kconfig: add optional warnings for changed input values

When reading .config input, Kconfig stores user-provided values first
and then resolves the final value after applying dependencies, ranges,
and other constraints.

If the final value differs from the user input, Kconfig already tracks
that state internally, but it does not provide a focused diagnostic to
show which explicit inputs were adjusted. This is particularly confusing
for requested values that get forced down by unmet dependencies or
clamped by ranges.

Add an opt-in diagnostic controlled by KCONFIG_WARN_CHANGED_INPUT. Emit
the warnings from conf_write() and conf_write_defconfig() after value
resolution. Print the diagnostic to stderr directly, not through the
normal message callback, so it remains visible when conf is run with -s,
such as from make -s.

Keep the diagnostic out of the conf_message() formatting buffer so long
warning lists are not truncated, and mark processed symbols as written
before the SYMBOL_WRITE check so duplicate menu nodes cannot emit
duplicate warnings.

Document the new environment variable and add tests for olddefconfig,
savedefconfig, and the silent-conf path.

Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Tested-by: Julian Braha <julianbraha@gmail.com>
Link: https://patch.msgid.link/20260611060000.23858-1-pengpeng@iscas.ac.cn
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

1 files changed, 102 insertions, 4 deletions

diff --git a/scripts/kconfig/confdata.c b/scripts/kconfig/confdata.c
index 9599a0408862..4234a51d16fd 100644
--- a/scripts/kconfig/confdata.c
+++ b/scripts/kconfig/confdata.c
@@ -206,6 +206,78 @@ static void conf_message(const char *fmt, ...)
 	va_end(ap);
 }
 
+static void conf_changed_input_warning(const char *s)
+{
+	fputs(s, stderr);
+}
+
+static bool conf_warn_changed_input_enabled(void)
+{
+	const char *env = getenv("KCONFIG_WARN_CHANGED_INPUT");
+
+	return env && *env;
+}
+
+static const char *sym_get_user_value_string(struct symbol *sym)
+{
+	switch (sym->type) {
+	case S_BOOLEAN:
+	case S_TRISTATE:
+		switch (sym->def[S_DEF_USER].tri) {
+		case yes:
+			return "y";
+		case mod:
+			return "m";
+		default:
+			return "n";
+		}
+	default:
+		return sym->def[S_DEF_USER].val ?: "";
+	}
+}
+
+static bool sym_user_value_changed(struct symbol *sym)
+{
+	if (!sym_has_value(sym) || sym->type == S_UNKNOWN)
+		return false;
+
+	switch (sym->type) {
+	case S_BOOLEAN:
+	case S_TRISTATE:
+		return sym->def[S_DEF_USER].tri != sym_get_tristate_value(sym);
+	default:
+		return strcmp(sym_get_user_value_string(sym),
+			      sym_get_string_value(sym));
+	}
+}
+
+static void conf_clear_written_flags(void)
+{
+	struct symbol *sym;
+
+	for_all_symbols(sym)
+		sym->flags &= ~SYMBOL_WRITTEN;
+}
+
+static void conf_append_changed_input_warning(struct gstr *gs,
+					      struct symbol *sym,
+					      bool *changed_input_found)
+{
+	if (!sym_user_value_changed(sym))
+		return;
+
+	if (!*changed_input_found) {
+		str_printf(gs,
+			   "warning: user-provided values changed by Kconfig:\n");
+		*changed_input_found = true;
+	}
+
+	str_printf(gs, "  %s%s: %s -> %s\n",
+		   CONFIG_, sym->name,
+		   sym_get_user_value_string(sym),
+		   sym_get_string_value(sym));
+}
+
 const char *conf_get_configname(void)
 {
 	char *name = getenv("KCONFIG_CONFIG");
@@ -759,11 +831,15 @@ int conf_write_defconfig(const char *filename)
 {
 	struct symbol *sym;
 	struct menu *menu;
+	struct gstr gs;
 	FILE *out;
+	bool warn_changed_input = conf_warn_changed_input_enabled();
+	bool changed_input_found = false;
 
 	out = fopen(filename, "w");
 	if (!out)
 		return 1;
+	gs = str_new();
 
 	sym_clear_all_valid();
 
@@ -772,10 +848,14 @@ int conf_write_defconfig(const char *filename)
 
 		sym = menu->sym;
 
-		if (!sym || sym_is_choice(sym))
+		if (!sym || sym_is_choice(sym) || sym->flags & SYMBOL_WRITTEN)
 			continue;
 
 		sym_calc_value(sym);
+		if (warn_changed_input)
+			conf_append_changed_input_warning(&gs, sym,
+							  &changed_input_found);
+		sym->flags |= SYMBOL_WRITTEN;
 		if (!(sym->flags & SYMBOL_WRITE))
 			continue;
 		sym->flags &= ~SYMBOL_WRITE;
@@ -798,6 +878,13 @@ int conf_write_defconfig(const char *filename)
 		print_symbol_for_dotconfig(out, sym);
 	}
 	fclose(out);
+
+	conf_clear_written_flags();
+
+	if (changed_input_found)
+		conf_changed_input_warning(str_get(&gs));
+
+	str_free(&gs);
 	return 0;
 }
 
@@ -809,7 +896,10 @@ int conf_write(const char *name)
 	const char *str;
 	char tmpname[PATH_MAX + 1], oldname[PATH_MAX + 1];
 	char *env;
+	struct gstr gs;
 	bool need_newline = false;
+	bool warn_changed_input = conf_warn_changed_input_enabled();
+	bool changed_input_found = false;
 
 	if (!name)
 		name = conf_get_configname();
@@ -838,6 +928,7 @@ int conf_write(const char *name)
 	}
 	if (!out)
 		return 1;
+	gs = str_new();
 
 	conf_write_heading(out, &comment_style_pound);
 
@@ -859,13 +950,16 @@ int conf_write(const char *name)
 		} else if (!sym_is_choice(sym) &&
 			   !(sym->flags & SYMBOL_WRITTEN)) {
 			sym_calc_value(sym);
+			if (warn_changed_input)
+				conf_append_changed_input_warning(&gs, sym,
+								  &changed_input_found);
+			sym->flags |= SYMBOL_WRITTEN;
 			if (!(sym->flags & SYMBOL_WRITE))
 				goto next;
 			if (need_newline) {
 				fprintf(out, "\n");
 				need_newline = false;
 			}
-			sym->flags |= SYMBOL_WRITTEN;
 			print_symbol_for_dotconfig(out, sym);
 		}
 
@@ -892,8 +986,12 @@ end_check:
 	}
 	fclose(out);
 
-	for_all_symbols(sym)
-		sym->flags &= ~SYMBOL_WRITTEN;
+	conf_clear_written_flags();
+
+	if (changed_input_found)
+		conf_changed_input_warning(str_get(&gs));
+
+	str_free(&gs);
 
 	if (*tmpname) {
 		if (is_same(name, tmpname)) {