2021-03-23 13:18:09

by Arnd Bergmann

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

From: Arnd Bergmann <[email protected]>

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 <[email protected]>
---
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

2021-03-23 13:42:32

by Rasmus Villemoes

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

On 23/03/2021 14.14, Arnd Bergmann wrote:
> From: Arnd Bergmann <[email protected]>
>
> overlapping input and output arguments to snprintf() are
> undefined behavior in C99:
>

Good luck:
https://lore.kernel.org/lkml/[email protected]/

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

2021-03-23 18:41:05

by Dmitry Torokhov

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

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 <[email protected]>
> >
> > overlapping input and output arguments to snprintf() are
> > undefined behavior in C99:
> >
>
> Good luck:
> https://lore.kernel.org/lkml/[email protected]/
>
> 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