMBOL_TRANS))
+		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 +602,9 @@ 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))

scripts/kconfig/symbol.c

@@ -214,6 +214,11 @@ static void sym_calc_visibility(struct symbol *sym)
 	struct property *prop;
 	tristate tri;
 
+	if (sym->flags & SYMBOL_TRANS) {
+		sym->visible = yes;
+		return;
+	}
+
 	/* any prompt visible? */
 	tri = no;
 	for_all_prompts(sym, prop) {
@@ -526,7 +531,7 @@ void sym_calc_value(struct symbol *sym)
 		}
 	}
 
-	if (sym_is_choice(sym))
+	if (sym_is_choice(sym) || sym->flags & SYMBOL_TRANS)
 		sym->flags &= ~SYMBOL_WRITE;
 }
 

scripts/kconfig/tests/err_transitional/Kconfig

@@ -0,0 +1,52 @@
+# SPDX-License-Identifier: GPL-2.0
+# Test that transitional symbols cannot have properties other than help
+
+config BAD_DEFAULT
+	bool
+	transitional
+	default y
+	help
+	  This transitional symbol illegally has a default property.
+
+config BAD_PROMPT
+	bool
+	transitional
+	prompt "Bad prompt"
+	help
+	  This transitional symbol illegally has a prompt.
+
+config BAD_SELECT
+	bool
+	transitional
+	select OTHER_SYMBOL
+	help
+	  This transitional symbol illegally has a select.
+
+config BAD_IMPLY
+	bool
+	transitional
+	imply OTHER_SYMBOL
+	help
+	  This transitional symbol illegally has an imply.
+
+config BAD_DEPENDS
+	bool
+	transitional
+	depends on OTHER_SYMBOL
+	help
+	  This transitional symbol illegally has a depends.
+
+config BAD_RANGE
+	int
+	transitional
+	range 1 10
+	help
+	  This transitional symbol illegally has a range.
+
+config BAD_NO_TYPE
+	transitional
+	help
+	  This transitional symbol illegally has no type specified.
+
+config OTHER_SYMBOL
+	bool

scripts/kconfig/tests/err_transitional/__init__.py

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0
+"""
+Test that transitional symbols with invalid properties are rejected.
+
+Transitional symbols can only have help sections. Any other properties
+(default, select, depends, etc.) should cause a parser error.
+"""
+
+def test(conf):
+    # This should fail with exit code 1 due to invalid transitional symbol
+    assert conf.olddefconfig() == 1
+
+    # Check that the error message is about transitional symbols
+    assert conf.stderr_contains('expected_stderr')

scripts/kconfig/tests/err_transitional/expected_stderr

@@ -0,0 +1,7 @@
+Kconfig:46:warning: config symbol defined without type
+Kconfig:7: error: transitional symbols can only have help sections
+Kconfig:14: error: transitional symbols can only have help sections
+Kconfig:21: error: transitional symbols can only have help sections
+Kconfig:28: error: transitional symbols can only have help sections
+Kconfig:32: error: transitional symbols can only have help sections
+Kconfig:42: error: transitional symbols can only have help sections

scripts/kconfig/tests/transitional/Kconfig

@@ -0,0 +1,100 @@
+# SPDX-License-Identifier: GPL-2.0
+# Test transitional symbols for config migration with all Kconfig types
+
+# Enable module support for tristate testing
+config MODULES
+	bool "Enable loadable module support"
+	modules
+	default y
+
+# Basic migration tests for all types
+config NEW_BOOL
+	bool "New bool option"
+	default OLD_BOOL
+
+config OLD_BOOL
+	bool
+	transitional
+
+config NEW_TRISTATE
+	tristate "New tristate option"
+	default OLD_TRISTATE
+
+config OLD_TRISTATE
+	tristate
+	transitional
+
+config NEW_STRING
+	string "New string option"
+	default OLD_STRING
+
+config OLD_STRING
+	string
+	transitional
+
+config NEW_HEX
+	hex "New hex option"
+	default OLD_HEX
+
+config OLD_HEX
+	hex
+	transitional
+
+config NEW_INT
+	int "New int option"
+	default OLD_INT
+
+config OLD_INT
+	int
+	transitional
+
+# Precedence tests for all types
+config NEW_BOOL_PRECEDENCE
+	bool "New bool option with precedence"
+	default OLD_BOOL_PRECEDENCE
+
+config OLD_BOOL_PRECEDENCE
+	bool
+	transitional
+
+config NEW_STRING_PRECEDENCE
+	string "New string option with precedence"
+	default OLD_STRING_PRECEDENCE
+
+config OLD_STRING_PRECEDENCE
+	string
+	transitional
+
+config NEW_TRISTATE_PRECEDENCE
+	tristate "New tristate option with precedence"
+	default OLD_TRISTATE_PRECEDENCE
+
+config OLD_TRISTATE_PRECEDENCE
+	tristate
+	transitional
+
+config NEW_HEX_PRECEDENCE
+	hex "New hex option with precedence"
+	default OLD_HEX_PRECEDENCE
+
+config OLD_HEX_PRECEDENCE
+	hex
+	transitional
+
+config NEW_INT_PRECEDENCE
+	int "New int option with precedence"
+	default OLD_INT_PRECEDENCE
+
+config OLD_INT_PRECEDENCE
+	int
+	transitional
+
+# Test that help sections are allowed for transitional symbols
+config OLD_WITH_HELP
+	bool
+	transitional
+	help
+	  This transitional symbol has a help section to validate that help is allowed.
+
+config REGULAR_OPTION
+	bool "Regular option"

scripts/kconfig/tests/transitional/__init__.py

@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: GPL-2.0
+"""
+Test transitional symbol migration functionality for all Kconfig types.
+
+This tests that:
+- OLD_* options in existing .config cause NEW_* options to be set
+- OLD_* options are not written to the new .config file
+- NEW_* options appear in the new .config file with correct values
+- All Kconfig types work correctly: bool, tristate, string, hex, int
+- User-set NEW values take precedence over conflicting OLD transitional values
+"""
+
+def test(conf):
+    # Run olddefconfig to process the migration with the initial config
+    assert conf.olddefconfig(dot_config='initial_config') == 0
+
+    # Check that the configuration matches expected output
+    assert conf.config_contains('expected_config')