driver core: bus: add bus_get_dev_root() function

Instead of poking around in the struct bus_type directly for the
dev_root pointer, provide a function to return it properly reference
counted, if it is present in the bus.  This will be needed to move the
pointer out of struct bus_type in the future.

Use the function in the driver core code at the same time it is
introduced to verify that it works properly.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230209093556.19132-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

3 files changed, 38 insertions, 6 deletions

diff --git a/drivers/base/bus.c b/drivers/base/bus.c
index 301b9c6ece86..def1e009ad9d 100644
--- a/drivers/base/bus.c
+++ b/drivers/base/bus.c
@@ -1334,6 +1334,26 @@ bool bus_is_registered(const struct bus_type *bus)
 	return is_initialized;
 }
 
+/**
+ * bus_get_dev_root - return a pointer to the "device root" of a bus
+ * @bus: bus to return the device root of.
+ *
+ * If a bus has a "device root" structure, return it, WITH THE REFERENCE
+ * COUNT INCREMENTED.
+ *
+ * Note, when finished with the device, a call to put_device() is required.
+ *
+ * If the device root is not present (or bus is not a valid pointer), NULL
+ * will be returned.
+ */
+struct device *bus_get_dev_root(const struct bus_type *bus)
+{
+	if (bus)
+		return get_device(bus->dev_root);
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(bus_get_dev_root);
+
 int __init buses_init(void)
 {
 	bus_kset = kset_create_and_add("bus", &bus_uevent_ops, NULL);
diff --git a/drivers/base/core.c b/drivers/base/core.c
index 2712a1a1e959..f9297c68214a 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -3173,8 +3173,9 @@ static DEFINE_MUTEX(gdp_mutex);
 static struct kobject *get_device_parent(struct device *dev,
 					 struct device *parent)
 {
+	struct kobject *kobj = NULL;
+
 	if (dev->class) {
-		struct kobject *kobj = NULL;
 		struct kobject *parent_kobj;
 		struct kobject *k;
 
@@ -3222,8 +3223,15 @@ static struct kobject *get_device_parent(struct device *dev,
 	}
 
 	/* subsystems can specify a default root directory for their devices */
-	if (!parent && dev->bus && dev->bus->dev_root)
-		return &dev->bus->dev_root->kobj;
+	if (!parent && dev->bus) {
+		struct device *dev_root = bus_get_dev_root(dev->bus);
+
+		if (dev_root) {
+			kobj = &dev_root->kobj;
+			put_device(dev_root);
+			return kobj;
+		}
+	}
 
 	if (parent)
 		return &parent->kobj;
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 441eb5bdec7d..8bb623039bb2 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -610,9 +610,13 @@ static const struct attribute_group cpu_root_vulnerabilities_group = {
 
 static void __init cpu_register_vulnerabilities(void)
 {
-	if (sysfs_create_group(&cpu_subsys.dev_root->kobj,
-			       &cpu_root_vulnerabilities_group))
-		pr_err("Unable to register CPU vulnerabilities\n");
+	struct device *dev = bus_get_dev_root(&cpu_subsys);
+
+	if (dev) {
+		if (sysfs_create_group(&dev->kobj, &cpu_root_vulnerabilities_group))
+			pr_err("Unable to register CPU vulnerabilities\n");
+		put_device(dev);
+	}
 }
 
 #else