fpga: region: add owner module and take its refcount

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.

Fixes: 0fa20cdfcc1f ("fpga: fpga-region: device tree control for FPGA")
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Xu Yilun <yilun.xu@intel.com>
Reviewed-by: Russ Weight <russ.weight@linux.dev>
Signed-off-by: Marco Pagani <marpagan@redhat.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20240419083601.77403-1-marpagan@redhat.com
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>

1 files changed, 14 insertions, 10 deletions

diff --git a/drivers/fpga/fpga-region.c b/drivers/fpga/fpga-region.c
index b364a929425c..753cd142503e 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->ops_owner)) {
 		put_device(dev);
 		mutex_unlock(&region->mutex);
 		return ERR_PTR(-ENODEV);
@@ -75,7 +75,7 @@ static void fpga_region_put(struct fpga_region *region)
 
 	dev_dbg(dev, "put\n");
 
-	module_put(dev->parent->driver->owner);
+	module_put(region->ops_owner);
 	put_device(dev);
 	mutex_unlock(&region->mutex);
 }
@@ -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: 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->ops_owner = owner;
 
 	mutex_init(&region->mutex);
 	INIT_LIST_HEAD(&region->bridge_list);
@@ -241,13 +244,14 @@ err_free:
 
 	return ERR_PTR(ret);
 }
-EXPORT_SYMBOL_GPL(fpga_region_register_full);
+EXPORT_SYMBOL_GPL(__fpga_region_register_full);
 
 /**
- * fpga_region_register - create and register an FPGA Region device
+ * __fpga_region_register - create and register an FPGA Region device
  * @parent: device parent
  * @mgr: manager that programs this region
  * @get_bridges: optional function to get bridges to a list
+ * @owner: module containing the get_bridges function
  *
  * This simple version of the register function should be sufficient for most users.
  * The fpga_region_register_full() function is available for users that need to
@@ -256,17 +260,17 @@ EXPORT_SYMBOL_GPL(fpga_region_register_full);
  * Return: struct fpga_region or ERR_PTR()
  */
 struct fpga_region *
-fpga_region_register(struct device *parent, struct fpga_manager *mgr,
-		     int (*get_bridges)(struct fpga_region *))
+__fpga_region_register(struct device *parent, struct fpga_manager *mgr,
+		       int (*get_bridges)(struct fpga_region *), struct module *owner)
 {
 	struct fpga_region_info info = { 0 };
 
 	info.mgr = mgr;
 	info.get_bridges = get_bridges;
 
-	return fpga_region_register_full(parent, &info);
+	return __fpga_region_register_full(parent, &info, owner);
 }
-EXPORT_SYMBOL_GPL(fpga_region_register);
+EXPORT_SYMBOL_GPL(__fpga_region_register);
 
 /**
  * fpga_region_unregister - unregister an FPGA region