[PATCH] 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. When the symbol is not changeable, the code calls
printf("%s\n", def), which leads to a segmentation fault on certain
systems/libc implementations when passing a NULL pointer to %s.

This patch adds a check to ensure 'def' is not NULL before printing.
Additionally, it removes the redundant re-initialization of the 'line'
buffer inside the !sym_is_changeable(sym) block, as it is already
initialized at the beginning of the function.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Xingjing Deng <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..2771bc84e 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 ? 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 ? def : "");
 			return 0;
 		}
 		/* fall through */
-- 
2.25.1