iio: at91-sama5d2_adc: Fix unsafe buffer attributes

The iio_triggered_buffer_setup_ext() was changed by
commit 15097c7 ("iio: buffer: wrap all buffer attributes into iio_dev_attr")
to silently expect that all attributes given in buffer_attrs array are
device-attributes. This expectation was not forced by the API - and some
drivers did register attributes created by IIO_CONST_ATTR().

The added attribute "wrapping" does not copy the pointer to stored
string constant and when the sysfs file is read the kernel will access
to invalid location.

Change the IIO_CONST_ATTRs from the driver to IIO_DEVICE_ATTR in order
to prevent the invalid memory access.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Fixes: 15097c7 ("iio: buffer: wrap all buffer attributes into iio_dev_attr")
Tested-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/be69775aa302159f088b8b91894e6ec449bca65b.1664782676.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: M-Vaittinen

drivers/iio/adc/at91-sama5d2_adc.c

@@ -2193,17 +2193,30 @@ static ssize_t at91_adc_get_watermark(struct device *dev,
 	return scnprintf(buf, PAGE_SIZE, "%d\n", st->dma_st.watermark);
 }
 
+static ssize_t hwfifo_watermark_min_show(struct device *dev,
+					 struct device_attribute *attr,
+					 char *buf)
+{
+	return sysfs_emit(buf, "%s\n", "2");
+}
+
+static ssize_t hwfifo_watermark_max_show(struct device *dev,
+					 struct device_attribute *attr,
+					 char *buf)
+{
+	return sysfs_emit(buf, "%s\n", AT91_HWFIFO_MAX_SIZE_STR);
+}
+
 static IIO_DEVICE_ATTR(hwfifo_enabled, 0444,
 		       at91_adc_get_fifo_state, NULL, 0);
 static IIO_DEVICE_ATTR(hwfifo_watermark, 0444,
 		       at91_adc_get_watermark, NULL, 0);
-
-static IIO_CONST_ATTR(hwfifo_watermark_min, "2");
-static IIO_CONST_ATTR(hwfifo_watermark_max, AT91_HWFIFO_MAX_SIZE_STR);
+static IIO_DEVICE_ATTR_RO(hwfifo_watermark_min, 0);
+static IIO_DEVICE_ATTR_RO(hwfifo_watermark_max, 0);
 
 static const struct attribute *at91_adc_fifo_attributes[] = {
-	&iio_const_attr_hwfifo_watermark_min.dev_attr.attr,
-	&iio_const_attr_hwfifo_watermark_max.dev_attr.attr,
+	&iio_dev_attr_hwfifo_watermark_min.dev_attr.attr,
+	&iio_dev_attr_hwfifo_watermark_max.dev_attr.attr,
 	&iio_dev_attr_hwfifo_watermark.dev_attr.attr,
 	&iio_dev_attr_hwfifo_enabled.dev_attr.attr,
 	NULL,