[PATCH v3] kconfig: fix potential NULL pointer dereference in conf_askvalue

[Xingjing Deng]

In conf_askvalue(), the 'def' argument (retrieved via sym_get_string_value)
can be NULL. While current call sites ensure that 'def' is valid,
calling printf("%s\n", def) is technically undefined behavior and could
lead to a segmentation fault on certain libc implementations if the
function were called with a NULL pointer in the future.

Improve the robustness of conf_askvalue() by providing an empty string
as a fallback.

Additionally, remove the redundant re-initialization of the 'line'
buffer inside the !sym_is_changeable(sym) block, as it is already
properly initialized at the function entry.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Xingjing Deng <micro6947@xxxxxxxxx>
Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
v3:
- fix compile errors.
- Link to v2: https://lore.kernel.org/all/20260301053035.1950087-1-micro6947@xxxxxxxxx/

v2:
- Change commit message and use fewer characters in modify.
- Link to v1: https://lore.kernel.org/all/20260225072246.3475275-1-micro6947@xxxxxxxxx/

 scripts/kconfig/conf.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/scripts/kconfig/conf.c b/scripts/kconfig/conf.c
index a7b44cd8a..c368bec5a 100644
--- a/scripts/kconfig/conf.c
+++ b/scripts/kconfig/conf.c
@@ -297,9 +297,7 @@ static int conf_askvalue(struct symbol *sym, const char *def)
 	line[1] = 0;
 
 	if (!sym_is_changeable(sym)) {
-		printf("%s\n", def);
-		line[0] = '\n';
-		line[1] = 0;
+		printf("%s\n", def ?: "");
 		return 0;
 	}
 
@@ -307,7 +305,7 @@ static int conf_askvalue(struct symbol *sym, const char *def)
 	case oldconfig:
 	case syncconfig:
 		if (sym_has_value(sym)) {
-			printf("%s\n", def);
+			printf("%s\n", def ?: "");
 			return 0;
 		}
 		/* fall through */
-- 
2.25.1