[PATCH v3 2/4] iio: core: Add opaque_struct_size() helper and use it

[Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>]

Introduce opaque_struct_size() helper, which may be moved
to overflow.h in the future, and use it in the IIO core.

Potential users could be (among possible others):

	__spi_alloc_controller() in drivers/spi/spi.c
	alloc_netdev_mqs in net/core/dev.c

Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Cc: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Nuno Sa <nuno.sa@xxxxxxxxxx>
---
 drivers/iio/industrialio-core.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index b153adc5bc84..118ca6b59504 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -1607,6 +1607,24 @@ const struct device_type iio_device_type = {
 	.release = iio_dev_release,
 };
 
+/**
+ * opaque_struct_size() - Calculate size of opaque structure with trailing aligned data.
+ * @p: Pointer to the opaque structure.
+ * @a: Alignment in bytes before trailing data.
+ * @s: Data size in bytes (preferred not to be 0).
+ *
+ * Calculates size of memory needed for structure of @p followed by
+ * the aligned data of size @s.
+ *
+ * Note, when @s is 0, the alignment @a is added to the sizeof(*(@p))
+ * and the result, depending on the @a, may be way off the initial size.
+ *
+ * Returns: Number of bytes needed or SIZE_MAX on overflow.
+ */
+#define opaque_struct_size(p, a, s)	size_add(ALIGN(sizeof(*(p)), (a)), (s))
+
+#define opaque_struct_data(p, a)	PTR_ALIGN((void *)((p) + 1), (a))
+
 /**
  * iio_device_alloc() - allocate an iio_dev from a driver
  * @parent:		Parent device.
@@ -1618,19 +1636,13 @@ struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv)
 	struct iio_dev *indio_dev;
 	size_t alloc_size;
 
-	alloc_size = sizeof(struct iio_dev_opaque);
-	if (sizeof_priv) {
-		alloc_size = ALIGN(alloc_size, IIO_DMA_MINALIGN);
-		alloc_size += sizeof_priv;
-	}
-
+	alloc_size = opaque_struct_size(iio_dev_opaque, IIO_DMA_MINALIGN, sizeof_priv);
 	iio_dev_opaque = kzalloc(alloc_size, GFP_KERNEL);
 	if (!iio_dev_opaque)
 		return NULL;
 
 	indio_dev = &iio_dev_opaque->indio_dev;
-	indio_dev->priv = (char *)iio_dev_opaque +
-		ALIGN(sizeof(struct iio_dev_opaque), IIO_DMA_MINALIGN);
+	indio_dev->priv = opaque_struct_data(iio_dev_opaque, IIO_DMA_MINALIGN);
 
 	indio_dev->dev.parent = parent;
 	indio_dev->dev.type = &iio_device_type;
-- 
2.40.0.1.gaa8946217a0b