iio:adc:ti-adc081c Fix alignment and data leak issues

One of a class of bugs pointed out by Lars in a recent review.
iio_push_to_buffers_with_timestamp assumes the buffer used is aligned
to the size of the timestamp (8 bytes).  This is not guaranteed in
this driver which uses an array of smaller elements on the stack.
As Lars also noted this anti pattern can involve a leak of data to
userspace and that indeed can happen here.  We close both issues by
moving to a suitable structure in the iio_priv().

This data is allocated with kzalloc so no data can leak apart
from previous readings.

The eplicit alignment of ts is necessary to ensure correct padding
on x86_32 where s64 is only aligned to 4 bytes.

Fixes: 08e05d1 ("ti-adc081c: Initial triggered buffer support")
Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: <Stable@vger.kernel.org>

Author: jic23

drivers/iio/adc/ti-adc081c.c

@@ -33,6 +33,12 @@ struct adc081c {
 
 	/* 8, 10 or 12 */
 	int bits;
+
+	/* Ensure natural alignment of buffer elements */
+	struct {
+		u16 channel;
+		s64 ts __aligned(8);
+	} scan;
 };
 
 #define REG_CONV_RES 0x00
@@ -128,14 +134,13 @@ static irqreturn_t adc081c_trigger_handler(int irq, void *p)
 	struct iio_poll_func *pf = p;
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct adc081c *data = iio_priv(indio_dev);
-	u16 buf[8]; /* 2 bytes data + 6 bytes padding + 8 bytes timestamp */
 	int ret;
 
 	ret = i2c_smbus_read_word_swapped(data->i2c, REG_CONV_RES);
 	if (ret < 0)
 		goto out;
-	buf[0] = ret;
-	iio_push_to_buffers_with_timestamp(indio_dev, buf,
+	data->scan.channel = ret;
+	iio_push_to_buffers_with_timestamp(indio_dev, &data->scan,
 					   iio_get_time_ns(indio_dev));
 out:
 	iio_trigger_notify_done(indio_dev->trig);