kconfig/menu.c: fix multiple references to expressions in menu_add_prop()

menu_add_prop() applies upper menus' visibilities to actual prompts
by AND-ing the prompts visibilities with the upper menus ones.

This creates a further reference to the menu's visibilities and when
the expression reduction functions do their work, they may remove or
modify expressions that have multiple references, thus causing
unpredictable side-effects.

The following example Kconfig constructs a case where this causes
problems: a menu and a prompt which's visibilities depend on the same
symbol.  When invoking mconf with this Kconfig and pressing "Z" we
see a problem caused by a free'd expression still referenced by the
menu's visibility:

------------------------------------------------------------------------
mainmenu "Kconfig Testing Configuration"

config VISIBLE
	def_bool n

config Placeholder
	bool "Place holder"

menu "Invisible"
	visible if VISIBLE

config TEST_VAR
	bool "Test option" if VISIBLE

endmenu
------------------------------------------------------------------------

This patch fixes this problem by creating copies of the menu's
visibility expressions before AND-ing them with the prompt's one.

Signed-off-by: Dirk Gouders <dirk@gouders.net>
[yann.morin.1998@free.fr: move variable into its block-scope,
                          keep lines <80 chars, typo]
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

scripts/kconfig/menu.c

@@ -146,11 +146,24 @@ struct property *menu_add_prop(enum prop_type type, char *prompt, struct expr *e
 			struct menu *menu = current_entry;
 
 			while ((menu = menu->parent) != NULL) {
+				struct expr *dup_expr;
+
 				if (!menu->visibility)
 					continue;
+				/*
+				 * Do not add a reference to the
+				 * menu's visibility expression but
+				 * use a copy of it.  Otherwise the
+				 * expression reduction functions
+				 * will modify expressions that have
+				 * multiple references which can
+				 * cause unwanted side effects.
+				 */
+				dup_expr = expr_copy(menu->visibility);
+
 				prop->visible.expr
 					= expr_alloc_and(prop->visible.expr,
-							 menu->visibility);
+							 dup_expr);
 			}
 		}