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Message-ID: <20240226212925.3781744-1-jacob.e.keller@intel.com>
Date: Mon, 26 Feb 2024 13:29:25 -0800
From: Jacob Keller <jacob.e.keller@...el.com>
To: Kees Cook <keescook@...omium.org>,
linux-i2c@...r.kernel.org,
linux-kernel@...r.kernel.org
Cc: gustavoars@...nel.org,
Peter Rosin <peda@...ntia.se>,
Jesse Brandeburg <jesse.brandeburg@...el.com>,
Jacob Keller <jacob.e.keller@...el.com>
Subject: [PATCH v2] mux: convert mux_chip->mux to flexible array
The mux_chip structure size is over allocated to additionally include both
the array of mux controllers as well as a device specific private area.
The controllers array is then pointed to by assigning mux_chip->mux to the
first block of extra memory, while the private area is extracted via
mux_chip_priv() and points to the area just after the controllers.
The size of the mux_chip allocation uses direct multiplication and addition
rather than the <linux/overflow.h> helpers. In addition, the mux_chip->mux
struct member wastes space by having to store the pointer as part of the
structures.
Convert struct mux_chip to use a flexible array member for the mux
controller array. Use struct_size() and size_add() to compute the size of
the structure while protecting against overflow.
After converting the mux pointer, notice that two 4-byte holes remain in
the structure layout due to the alignment requirements for the dev
sub-structure and the ops pointer.
These can be easily fixed through re-ordering the id field to the 4-byte
hole just after the controllers member.
This changes the layout from:
struct mux_chip {
unsigned int controllers; /* 0 4 */
/* XXX 4 bytes hole, try to pack */
struct mux_control * mux; /* 8 8 */
struct device dev __attribute__((__aligned__(8))); /* 16 1488 */
/* XXX last struct has 3 bytes of padding */
/* --- cacheline 23 boundary (1472 bytes) was 32 bytes ago --- */
int id; /* 1504 4 */
/* XXX 4 bytes hole, try to pack */
const struct mux_control_ops * ops; /* 1512 8 */
/* size: 1520, cachelines: 24, members: 5 */
/* sum members: 1512, holes: 2, sum holes: 8 */
/* paddings: 1, sum paddings: 3 */
/* forced alignments: 1 */
/* last cacheline: 48 bytes */
} __attribute__((__aligned__(8)));
To the following:
struct mux_chip {
unsigned int controllers; /* 0 4 */
int id; /* 4 4 */
struct device dev __attribute__((__aligned__(8))); /* 8 1488 */
/* XXX last struct has 3 bytes of padding */
/* --- cacheline 23 boundary (1472 bytes) was 24 bytes ago --- */
const struct mux_control_ops * ops; /* 1496 8 */
struct mux_control mux[]; /* 1504 0 */
/* size: 1504, cachelines: 24, members: 5 */
/* paddings: 1, sum paddings: 3 */
/* forced alignments: 1 */
/* last cacheline: 32 bytes */
} __attribute__((__aligned__(8)));
This both removes risk of overflowing and performing an under-allocation,
as well as saves 16 bytes of otherwise wasted space for every mux_chip.
Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
---
Changes since v1:
* Rebased and updated the commit message slightly.
drivers/mux/core.c | 7 +++----
include/linux/mux/driver.h | 10 +++++-----
2 files changed, 8 insertions(+), 9 deletions(-)
diff --git a/drivers/mux/core.c b/drivers/mux/core.c
index 775816112932..9225abca7897 100644
--- a/drivers/mux/core.c
+++ b/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;
diff --git a/include/linux/mux/driver.h b/include/linux/mux/driver.h
index 18824064f8c0..84dc0d3e79d6 100644
--- a/include/linux/mux/driver.h
+++ b/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.
+ * @dev: Device structure.
* @ops: Mux controller operations.
+ * @mux: Flexible array of mux controllers that are handled.
*/
struct mux_chip {
unsigned int controllers;
- struct mux_control *mux;
- struct device dev;
int id;
-
+ struct device dev;
const struct mux_control_ops *ops;
+
+ struct mux_control mux[];
};
#define to_mux_chip(x) container_of((x), struct mux_chip, dev)
base-commit: 45ec2f5f6ed3ec3a79ba1329ad585497cdcbe663
--
2.41.0
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