[PATCH] mtd: rawnand: rockchip: Use struct_size()

[Christophe JAILLET <christophe.jaillet@wanadoo.fr>]

Use struct_size() instead of hand writing it.
This is less verbose and more robust.

While at it, prepare for the coming implementation by GCC and Clang of the
__counted_by attribute. Flexible array members annotated with __counted_by
can have their accesses bounds-checked at run-time checking via
CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
strcpy/memcpy-family functions).

Also remove a useless comment about the position of a flex-array in a
structure.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
This patch is part of a work done in parallel of what is currently worked
on by Kees Cook.

My patches are only related to corner cases that do NOT match the
semantic of his Coccinelle script[1].

In this case, struct_size() was not used to compute the size needed for the
structure and its flex array.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci
---
 drivers/mtd/nand/raw/rockchip-nand-controller.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/mtd/nand/raw/rockchip-nand-controller.c b/drivers/mtd/nand/raw/rockchip-nand-controller.c
index 5bc90ffa721f..596cf9a78274 100644
--- a/drivers/mtd/nand/raw/rockchip-nand-controller.c
+++ b/drivers/mtd/nand/raw/rockchip-nand-controller.c
@@ -158,8 +158,7 @@ struct rk_nfc_nand_chip {
 	u32 timing;
 
 	u8 nsels;
-	u8 sels[];
-	/* Nothing after this field. */
+	u8 sels[] __counted_by(nsels);
 };
 
 struct rk_nfc {
@@ -1119,7 +1118,7 @@ static int rk_nfc_nand_chip_init(struct device *dev, struct rk_nfc *nfc,
 		return -EINVAL;
 	}
 
-	rknand = devm_kzalloc(dev, sizeof(*rknand) + nsels * sizeof(u8),
+	rknand = devm_kzalloc(dev, struct_size(rknand, sels, nsels),
 			      GFP_KERNEL);
 	if (!rknand)
 		return -ENOMEM;
-- 
2.34.1