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Message-ID: <20240321173624.c3p7r3l2rvjq6vvt@4VRSMR2-DT.corp.robot.car>
Date: Thu, 21 Mar 2024 10:36:24 -0700
From: Russ Weight <russ.weight@...ux.dev>
To: Marco Pagani <marpagan@...hat.com>
Cc: Moritz Fischer <mdf@...nel.org>, Wu Hao <hao.wu@...el.com>,
Xu Yilun <yilun.xu@...el.com>, Tom Rix <trix@...hat.com>,
Jonathan Corbet <corbet@....net>,
Alan Tull <atull@...nsource.altera.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-fpga@...r.kernel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fpga: bridge: add owner module and take its refcount
On Tue, Mar 19, 2024 at 06:20:24PM +0100, Marco Pagani wrote:
> The current implementation of the fpga bridge assumes that the low-level
> module registers a driver for the parent device and uses its owner pointer
> to take the module's refcount. This approach is problematic since it can
> lead to a null pointer dereference while attempting to get the bridge if
> the parent device does not have a driver.
>
> To address this problem, add a module owner pointer to the fpga_bridge
> struct and use it to take the module's refcount. Modify the function for
> registering a bridge to take an additional owner module parameter and
> rename it to avoid conflicts. Use the old function name for a helper macro
> that automatically sets the module that registers the bridge as the owner.
> This ensures compatibility with existing low-level control modules and
> reduces the chances of registering a bridge without setting the owner.
>
> Also, update the documentation to keep it consistent with the new interface
> for registering an fpga bridge.
>
> Other changes: opportunistically move put_device() from __fpga_bridge_get()
> to fpga_bridge_get() and of_fpga_bridge_get() to improve code clarity since
> the bridge device is taken in these functions.
>
> Fixes: 21aeda950c5f ("fpga: add fpga bridge framework")
> Suggested-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Suggested-by: Xu Yilun <yilun.xu@...el.com>
Reviewed-by: Russ Weight <russ.weight@...ux.dev>
> Signed-off-by: Marco Pagani <marpagan@...hat.com>
> ---
>
> v2:
> - Split out protection against races while taking the mod's refcount
> ---
> Documentation/driver-api/fpga/fpga-bridge.rst | 7 ++-
> drivers/fpga/fpga-bridge.c | 57 ++++++++++---------
> include/linux/fpga/fpga-bridge.h | 10 +++-
> 3 files changed, 43 insertions(+), 31 deletions(-)
>
> diff --git a/Documentation/driver-api/fpga/fpga-bridge.rst b/Documentation/driver-api/fpga/fpga-bridge.rst
> index 604208534095..d831d5ab6b0d 100644
> --- a/Documentation/driver-api/fpga/fpga-bridge.rst
> +++ b/Documentation/driver-api/fpga/fpga-bridge.rst
> @@ -6,9 +6,12 @@ API to implement a new FPGA bridge
>
> * struct fpga_bridge - The FPGA Bridge structure
> * struct fpga_bridge_ops - Low level Bridge driver ops
> -* fpga_bridge_register() - Create and register a bridge
> +* __fpga_bridge_register() - Create and register a bridge
> * fpga_bridge_unregister() - Unregister a bridge
>
> +The helper macro ``fpga_bridge_register()`` automatically sets
> +the module that registers the bridge as the owner.
> +
> .. kernel-doc:: include/linux/fpga/fpga-bridge.h
> :functions: fpga_bridge
>
> @@ -16,7 +19,7 @@ API to implement a new FPGA bridge
> :functions: fpga_bridge_ops
>
> .. kernel-doc:: drivers/fpga/fpga-bridge.c
> - :functions: fpga_bridge_register
> + :functions: __fpga_bridge_register
>
> .. kernel-doc:: drivers/fpga/fpga-bridge.c
> :functions: fpga_bridge_unregister
> diff --git a/drivers/fpga/fpga-bridge.c b/drivers/fpga/fpga-bridge.c
> index 79c473b3c7c3..8ef395b49bf8 100644
> --- a/drivers/fpga/fpga-bridge.c
> +++ b/drivers/fpga/fpga-bridge.c
> @@ -55,33 +55,26 @@ int fpga_bridge_disable(struct fpga_bridge *bridge)
> }
> EXPORT_SYMBOL_GPL(fpga_bridge_disable);
>
> -static struct fpga_bridge *__fpga_bridge_get(struct device *dev,
> +static struct fpga_bridge *__fpga_bridge_get(struct device *bridge_dev,
> struct fpga_image_info *info)
> {
> struct fpga_bridge *bridge;
> - int ret = -ENODEV;
>
> - bridge = to_fpga_bridge(dev);
> + bridge = to_fpga_bridge(bridge_dev);
>
> bridge->info = info;
>
> - if (!mutex_trylock(&bridge->mutex)) {
> - ret = -EBUSY;
> - goto err_dev;
> - }
> + if (!mutex_trylock(&bridge->mutex))
> + return ERR_PTR(-EBUSY);
>
> - if (!try_module_get(dev->parent->driver->owner))
> - goto err_ll_mod;
> + if (!try_module_get(bridge->br_ops_owner)) {
> + mutex_unlock(&bridge->mutex);
> + return ERR_PTR(-ENODEV);
> + }
>
> dev_dbg(&bridge->dev, "get\n");
>
> return bridge;
> -
> -err_ll_mod:
> - mutex_unlock(&bridge->mutex);
> -err_dev:
> - put_device(dev);
> - return ERR_PTR(ret);
> }
>
> /**
> @@ -98,13 +91,18 @@ static struct fpga_bridge *__fpga_bridge_get(struct device *dev,
> struct fpga_bridge *of_fpga_bridge_get(struct device_node *np,
> struct fpga_image_info *info)
> {
> - struct device *dev;
> + struct fpga_bridge *bridge;
> + struct device *bridge_dev;
>
> - dev = class_find_device_by_of_node(&fpga_bridge_class, np);
> - if (!dev)
> + bridge_dev = class_find_device_by_of_node(&fpga_bridge_class, np);
> + if (!bridge_dev)
> return ERR_PTR(-ENODEV);
>
> - return __fpga_bridge_get(dev, info);
> + bridge = __fpga_bridge_get(bridge_dev, info);
> + if (IS_ERR(bridge))
> + put_device(bridge_dev);
> +
> + return bridge;
> }
> EXPORT_SYMBOL_GPL(of_fpga_bridge_get);
>
> @@ -125,6 +123,7 @@ static int fpga_bridge_dev_match(struct device *dev, const void *data)
> struct fpga_bridge *fpga_bridge_get(struct device *dev,
> struct fpga_image_info *info)
> {
> + struct fpga_bridge *bridge;
> struct device *bridge_dev;
>
> bridge_dev = class_find_device(&fpga_bridge_class, NULL, dev,
> @@ -132,7 +131,11 @@ struct fpga_bridge *fpga_bridge_get(struct device *dev,
> if (!bridge_dev)
> return ERR_PTR(-ENODEV);
>
> - return __fpga_bridge_get(bridge_dev, info);
> + bridge = __fpga_bridge_get(bridge_dev, info);
> + if (IS_ERR(bridge))
> + put_device(bridge_dev);
> +
> + return bridge;
> }
> EXPORT_SYMBOL_GPL(fpga_bridge_get);
>
> @@ -146,7 +149,7 @@ void fpga_bridge_put(struct fpga_bridge *bridge)
> dev_dbg(&bridge->dev, "put\n");
>
> bridge->info = NULL;
> - module_put(bridge->dev.parent->driver->owner);
> + module_put(bridge->br_ops_owner);
> mutex_unlock(&bridge->mutex);
> put_device(&bridge->dev);
> }
> @@ -316,18 +319,19 @@ static struct attribute *fpga_bridge_attrs[] = {
> ATTRIBUTE_GROUPS(fpga_bridge);
>
> /**
> - * fpga_bridge_register - create and register an FPGA Bridge device
> + * __fpga_bridge_register - create and register an FPGA Bridge device
> * @parent: FPGA bridge device from pdev
> * @name: FPGA bridge name
> * @br_ops: pointer to structure of fpga bridge ops
> * @priv: FPGA bridge private data
> + * @owner: owner module containing the br_ops
> *
> * Return: struct fpga_bridge pointer or ERR_PTR()
> */
> struct fpga_bridge *
> -fpga_bridge_register(struct device *parent, const char *name,
> - const struct fpga_bridge_ops *br_ops,
> - void *priv)
> +__fpga_bridge_register(struct device *parent, const char *name,
> + const struct fpga_bridge_ops *br_ops,
> + void *priv, struct module *owner)
> {
> struct fpga_bridge *bridge;
> int id, ret;
> @@ -357,6 +361,7 @@ fpga_bridge_register(struct device *parent, const char *name,
>
> bridge->name = name;
> bridge->br_ops = br_ops;
> + bridge->br_ops_owner = owner;
> bridge->priv = priv;
>
> bridge->dev.groups = br_ops->groups;
> @@ -386,7 +391,7 @@ fpga_bridge_register(struct device *parent, const char *name,
>
> return ERR_PTR(ret);
> }
> -EXPORT_SYMBOL_GPL(fpga_bridge_register);
> +EXPORT_SYMBOL_GPL(__fpga_bridge_register);
>
> /**
> * fpga_bridge_unregister - unregister an FPGA bridge
> diff --git a/include/linux/fpga/fpga-bridge.h b/include/linux/fpga/fpga-bridge.h
> index 223da48a6d18..94c4edd047e5 100644
> --- a/include/linux/fpga/fpga-bridge.h
> +++ b/include/linux/fpga/fpga-bridge.h
> @@ -45,6 +45,7 @@ struct fpga_bridge_info {
> * @dev: FPGA bridge device
> * @mutex: enforces exclusive reference to bridge
> * @br_ops: pointer to struct of FPGA bridge ops
> + * @br_ops_owner: module containing the br_ops
> * @info: fpga image specific information
> * @node: FPGA bridge list node
> * @priv: low level driver private date
> @@ -54,6 +55,7 @@ struct fpga_bridge {
> struct device dev;
> struct mutex mutex; /* for exclusive reference to bridge */
> const struct fpga_bridge_ops *br_ops;
> + struct module *br_ops_owner;
> struct fpga_image_info *info;
> struct list_head node;
> void *priv;
> @@ -79,10 +81,12 @@ int of_fpga_bridge_get_to_list(struct device_node *np,
> struct fpga_image_info *info,
> struct list_head *bridge_list);
>
> +#define fpga_bridge_register(parent, name, br_ops, priv) \
> + __fpga_bridge_register(parent, name, br_ops, priv, THIS_MODULE)
> struct fpga_bridge *
> -fpga_bridge_register(struct device *parent, const char *name,
> - const struct fpga_bridge_ops *br_ops,
> - void *priv);
> +__fpga_bridge_register(struct device *parent, const char *name,
> + const struct fpga_bridge_ops *br_ops, void *priv,
> + struct module *owner);
> void fpga_bridge_unregister(struct fpga_bridge *br);
>
> #endif /* _LINUX_FPGA_BRIDGE_H */
>
> base-commit: b1a91ca25f15b6d7b311de4465854a5981dee3d3
> --
> 2.44.0
>
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