kconfig: add warn-unknown-symbols sanity check

Introduce KCONFIG_WARN_UNKNOWN_SYMBOLS environment variable,
which makes Kconfig warn about unknown config symbols.

This is especially useful for continuous kernel uprevs when
some symbols can be either removed or renamed between kernel
releases (which can go unnoticed otherwise).

By default KCONFIG_WARN_UNKNOWN_SYMBOLS generates warnings,
which are non-terminal. There is an additional environment
variable KCONFIG_WERROR that overrides this behaviour and
turns warnings into errors.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/kconfig.rst

@@ -54,6 +54,15 @@ KCONFIG_OVERWRITECONFIG
 If you set KCONFIG_OVERWRITECONFIG in the environment, Kconfig will not
 break symlinks when .config is a symlink to somewhere else.
 
+KCONFIG_WARN_UNKNOWN_SYMBOLS
+----------------------------
+This environment variable makes Kconfig warn about all unrecognized
+symbols in the config input.
+
+KCONFIG_WERROR
+--------------
+If set, Kconfig treats warnings as errors.
+
 `CONFIG_`
 ---------
 If you set `CONFIG_` in the environment, Kconfig will prefix all symbols

scripts/kconfig/confdata.c

@@ -349,7 +349,11 @@ int conf_read_simple(const char *name, int def)
 	char *p, *p2;
 	struct symbol *sym;
 	int i, def_flags;
+	const char *warn_unknown;
+	const char *werror;
 
+	warn_unknown = getenv("KCONFIG_WARN_UNKNOWN_SYMBOLS");
+	werror = getenv("KCONFIG_WERROR");
 	if (name) {
 		in = zconf_fopen(name);
 	} else {
@@ -437,6 +441,10 @@ load:
 			if (def == S_DEF_USER) {
 				sym = sym_find(line + 2 + strlen(CONFIG_));
 				if (!sym) {
+					if (warn_unknown)
+						conf_warning("unknown symbol: %s",
+							     line + 2 + strlen(CONFIG_));
+
 					conf_set_changed(true);
 					continue;
 				}
@@ -471,7 +479,7 @@ load:
 
 			sym = sym_find(line + strlen(CONFIG_));
 			if (!sym) {
-				if (def == S_DEF_AUTO)
+				if (def == S_DEF_AUTO) {
 					/*
 					 * Reading from include/config/auto.conf
 					 * If CONFIG_FOO previously existed in
@@ -479,8 +487,13 @@ load:
 					 * include/config/FOO must be touched.
 					 */
 					conf_touch_dep(line + strlen(CONFIG_));
-				else
+				} else {
+					if (warn_unknown)
+						conf_warning("unknown symbol: %s",
+							     line + strlen(CONFIG_));
+
 					conf_set_changed(true);
+				}
 				continue;
 			}
 
@@ -519,6 +532,10 @@ load:
 	}
 	free(line);
 	fclose(in);
+
+	if (conf_warnings && werror)
+		exit(1);
+
 	return 0;
 }