gpio: GPIO_GET_LINEHANDLE_IOCTL: Reject invalid line flags

The GPIO_GET_LINEHANDLE_IOCTL currently ignores unknown or undefined
linehandle flags. From a backwards and forwards compatibility viewpoint it
is highly desirable to reject unknown flags though.

On one hand an application that is using newer flags and is running on
an older kernel has no way to detect if the new flags were handled
correctly if they are silently discarded.

On the other hand an application that (accidentally) passes undefined flags
will run fine on an older kernel, but may break on a newer kernel when
these flags get defined.

Ensure that requests that have undefined flags set are rejected with an
error, rather than silently discarding the undefined flags.

Cc: stable@vger.kernel.org
Fixes: d7c51b47ac ("gpio: userspace ABI for reading/writing GPIO lines")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -333,6 +333,13 @@ struct linehandle_state {
 	u32 numdescs;
 };
 
+#define GPIOHANDLE_REQUEST_VALID_FLAGS \
+	(GPIOHANDLE_REQUEST_INPUT | \
+	GPIOHANDLE_REQUEST_OUTPUT | \
+	GPIOHANDLE_REQUEST_ACTIVE_LOW | \
+	GPIOHANDLE_REQUEST_OPEN_DRAIN | \
+	GPIOHANDLE_REQUEST_OPEN_SOURCE)
+
 static long linehandle_ioctl(struct file *filep, unsigned int cmd,
 			     unsigned long arg)
 {
@@ -451,6 +458,12 @@ static int linehandle_create(struct gpio_device *gdev, void __user *ip)
 			goto out_free_descs;
 		}
 
+		/* Return an error if a unknown flag is set */
+		if (lflags & ~GPIOHANDLE_REQUEST_VALID_FLAGS) {
+			ret = -EINVAL;
+			goto out_free_descs;
+		}
+
 		desc = &gdev->descs[offset];
 		ret = gpiod_request(desc, lh->label);
 		if (ret)