snprintf() returns the "number of characters which *would* be generated for
the given input", not the size *really* generated.

In order to avoid too large values for 'len' (and potential negative
values for "sizeof(buf) - (len - 1)") use scnprintf() instead of
snprintf().

Fixes: c6e3fd22cd53 ("Staging: add speakup to the staging directory")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
I guess that the -1 in the 2nd snprintf() call is here to overwrite the
ending \n.
---
 drivers/accessibility/speakup/kobjects.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/accessibility/speakup/kobjects.c b/drivers/accessibility/speakup/kobjects.c
index a7522d409802..1eea7c492875 100644
--- a/drivers/accessibility/speakup/kobjects.c
+++ b/drivers/accessibility/speakup/kobjects.c
@@ -92,9 +92,9 @@ static void report_char_chartab_status(int reset, int received, int used,
 	if (reset) {
 		pr_info("%s reset to defaults\n", object_type[do_characters]);
 	} else if (received) {
-		len = snprintf(buf, sizeof(buf),
-			       " updated %d of %d %s\n",
-			       used, received, object_type[do_characters]);
+		len = scnprintf(buf, sizeof(buf),
+				" updated %d of %d %s\n",
+				used, received, object_type[do_characters]);
 		if (rejected)
 			snprintf(buf + (len - 1), sizeof(buf) - (len - 1),
 				 " with %d reject%s\n",
-- 
2.34.1

On Mon, Sep 04, 2023 at 09:31:44PM +0200, Christophe JAILLET wrote:
> snprintf() returns the "number of characters which *would* be generated for
> the given input", not the size *really* generated.
> 
> In order to avoid too large values for 'len' (and potential negative
> values for "sizeof(buf) - (len - 1)") use scnprintf() instead of
> snprintf().
> 

A bunch of kernel-janitors mail didn't reach my inbox...  Like this
one.  So weird.

Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>

The impact of this bug is a WARN() btw.  snprintf() will refuse to
print negative bytes.

regards,
dan carpenter

Christophe JAILLET, le lun. 04 sept. 2023 21:31:44 +0200, a ecrit:
> snprintf() returns the "number of characters which *would* be generated for
> the given input", not the size *really* generated.
> 
> In order to avoid too large values for 'len' (and potential negative
> values for "sizeof(buf) - (len - 1)") use scnprintf() instead of
> snprintf().
> 
> Fixes: c6e3fd22cd53 ("Staging: add speakup to the staging directory")
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>

Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

Thanks!

> ---
> I guess that the -1 in the 2nd snprintf() call is here to overwrite the
> ending \n.
> ---
>  drivers/accessibility/speakup/kobjects.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/accessibility/speakup/kobjects.c b/drivers/accessibility/speakup/kobjects.c
> index a7522d409802..1eea7c492875 100644
> --- a/drivers/accessibility/speakup/kobjects.c
> +++ b/drivers/accessibility/speakup/kobjects.c
> @@ -92,9 +92,9 @@ static void report_char_chartab_status(int reset, int received, int used,
>  	if (reset) {
>  		pr_info("%s reset to defaults\n", object_type[do_characters]);
>  	} else if (received) {
> -		len = snprintf(buf, sizeof(buf),
> -			       " updated %d of %d %s\n",
> -			       used, received, object_type[do_characters]);
> +		len = scnprintf(buf, sizeof(buf),
> +				" updated %d of %d %s\n",
> +				used, received, object_type[do_characters]);
>  		if (rejected)
>  			snprintf(buf + (len - 1), sizeof(buf) - (len - 1),
>  				 " with %d reject%s\n",
> -- 
> 2.34.1