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Message-ID: <202402261647.CCCF97AF@keescook>
Date: Mon, 26 Feb 2024 16:48:38 -0800
From: Kees Cook <keescook@...omium.org>
To: Jacob Keller <jacob.e.keller@...el.com>
Cc: linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
gustavoars@...nel.org, Peter Rosin <peda@...ntia.se>,
Jesse Brandeburg <jesse.brandeburg@...el.com>
Subject: Re: [PATCH v2] mux: convert mux_chip->mux to flexible array
On Mon, Feb 26, 2024 at 01:29:25PM -0800, Jacob Keller wrote:
> 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.
This looks like a great clean-up, thank you!
>
> 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[];
This can gain the __counted_by annotation as well:
struct mux_control mux[] __counted_by(controllers);
With that:
Reviewed-by: Kees Cook <keescook@...omium.org>
-Kees
--
Kees Cook
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