[PATCH] Input: analog - fix invalid snprintf() call

[PATCH] Input: analog - fix invalid snprintf() call

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

overlapping input and output arguments to snprintf() are
undefined behavior in C99:

drivers/input/joystick/analog.c: In function 'analog_name':
drivers/input/joystick/analog.c:428:3: error: 'snprintf' argument 4 overlaps destination object 'analog' [-Werror=restrict]
  428 |   snprintf(analog->name, sizeof(analog->name), "%s %d-hat",
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  429 |     analog->name, hweight16(analog->mask & ANALOG_HATS_ALL));
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/input/joystick/analog.c:420:40: note: destination object referenced by 'restrict'-qualified argument 1 was declared here
  420 | static void analog_name(struct analog *analog)
      |                         ~~~~~~~~~~~~~~~^~~~~~

Change this function to just use the offset it already knows.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/input/joystick/analog.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/input/joystick/analog.c b/drivers/input/joystick/analog.c
index f798922a4598..8c9fed3f13e2 100644
--- a/drivers/input/joystick/analog.c
+++ b/drivers/input/joystick/analog.c
@@ -419,14 +419,16 @@ static void analog_calibrate_timer(struct analog_port *port)
 
 static void analog_name(struct analog *analog)
 {
-	snprintf(analog->name, sizeof(analog->name), "Analog %d-axis %d-button",
+	int len;
+
+	len = snprintf(analog->name, sizeof(analog->name), "Analog %d-axis %d-button",
 		 hweight8(analog->mask & ANALOG_AXES_STD),
 		 hweight8(analog->mask & ANALOG_BTNS_STD) + !!(analog->mask & ANALOG_BTNS_CHF) * 2 +
 		 hweight16(analog->mask & ANALOG_BTNS_GAMEPAD) + !!(analog->mask & ANALOG_HBTN_CHF) * 4);
 
 	if (analog->mask & ANALOG_HATS_ALL)
-		snprintf(analog->name, sizeof(analog->name), "%s %d-hat",
-			 analog->name, hweight16(analog->mask & ANALOG_HATS_ALL));
+		len += snprintf(analog->name + len, sizeof(analog->name) - len, "%d-hat",
+			 hweight16(analog->mask & ANALOG_HATS_ALL));
 
 	if (analog->mask & ANALOG_HAT_FCS)
 		strlcat(analog->name, " FCS", sizeof(analog->name));
-- 
2.29.2

Re: [PATCH] Input: analog - fix invalid snprintf() call

[Rasmus Villemoes]

On 23/03/2021 14.14, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> overlapping input and output arguments to snprintf() are
> undefined behavior in C99:
> 

Good luck:
https://lore.kernel.org/lkml/1457469654-17059-1-git-send-email-linux@rasmusvillemoes.dk/

At least 5 years ago the consensus from old-timers was that "the
kernel's snprintf supports this use case, just keep it working that way".

> diff --git a/drivers/input/joystick/analog.c b/drivers/input/joystick/analog.c
> index f798922a4598..8c9fed3f13e2 100644
> --- a/drivers/input/joystick/analog.c
> +++ b/drivers/input/joystick/analog.c
> @@ -419,14 +419,16 @@ static void analog_calibrate_timer(struct analog_port *port)
>  
>  static void analog_name(struct analog *analog)
>  {
> -	snprintf(analog->name, sizeof(analog->name), "Analog %d-axis %d-button",
> +	int len;
> +
> +	len = snprintf(analog->name, sizeof(analog->name), "Analog %d-axis %d-button",
>  		 hweight8(analog->mask & ANALOG_AXES_STD),
>  		 hweight8(analog->mask & ANALOG_BTNS_STD) + !!(analog->mask & ANALOG_BTNS_CHF) * 2 +
>  		 hweight16(analog->mask & ANALOG_BTNS_GAMEPAD) + !!(analog->mask & ANALOG_HBTN_CHF) * 4);
>  
>  	if (analog->mask & ANALOG_HATS_ALL)
> -		snprintf(analog->name, sizeof(analog->name), "%s %d-hat",
> -			 analog->name, hweight16(analog->mask & ANALOG_HATS_ALL));
> +		len += snprintf(analog->name + len, sizeof(analog->name) - len, "%d-hat",
> +			 hweight16(analog->mask & ANALOG_HATS_ALL));

Use scnprintf, this is too fragile and hard to verify. If the first
snprintf overflows, the second passes a huge size_t to snprintf which
will WARN.

Rasmus

Re: [PATCH] Input: analog - fix invalid snprintf() call

[Dmitry Torokhov]

On Tue, Mar 23, 2021 at 02:29:15PM +0100, Rasmus Villemoes wrote:
> On 23/03/2021 14.14, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@arndb.de>
> > 
> > overlapping input and output arguments to snprintf() are
> > undefined behavior in C99:
> > 
> 
> Good luck:
> https://lore.kernel.org/lkml/1457469654-17059-1-git-send-email-linux@rasmusvillemoes.dk/
> 
> At least 5 years ago the consensus from old-timers was that "the
> kernel's snprintf supports this use case, just keep it working that way".
> 
> > diff --git a/drivers/input/joystick/analog.c b/drivers/input/joystick/analog.c
> > index f798922a4598..8c9fed3f13e2 100644
> > --- a/drivers/input/joystick/analog.c
> > +++ b/drivers/input/joystick/analog.c
> > @@ -419,14 +419,16 @@ static void analog_calibrate_timer(struct analog_port *port)
> >  
> >  static void analog_name(struct analog *analog)
> >  {
> > -	snprintf(analog->name, sizeof(analog->name), "Analog %d-axis %d-button",
> > +	int len;
> > +
> > +	len = snprintf(analog->name, sizeof(analog->name), "Analog %d-axis %d-button",
> >  		 hweight8(analog->mask & ANALOG_AXES_STD),
> >  		 hweight8(analog->mask & ANALOG_BTNS_STD) + !!(analog->mask & ANALOG_BTNS_CHF) * 2 +
> >  		 hweight16(analog->mask & ANALOG_BTNS_GAMEPAD) + !!(analog->mask & ANALOG_HBTN_CHF) * 4);
> >  
> >  	if (analog->mask & ANALOG_HATS_ALL)
> > -		snprintf(analog->name, sizeof(analog->name), "%s %d-hat",
> > -			 analog->name, hweight16(analog->mask & ANALOG_HATS_ALL));
> > +		len += snprintf(analog->name + len, sizeof(analog->name) - len, "%d-hat",
> > +			 hweight16(analog->mask & ANALOG_HATS_ALL));
> 
> Use scnprintf, this is too fragile and hard to verify. If the first
> snprintf overflows, the second passes a huge size_t to snprintf which
> will WARN.

Also the format needs to be " %d-hat" (note the leading space).

Thanks.

-- 
Dmitry