driver core: generalize driver_override in struct device

Currently, there are 12 busses (including platform and PCI) that
duplicate the driver_override logic for their individual devices.

All of them seem to be prone to the bug described in [1].

While this could be solved for every bus individually using a separate
lock, solving this in the driver-core generically results in less (and
cleaner) changes overall.

Thus, move driver_override to struct device, provide corresponding
accessors for busses and handle locking with a separate lock internally.

In particular, add device_set_driver_override(),
device_has_driver_override(), device_match_driver_override() and
generalize the sysfs store() and show() callbacks via a driver_override
feature flag in struct bus_type.

Until all busses have migrated, keep driver_set_override() in place.

Note that we can't use the device lock for the reasons described in [2].

Link: https://bugzilla.kernel.org/show_bug.cgi?id=220789 [1]
Link: https://lore.kernel.org/driver-core/DGRGTIRHA62X.3RY09D9SOK77P@kernel.org/ [2]
Tested-by: Gui-Dong Han <hanguidong02@gmail.com>
Co-developed-by: Gui-Dong Han <hanguidong02@gmail.com>
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/20260303115720.48783-2-dakr@kernel.org
[ Use dev->bus instead of sp->bus for consistency; fix commit message to
  refer to the struct bus_type's driver_override feature flag. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

2 files changed, 58 insertions, 0 deletions

diff --git a/include/linux/device.h b/include/linux/device.h
index 0be95294b6e6..e65d564f01cd 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -483,6 +483,8 @@ struct device_physical_location {
  * 		on.  This shrinks the "Board Support Packages" (BSPs) and
  * 		minimizes board-specific #ifdefs in drivers.
  * @driver_data: Private pointer for driver specific info.
+ * @driver_override: Driver name to force a match.  Do not touch directly; use
+ *		     device_set_driver_override() instead.
  * @links:	Links to suppliers and consumers of this device.
  * @power:	For device power management.
  *		See Documentation/driver-api/pm/devices.rst for details.
@@ -576,6 +578,10 @@ struct device {
 					   core doesn't touch it */
 	void		*driver_data;	/* Driver data, set and get with
 					   dev_set_drvdata/dev_get_drvdata */
+	struct {
+		const char	*name;
+		spinlock_t	lock;
+	} driver_override;
 	struct mutex		mutex;	/* mutex to synchronize calls to
 					 * its driver.
 					 */
@@ -701,6 +707,54 @@ struct device_link {
 
 #define kobj_to_dev(__kobj)	container_of_const(__kobj, struct device, kobj)
 
+int __device_set_driver_override(struct device *dev, const char *s, size_t len);
+
+/**
+ * device_set_driver_override() - Helper to set or clear driver override.
+ * @dev: Device to change
+ * @s: NUL-terminated string, new driver name to force a match, pass empty
+ *     string to clear it ("" or "\n", where the latter is only for sysfs
+ *     interface).
+ *
+ * Helper to set or clear driver override of a device.
+ *
+ * Returns: 0 on success or a negative error code on failure.
+ */
+static inline int device_set_driver_override(struct device *dev, const char *s)
+{
+	return __device_set_driver_override(dev, s, s ? strlen(s) : 0);
+}
+
+/**
+ * device_has_driver_override() - Check if a driver override has been set.
+ * @dev: device to check
+ *
+ * Returns true if a driver override has been set for this device.
+ */
+static inline bool device_has_driver_override(struct device *dev)
+{
+	guard(spinlock)(&dev->driver_override.lock);
+	return !!dev->driver_override.name;
+}
+
+/**
+ * device_match_driver_override() - Match a driver against the device's driver_override.
+ * @dev: device to check
+ * @drv: driver to match against
+ *
+ * Returns > 0 if a driver override is set and matches the given driver, 0 if a
+ * driver override is set but does not match, or < 0 if a driver override is not
+ * set at all.
+ */
+static inline int device_match_driver_override(struct device *dev,
+					       const struct device_driver *drv)
+{
+	guard(spinlock)(&dev->driver_override.lock);
+	if (dev->driver_override.name)
+		return !strcmp(dev->driver_override.name, drv->name);
+	return -1;
+}
+
 /**
  * device_iommu_mapped - Returns true when the device DMA is translated
  *			 by an IOMMU
diff --git a/include/linux/device/bus.h b/include/linux/device/bus.h
index 63de5f053c33..c1b463cd6464 100644
--- a/include/linux/device/bus.h
+++ b/include/linux/device/bus.h
@@ -65,6 +65,9 @@ struct fwnode_handle;
  *			this bus.
  * @pm:		Power management operations of this bus, callback the specific
  *		device driver's pm-ops.
+ * @driver_override:	Set to true if this bus supports the driver_override
+ *			mechanism, which allows userspace to force a specific
+ *			driver to bind to a device via a sysfs attribute.
  * @need_parent_lock:	When probing or removing a device on this bus, the
  *			device core should lock the device's parent.
  *
@@ -106,6 +109,7 @@ struct bus_type {
 
 	const struct dev_pm_ops *pm;
 
+	bool driver_override;
 	bool need_parent_lock;
 };