[PATCH v2] kconfig/nconf: Replace strncpy() with snprintf()

[PATCH v2] kconfig/nconf: Replace strncpy() with snprintf()

[Shankari Anand]

I'm following up on my patch titled: [PATCH v2] kconfig/nconf: Replace strncpy() with snprintf() for safe string copy
Link: https://lore.kernel.org/linux-kbuild/CAPRMd3kWRHvUE=FuRoGBXYR1POCL05sec4-9u6_Pb667TBFaiA@mail.gmail.com/T/#t
I wanted to check whether this change is considered valid and applicable, or if there are any concerns about the direction or scope.

Why I have proposed this change -
1. item_add_str() in nconf.c: k_menu_items[index].str is a fixed-size char array. To copy the full string whilemaking sure it’s always null-terminated, snprintf() writes up to N-1 bytes + null terminator. strncpy() could leave the destination unterminated if tmp_str is too long which might be unsafe for new_item().
2. fill_window() in nconf.gui.c: The intent is to print at most len characters, clipped by the window width. Using snprintf() with precision (%.*s) ensures that we never read/write beyond bounds. With strncpy(), again, we risk lack of null termination unless manually handled (as it was done with tmp[len] = '\0';), which is brittle.
3. dialog_inputbox() in nconf.gui.c: result is a user-editable string buffer, pre-initialized with init. *result_len is the max buffer size. snprintf() will write up to result_len - 1 chars and null-terminate; strncpy() could fail to null-terminate if init is too long.

Thank you for your time and guidance.

Best regards,  
Shankari Anand (1):

[PATCH v2] kconfig/nconf: Replace strncpy() with snprintf()

[Shankari Anand]

Replace strncpy() calls with snprintf() in nconf.c and nconf.gui.c,
which are part of the userspace Kconfig frontend.
The affected functions are - item_add_str() in nconf.c, 
fill_window() and dialog_inputbox() in nconf.gui.c

As strscpy is not supported in userspace, snprintf can be used.

Signed-off-by: Shankari Anand <shankari.ak0208@gmail.com>
---
v1 -> v2 : Changed strscpy to snprintf as strscpy is not supported in user-space
---
 scripts/kconfig/nconf.c     | 6 +++---
 scripts/kconfig/nconf.gui.c | 5 ++---
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/scripts/kconfig/nconf.c b/scripts/kconfig/nconf.c
index 67ee33fe16ef..42a78f0baf16 100644
--- a/scripts/kconfig/nconf.c
+++ b/scripts/kconfig/nconf.c
@@ -589,9 +589,9 @@ static void item_add_str(const char *fmt, ...)
 	va_end(ap);
 	snprintf(tmp_str, sizeof(tmp_str), "%s%s",
 			k_menu_items[index].str, new_str);
-	strncpy(k_menu_items[index].str,
-		tmp_str,
-		sizeof(k_menu_items[index].str));
+	snprintf(k_menu_items[index].str,
+		sizeof(k_menu_items[index].str),
+		"%s", tmp_str);
 
 	free_item(curses_menu_items[index]);
 	curses_menu_items[index] = new_item(
diff --git a/scripts/kconfig/nconf.gui.c b/scripts/kconfig/nconf.gui.c
index a1306fafd07f..d55518268f45 100644
--- a/scripts/kconfig/nconf.gui.c
+++ b/scripts/kconfig/nconf.gui.c
@@ -176,8 +176,7 @@ void fill_window(WINDOW *win, const char *text)
 		char tmp[x+10];
 		const char *line = get_line(text, i);
 		int len = get_line_length(line);
-		strncpy(tmp, line, min(len, x));
-		tmp[len] = '\0';
+		snprintf(tmp, sizeof(tmp), "%.*s", min(len, x), line);
 		mvwprintw(win, i, 0, "%s", tmp);
 	}
 }
@@ -358,7 +357,7 @@ int dialog_inputbox(WINDOW *main_window,
 	y = (lines-win_lines)/2;
 	x = (columns-win_cols)/2;
 
-	strncpy(result, init, *result_len);
+	snprintf(result, *result_len, "%s", init);
 
 	/* create the windows */
 	win = newwin(win_lines, win_cols, y, x);
-- 
2.34.1

Re: [PATCH v2] kconfig/nconf: Replace strncpy() with snprintf()

[Masahiro Yamada]

On Thu, Jun 12, 2025 at 4:48 PM Shankari Anand
<shankari.ak0208@gmail.com> wrote:
>
> Replace strncpy() calls with snprintf() in nconf.c and nconf.gui.c,
> which are part of the userspace Kconfig frontend.
> The affected functions are - item_add_str() in nconf.c,
> fill_window() and dialog_inputbox() in nconf.gui.c

This is completely unnecessary information.


> As strscpy is not supported in userspace, snprintf can be used.

This one is not necessary either.


What you really need to explain in the commit description is:

strncpy() does not null-terminate the destination buffer if it is not
large enough to hold the entire source string. Make sure to explicitly
terminate the buffer.


And, I do not think you need to use snprintf() - just terminate
the buffer.

> Signed-off-by: Shankari Anand <shankari.ak0208@gmail.com>
> ---
> v1 -> v2 : Changed strscpy to snprintf as strscpy is not supported in user-space
> ---
>  scripts/kconfig/nconf.c     | 6 +++---
>  scripts/kconfig/nconf.gui.c | 5 ++---
>  2 files changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/scripts/kconfig/nconf.c b/scripts/kconfig/nconf.c
> index 67ee33fe16ef..42a78f0baf16 100644
> --- a/scripts/kconfig/nconf.c
> +++ b/scripts/kconfig/nconf.c
> @@ -589,9 +589,9 @@ static void item_add_str(const char *fmt, ...)
>         va_end(ap);
>         snprintf(tmp_str, sizeof(tmp_str), "%s%s",
>                         k_menu_items[index].str, new_str);
> -       strncpy(k_menu_items[index].str,
> -               tmp_str,
> -               sizeof(k_menu_items[index].str));
> +       snprintf(k_menu_items[index].str,
> +               sizeof(k_menu_items[index].str),
> +               "%s", tmp_str);
>
>         free_item(curses_menu_items[index]);
>         curses_menu_items[index] = new_item(
> diff --git a/scripts/kconfig/nconf.gui.c b/scripts/kconfig/nconf.gui.c
> index a1306fafd07f..d55518268f45 100644
> --- a/scripts/kconfig/nconf.gui.c
> +++ b/scripts/kconfig/nconf.gui.c
> @@ -176,8 +176,7 @@ void fill_window(WINDOW *win, const char *text)
>                 char tmp[x+10];
>                 const char *line = get_line(text, i);
>                 int len = get_line_length(line);
> -               strncpy(tmp, line, min(len, x));
> -               tmp[len] = '\0';

This one can be fixed to

     tmp[sizeof(tmp) - 1] = '\0'



> +               snprintf(tmp, sizeof(tmp), "%.*s", min(len, x), line);
>                 mvwprintw(win, i, 0, "%s", tmp);
>         }
>  }
> @@ -358,7 +357,7 @@ int dialog_inputbox(WINDOW *main_window,
>         y = (lines-win_lines)/2;
>         x = (columns-win_cols)/2;
>
> -       strncpy(result, init, *result_len);
> +       snprintf(result, *result_len, "%s", init);
>
>         /* create the windows */
>         win = newwin(win_lines, win_cols, y, x);
> --
> 2.34.1
>


-- 
Best Regards
Masahiro Yamada