linux-hardening - [PATCH 1/2] mux: Turn 'mux' into a flexible array in 'struct mux

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Message-Id: <d17bd9b622dbe3f7cb2f18736ef3138a6927f86c.1700986053.git.christophe.jaillet@wanadoo.fr>
Date: Sun, 26 Nov 2023 09:08:11 +0100
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Peter Rosin <peda@...ntia.se>,
	Kees Cook <keescook@...omium.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org,
	Christophe JAILLET <christophe.jaillet@...adoo.fr>,
	linux-hardening@...r.kernel.org
Subject: [PATCH 1/2] mux: Turn 'mux' into a flexible array in 'struct mux_chip'

The 'mux' array stored in 'struct mux_chip' can be changed into a flexible
array.

This saves:
   - a pointer in the structure
   - an indirection when accessing the array
   - some pointer arithmetic when computing and storing the address in
     'mux'

It is also now possible to use __counted_by() and struct_size() for
additional safety.

The address for the 'priv' memory is computed with mux_chip_priv(). It
should work as good with a flexible array.

Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
The struct_size() goodies only work if sizeof_priv is 0. Adding an
additional size_add() would make it safe in all cases but would make code
less readable (IMHO).
---
 drivers/mux/core.c         | 4 +---
 include/linux/mux/driver.h | 5 +++--
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/mux/core.c b/drivers/mux/core.c
index 775816112932..80b2607b083b 100644
--- a/drivers/mux/core.c
+++ b/drivers/mux/core.c
@@ -98,13 +98,11 @@ 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) +
+	mux_chip = kzalloc(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;
diff --git a/include/linux/mux/driver.h b/include/linux/mux/driver.h
index 18824064f8c0..c29e9b7fb17b 100644
--- a/include/linux/mux/driver.h
+++ b/include/linux/mux/driver.h
@@ -56,18 +56,19 @@ 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)
-- 
2.34.1