[PATCH] Input: focaltech - use signed coordinates to prevent underflow

[PATCH] Input: focaltech - use signed coordinates to prevent underflow

[Dmitry Torokhov]

focaltech_finger_state stores finger coordinates x and y as unsigned
int. When processing relative packets, negative deltas can cause
unsigned integer underflow if the finger moves past the left or bottom
boundary of the touchpad, wrapping the coordinates to values near
UINT_MAX.

When clamping the coordinates in focaltech_report_state(), these
underflowed values are clamped against priv->x_max / priv->y_max instead
of 0, causing the cursor to jump erratically to the opposite edge of the
touchpad.

Change the coordinate variables and limits to signed int so that
negative values resulting from relative movements clamp correctly to 0.

Fixes: 05be1d079ec0 ("Input: psmouse - support for the FocalTech PS/2 protocol extensions")
Reported-by: sashiko-bot@kernel.org
Assisted-by: Antigravity:gemini-3.6-flash
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---
 drivers/input/mouse/focaltech.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/input/mouse/focaltech.c b/drivers/input/mouse/focaltech.c
index d3ad4af5aa09..c6f6540e3e29 100644
--- a/drivers/input/mouse/focaltech.c
+++ b/drivers/input/mouse/focaltech.c
@@ -78,8 +78,8 @@ struct focaltech_finger_state {
 	 * Absolute position (from the bottom left corner) of the
 	 * finger.
 	 */
-	unsigned int x;
-	unsigned int y;
+	int x;
+	int y;
 };
 
 /*
@@ -108,7 +108,7 @@ struct focaltech_hw_state {
 };
 
 struct focaltech_data {
-	unsigned int x_max, y_max;
+	int x_max, y_max;
 	struct focaltech_hw_state state;
 };
 
@@ -126,14 +126,14 @@ static void focaltech_report_state(struct psmouse *psmouse)
 		input_mt_slot(dev, i);
 		input_mt_report_slot_state(dev, MT_TOOL_FINGER, active);
 		if (active) {
-			unsigned int clamped_x, clamped_y;
+			int clamped_x, clamped_y;
 			/*
 			 * The touchpad might report invalid data, so we clamp
 			 * the resulting values so that we do not confuse
 			 * userspace.
 			 */
-			clamped_x = clamp(finger->x, 0U, priv->x_max);
-			clamped_y = clamp(finger->y, 0U, priv->y_max);
+			clamped_x = clamp(finger->x, 0, priv->x_max);
+			clamped_y = clamp(finger->y, 0, priv->y_max);
 			input_report_abs(dev, ABS_MT_POSITION_X, clamped_x);
 			input_report_abs(dev, ABS_MT_POSITION_Y,
 					 priv->y_max - clamped_y);
-- 
2.55.0.508.g3f0d502094-goog


-- 
Dmitry

Re: [PATCH] Input: focaltech - use signed coordinates to prevent underflow

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

New issues:
- [High] The patch changes internal coordinates to signed integers to prevent underflow jumps, but fails to write the clamped coordinates back to the internal state, creating a wind-up bug at the screen edges.

Pre-existing issues:
- [Critical] A signedness bug in `focaltech_process_rel_packet` allows an unused finger index to evaluate to -1 and bypass bounds checks, causing an out-of-bounds array write that silently corrupts the touchpad's physical bounds limits.
--

--- Patch [1]: [PATCH] Input: focaltech - use signed coordinates to prevent underflow ---
commit e1624dd1b0cb6c1665dcbaeda6430fe12e1a62aa
Author: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Input: focaltech - use signed coordinates to prevent underflow

This commit changes the internal finger coordinates from unsigned to signed
integers to prevent underflow when processing relative packets. It updates
the variables in focaltech_finger_state and the clamping limits in
focaltech_data to avoid erratic cursor jumps when moving past the touchpad
boundaries.

> diff --git a/drivers/input/mouse/focaltech.c b/drivers/input/mouse/focaltech.c
> index 43f9939b7c63c..4c909c4ca76c9 100644
> --- a/drivers/input/mouse/focaltech.c
> +++ b/drivers/input/mouse/focaltech.c

[ ... ]

> @@ -126,14 +126,14 @@ static void focaltech_report_state(struct psmouse *psmouse)
>  		input_mt_slot(dev, i);
>  		input_mt_report_slot_state(dev, MT_TOOL_FINGER, active);
>  		if (active) {
> -			unsigned int clamped_x, clamped_y;
> +			int clamped_x, clamped_y;
>  			/*
>  			 * The touchpad might report invalid data, so we clamp
>  			 * the resulting values so that we do not confuse
>  			 * userspace.
>  			 */
> -			clamped_x = clamp(finger->x, 0U, priv->x_max);
> -			clamped_y = clamp(finger->y, 0U, priv->y_max);
> +			clamped_x = clamp(finger->x, 0, priv->x_max);
> +			clamped_y = clamp(finger->y, 0, priv->y_max);

[Severity: High]
Does changing these coordinates to signed integers without updating the
underlying state create a wind-up effect at the screen edges?

When swiping past the left or bottom edge, finger->x or finger->y will now
be driven into negative values. Because the code clamps these values for
reporting but does not write the bounded coordinates back to finger->x or
finger->y, the out-of-bounds coordinates can accumulate wind-up.

Would the user then have to swipe a significant distance in the opposite
direction just to offset this accumulated negative value before the cursor
visibly moves away from the edge?

>  			input_report_abs(dev, ABS_MT_POSITION_X, clamped_x);
>  			input_report_abs(dev, ABS_MT_POSITION_Y,

[Severity: Critical]
This is a pre-existing issue, but does this out-of-bounds array write in
focaltech_process_rel_packet() explain the original erratic jumping behavior
the patch aims to fix?

When an odd number of fingers touches the pad, the hardware reports unused
fingers with an index of 0. In focaltech_process_rel_packet(), the index is
computed and checked like this:

drivers/input/mouse/focaltech.c:focaltech_process_rel_packet() {
    ...
    finger2 = ((packet[3] >> 4) & 0x7) - 1;
    if (finger2 < FOC_MAX_FINGERS) {
        state->fingers[finger2].x += (s8)packet[4];
    ...
}

Because finger2 is a signed int, an unused index of 0 evaluates to -1.
The bounds check -1 < FOC_MAX_FINGERS incorrectly evaluates to true,
bypassing the safety check. The exact same signedness flaw exists for
finger1.

Due to the struct layout in focaltech_data, accessing state->fingers[-1]
offsets backwards by exactly 12 bytes. This perfectly aliases
state->fingers[-1].x to priv->x_max and state->fingers[-1].y to priv->y_max.

Could this out-of-bounds write be silently corrupting the touchpad physical
bounds limits, continuously adding relative movement deltas to the device's
maximum coordinate limits and causing the cursor to jump erratically?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/am_tH_F938rK6ask@google.com?part=1

Re: [PATCH] Input: focaltech - use signed coordinates to prevent underflow

[Richard Davies]

Dmitry Torokhov wrote:
>focaltech_finger_state stores finger coordinates x and y as unsigned
>int. When processing relative packets, negative deltas can cause
>unsigned integer underflow if the finger moves past the left or bottom
>boundary of the touchpad, wrapping the coordinates to values near
>UINT_MAX.
>
>When clamping the coordinates in focaltech_report_state(), these
>underflowed values are clamped against priv->x_max / priv->y_max instead
>of 0, causing the cursor to jump erratically to the opposite edge of the
>touchpad.

I agree this is theoretically possible. For what it's worth, I haven't 
actually experienced the cursor jumping to the opposite edge of the screen 
on my laptop. That might mean the relative packets are actually safe, e.g. 
generated internally from an absolute position.

>Change the coordinate variables and limits to signed int so that
>negative values resulting from relative movements clamp correctly to 0.
>
>Fixes: 05be1d079ec0 ("Input: psmouse - support for the FocalTech PS/2 protocol extensions")
>Reported-by: sashiko-bot@kernel.org
>Assisted-by: Antigravity:gemini-3.6-flash
>Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
>---
> drivers/input/mouse/focaltech.c | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
>diff --git a/drivers/input/mouse/focaltech.c b/drivers/input/mouse/focaltech.c
>index d3ad4af5aa09..c6f6540e3e29 100644
>--- a/drivers/input/mouse/focaltech.c
>+++ b/drivers/input/mouse/focaltech.c
>@@ -78,8 +78,8 @@ struct focaltech_finger_state {
> 	 * Absolute position (from the bottom left corner) of the
> 	 * finger.
> 	 */
>-	unsigned int x;
>-	unsigned int y;
>+	int x;
>+	int y;
> };
>
> /*
>@@ -108,7 +108,7 @@ struct focaltech_hw_state {
> };
>
> struct focaltech_data {
>-	unsigned int x_max, y_max;
>+	int x_max, y_max;
> 	struct focaltech_hw_state state;
> };
>
>@@ -126,14 +126,14 @@ static void focaltech_report_state(struct psmouse *psmouse)
> 		input_mt_slot(dev, i);
> 		input_mt_report_slot_state(dev, MT_TOOL_FINGER, active);
> 		if (active) {
>-			unsigned int clamped_x, clamped_y;
>+			int clamped_x, clamped_y;
> 			/*
> 			 * The touchpad might report invalid data, so we clamp
> 			 * the resulting values so that we do not confuse
> 			 * userspace.
> 			 */
>-			clamped_x = clamp(finger->x, 0U, priv->x_max);
>-			clamped_y = clamp(finger->y, 0U, priv->y_max);
>+			clamped_x = clamp(finger->x, 0, priv->x_max);
>+			clamped_y = clamp(finger->y, 0, priv->y_max);
> 			input_report_abs(dev, ABS_MT_POSITION_X, clamped_x);
> 			input_report_abs(dev, ABS_MT_POSITION_Y,
> 					 priv->y_max - clamped_y);

If it's allowed and race-free to change finger->{x,y} in 
focaltech_report_state, then you might fix the Sashiko [High] issue on this 
patch by just doing:

finger->x = clamp(finger->x, 0, priv->x_max);
finger->y = clamp(finger->y, 0, priv->y_max);

not having the clamped_{x,y} variables at all, and updating the two 
input_report_abs lines below to use finger->{x,y}


The Sashiko [Critical] pre-existing issue is the same one fixed by my 
previous patch - thanks for applying that.

Richard.

Re: [PATCH] Input: focaltech - use signed coordinates to prevent underflow

[Dmitry Torokhov]

On Mon, Aug 03, 2026 at 01:29:32PM +0000, Richard Davies wrote:
> Dmitry Torokhov wrote:
> > focaltech_finger_state stores finger coordinates x and y as unsigned
> > int. When processing relative packets, negative deltas can cause
> > unsigned integer underflow if the finger moves past the left or bottom
> > boundary of the touchpad, wrapping the coordinates to values near
> > UINT_MAX.
> > 
> > When clamping the coordinates in focaltech_report_state(), these
> > underflowed values are clamped against priv->x_max / priv->y_max instead
> > of 0, causing the cursor to jump erratically to the opposite edge of the
> > touchpad.
> 
> I agree this is theoretically possible. For what it's worth, I haven't
> actually experienced the cursor jumping to the opposite edge of the screen
> on my laptop. That might mean the relative packets are actually safe, e.g.
> generated internally from an absolute position.
> 
> > Change the coordinate variables and limits to signed int so that
> > negative values resulting from relative movements clamp correctly to 0.
> > 
> > Fixes: 05be1d079ec0 ("Input: psmouse - support for the FocalTech PS/2 protocol extensions")
> > Reported-by: sashiko-bot@kernel.org
> > Assisted-by: Antigravity:gemini-3.6-flash
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > ---
> > drivers/input/mouse/focaltech.c | 12 ++++++------
> > 1 file changed, 6 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/input/mouse/focaltech.c b/drivers/input/mouse/focaltech.c
> > index d3ad4af5aa09..c6f6540e3e29 100644
> > --- a/drivers/input/mouse/focaltech.c
> > +++ b/drivers/input/mouse/focaltech.c
> > @@ -78,8 +78,8 @@ struct focaltech_finger_state {
> > 	 * Absolute position (from the bottom left corner) of the
> > 	 * finger.
> > 	 */
> > -	unsigned int x;
> > -	unsigned int y;
> > +	int x;
> > +	int y;
> > };
> > 
> > /*
> > @@ -108,7 +108,7 @@ struct focaltech_hw_state {
> > };
> > 
> > struct focaltech_data {
> > -	unsigned int x_max, y_max;
> > +	int x_max, y_max;
> > 	struct focaltech_hw_state state;
> > };
> > 
> > @@ -126,14 +126,14 @@ static void focaltech_report_state(struct psmouse *psmouse)
> > 		input_mt_slot(dev, i);
> > 		input_mt_report_slot_state(dev, MT_TOOL_FINGER, active);
> > 		if (active) {
> > -			unsigned int clamped_x, clamped_y;
> > +			int clamped_x, clamped_y;
> > 			/*
> > 			 * The touchpad might report invalid data, so we clamp
> > 			 * the resulting values so that we do not confuse
> > 			 * userspace.
> > 			 */
> > -			clamped_x = clamp(finger->x, 0U, priv->x_max);
> > -			clamped_y = clamp(finger->y, 0U, priv->y_max);
> > +			clamped_x = clamp(finger->x, 0, priv->x_max);
> > +			clamped_y = clamp(finger->y, 0, priv->y_max);
> > 			input_report_abs(dev, ABS_MT_POSITION_X, clamped_x);
> > 			input_report_abs(dev, ABS_MT_POSITION_Y,
> > 					 priv->y_max - clamped_y);
> 
> If it's allowed and race-free to change finger->{x,y} in
> focaltech_report_state, then you might fix the Sashiko [High] issue on this
> patch by just doing:
> 
> finger->x = clamp(finger->x, 0, priv->x_max);
> finger->y = clamp(finger->y, 0, priv->y_max);
> 
> not having the clamped_{x,y} variables at all, and updating the two
> input_report_abs lines below to use finger->{x,y}

Good idea, thank you. I'll update and apply.

Thanks.

-- 
Dmitry