linux-kernel - [PATCH 4.9 069/175] Input: ads7846 - fix integer overflow on Rt calculation

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Message-Id: <20201228124856.595791689@linuxfoundation.org>
Date:   Mon, 28 Dec 2020 13:48:42 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, David Jander <david@...tonic.nl>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.9 069/175] Input: ads7846 - fix integer overflow on Rt calculation

From: Oleksij Rempel <o.rempel@...gutronix.de>

[ Upstream commit 820830ec918f6c3dcd77a54a1c6198ab57407916 ]

In some rare cases the 32 bit Rt value will overflow if z2 and x is max,
z1 is minimal value and x_plate_ohms is relatively high (for example 800
ohm). This would happen on some screen age with low pressure.

There are two possible fixes:
- make Rt 64bit
- reorder calculation to avoid overflow

The second variant seems to be preferable, since 64 bit calculation on
32 bit system is a bit more expensive.

Fixes: ffa458c1bd9b6f653008d450f337602f3d52a646 ("spi: ads7846 driver")
Co-developed-by: David Jander <david@...tonic.nl>
Signed-off-by: David Jander <david@...tonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
Link: https://lore.kernel.org/r/20201113112240.1360-1-o.rempel@pengutronix.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/input/touchscreen/ads7846.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
index 1ce3ecbe37f89..9fa7ae12df80a 100644
--- a/drivers/input/touchscreen/ads7846.c
+++ b/drivers/input/touchscreen/ads7846.c
@@ -785,10 +785,11 @@ static void ads7846_report_state(struct ads7846 *ts)
 		/* compute touch pressure resistance using equation #2 */
 		Rt = z2;
 		Rt -= z1;
-		Rt *= x;
 		Rt *= ts->x_plate_ohms;
+		Rt = DIV_ROUND_CLOSEST(Rt, 16);
+		Rt *= x;
 		Rt /= z1;
-		Rt = (Rt + 2047) >> 12;
+		Rt = DIV_ROUND_CLOSEST(Rt, 256);
 	} else {
 		Rt = 0;
 	}
-- 
2.27.0