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

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Feb 25, 2026 at 07:22:46AM +0000, Xingjing Deng wrote:
> 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.

How do you reproduce this segmentation fault? Surely someone would have
hit this if it were a real problem given the Fixes tag? Or is this a
corner case?

> 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@gmail.com>
> ---
>  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
>