Re: [PATCH] fpga: region: add owner module and take its refcount

[Marco Pagani]

On 2024-03-26 18:28, Russ Weight wrote:
> 
> On Fri, Mar 22, 2024 at 06:19:30PM +0100, Marco Pagani wrote:
>> The current implementation of the fpga region 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 region
>> during programming if the parent device does not have a driver.
>>
>> To address this problem, add a module owner pointer to the fpga_region
>> struct and use it to take the module's refcount. Modify the functions for
>> registering a region to take an additional owner module parameter and
>> rename them to avoid conflicts. Use the old function names for helper
>> macros that automatically set the module that registers the region as the
>> owner. This ensures compatibility with existing low-level control modules
>> and reduces the chances of registering a region without setting the owner.
>>
>> Also, update the documentation to keep it consistent with the new interface
>> for registering an fpga region.
>>
>> Other changes: unlock the mutex before calling put_device() in
>> fpga_region_put() to avoid potential use after release issues.
>>
>> Fixes: 0fa20cdfcc1f ("fpga: fpga-region: device tree control for FPGA")
>> Suggested-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>> Suggested-by: Xu Yilun <yilun.xu@xxxxxxxxx>
>> Signed-off-by: Marco Pagani <marpagan@xxxxxxxxxx>
>> ---
>>  Documentation/driver-api/fpga/fpga-region.rst | 13 ++++++----
>>  drivers/fpga/fpga-region.c                    | 26 +++++++++++--------
>>  include/linux/fpga/fpga-region.h              | 13 +++++++---
>>  3 files changed, 33 insertions(+), 19 deletions(-)
>>
>> diff --git a/Documentation/driver-api/fpga/fpga-region.rst b/Documentation/driver-api/fpga/fpga-region.rst
>> index dc55d60a0b4a..3aff5199b6d8 100644
>> --- a/Documentation/driver-api/fpga/fpga-region.rst
>> +++ b/Documentation/driver-api/fpga/fpga-region.rst
>> @@ -46,13 +46,16 @@ API to add a new FPGA region
>>  ----------------------------
>>  
>>  * struct fpga_region - The FPGA region struct
>> -* struct fpga_region_info - Parameter structure for fpga_region_register_full()
>> -* fpga_region_register_full() -  Create and register an FPGA region using the
>> +* struct fpga_region_info - Parameter structure for __fpga_region_register_full()
>> +* __fpga_region_register_full() -  Create and register an FPGA region using the
>>    fpga_region_info structure to provide the full flexibility of options
>> -* fpga_region_register() -  Create and register an FPGA region using standard
>> +* __fpga_region_register() -  Create and register an FPGA region using standard
>>    arguments
>>  * fpga_region_unregister() -  Unregister an FPGA region
>>  
>> +Helper macros ``fpga_region_register()`` and ``fpga_region_register_full()``
>> +automatically sets the module that registers the FPGA region as the owner.
> 
> s/sets/set/

Nice catch.

> 
>> +
>>  The FPGA region's probe function will need to get a reference to the FPGA
>>  Manager it will be using to do the programming.  This usually would happen
>>  during the region's probe function.
>> @@ -82,10 +85,10 @@ following APIs to handle building or tearing down that list.
>>     :functions: fpga_region_info
>>  
>>  .. kernel-doc:: drivers/fpga/fpga-region.c
>> -   :functions: fpga_region_register_full
>> +   :functions: __fpga_region_register
>>  
>>  .. kernel-doc:: drivers/fpga/fpga-region.c
>> -   :functions: fpga_region_register
>> +   :functions: __fpga_region_register_full
>>  
>>  .. kernel-doc:: drivers/fpga/fpga-region.c
>>     :functions: fpga_region_unregister
>> diff --git a/drivers/fpga/fpga-region.c b/drivers/fpga/fpga-region.c
>> index b364a929425c..f8bbda024d59 100644
>> --- a/drivers/fpga/fpga-region.c
>> +++ b/drivers/fpga/fpga-region.c
>> @@ -53,7 +53,7 @@ static struct fpga_region *fpga_region_get(struct fpga_region *region)
>>  	}
>>  
>>  	get_device(dev);
>> -	if (!try_module_get(dev->parent->driver->owner)) {
>> +	if (!try_module_get(region->get_br_owner)) {
>>  		put_device(dev);
>>  		mutex_unlock(&region->mutex);
>>  		return ERR_PTR(-ENODEV);
>> @@ -75,9 +75,9 @@ static void fpga_region_put(struct fpga_region *region)
>>  
>>  	dev_dbg(dev, "put\n");
>>  
>> -	module_put(dev->parent->driver->owner);
>> -	put_device(dev);
>> +	module_put(region->get_br_owner);
>>  	mutex_unlock(&region->mutex);
>> +	put_device(dev);
>>  }
>>  
>>  /**
>> @@ -181,14 +181,16 @@ static struct attribute *fpga_region_attrs[] = {
>>  ATTRIBUTE_GROUPS(fpga_region);
>>  
>>  /**
>> - * fpga_region_register_full - create and register an FPGA Region device
>> + * __fpga_region_register_full - create and register an FPGA Region device
>>   * @parent: device parent
>>   * @info: parameters for FPGA Region
>> + * @owner: owner module containing the get_bridges function
>>   *
>>   * Return: struct fpga_region or ERR_PTR()
>>   */
>>  struct fpga_region *
>> -fpga_region_register_full(struct device *parent, const struct fpga_region_info *info)
>> +__fpga_region_register_full(struct device *parent, const struct fpga_region_info *info,
>> +			    struct module *owner)
>>  {
>>  	struct fpga_region *region;
>>  	int id, ret = 0;
>> @@ -213,6 +215,7 @@ fpga_region_register_full(struct device *parent, const struct fpga_region_info *
>>  	region->compat_id = info->compat_id;
>>  	region->priv = info->priv;
>>  	region->get_bridges = info->get_bridges;
>> +	region->get_br_owner = owner;
> 
> get_* implies a function. Maybe this would be better called br_owner?

You are right. I will change it to br_owner in v2.

> 
> - Russ
> 

[ ... ]
Thanks,
Marco