mux: Convert mux_control_ops to a flex array member in mux_chip

Convert mux_control_ops to a flexible array member at the end of the
mux_chip struct and add the __counted_by() compiler attribute to
improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Use struct_size() to calculate the number of bytes to allocate for a new
mux chip and to remove the following Coccinelle/coccicheck warning:

  WARNING: Use struct_size

Use size_add() to safely add any extra bytes.

No functional changes intended.

Link: KSPP/linux#83
Signed-off-by: Thorsten Blum <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: toblux

drivers/mux/core.c

@@ -98,13 +98,12 @@ struct mux_chip *mux_chip_alloc(struct device *dev,
 	if (WARN_ON(!dev || !controllers))
 		return ERR_PTR(-EINVAL);
 
-	mux_chip = kzalloc(sizeof(*mux_chip) +
-			   controllers * sizeof(*mux_chip->mux) +
-			   sizeof_priv, GFP_KERNEL);
+	mux_chip = kzalloc(size_add(struct_size(mux_chip, mux, controllers),
+				    sizeof_priv),
+			   GFP_KERNEL);
 	if (!mux_chip)
 		return ERR_PTR(-ENOMEM);
 
-	mux_chip->mux = (struct mux_control *)(mux_chip + 1);
 	mux_chip->dev.class = &mux_class;
 	mux_chip->dev.type = &mux_type;
 	mux_chip->dev.parent = dev;

include/linux/mux/driver.h

@@ -56,18 +56,18 @@ struct mux_control {
 /**
  * struct mux_chip -	Represents a chip holding mux controllers.
  * @controllers:	Number of mux controllers handled by the chip.
- * @mux:		Array of mux controllers that are handled.
  * @dev:		Device structure.
  * @id:			Used to identify the device internally.
  * @ops:		Mux controller operations.
+ * @mux:		Array of mux controllers that are handled.
  */
 struct mux_chip {
 	unsigned int controllers;
-	struct mux_control *mux;
 	struct device dev;
 	int id;
 
 	const struct mux_control_ops *ops;
+	struct mux_control mux[] __counted_by(controllers);
 };
 
 #define to_mux_chip(x) container_of((x), struct mux_chip, dev)