// SPDX-License-Identifier: GPL-2.0 #include "dm-core.h" /* * The kobject release method must not be placed in the module itself, * otherwise we are subject to module unload races. * * The release method is called when the last reference to the kobject is * dropped. It may be called by any other kernel code that drops the last * reference. * * The release method suffers from module unload race. We may prevent the * module from being unloaded at the start of the release method (using * increased module reference count or synchronizing against the release * method), however there is no way to prevent the module from being * unloaded at the end of the release method. * * If this code were placed in the dm module, the following race may * happen: * 1. Some other process takes a reference to dm kobject * 2. The user issues ioctl function to unload the dm device * 3. dm_sysfs_exit calls kobject_put, however the object is not released * because of the other reference taken at step 1 * 4. dm_sysfs_exit waits on the completion * 5. The other process that took the reference in step 1 drops it, * dm_kobject_release is called from this process * 6. dm_kobject_release calls complete() * 7. a reschedule happens before dm_kobject_release returns * 8. dm_sysfs_exit continues, the dm device is unloaded, module reference * count is decremented * 9. The user unloads the dm module * 10. The other process that was rescheduled in step 7 continues to run, * it is now executing code in unloaded module, so it crashes * * Note that if the process that takes the foreign reference to dm kobject * has a low priority and the system is sufficiently loaded with * higher-priority processes that prevent the low-priority process from * being scheduled long enough, this bug may really happen. * * In order to fix this module unload race, we place the release method * into a helper code that is compiled directly into the kernel. */ void dm_kobject_release(struct kobject *kobj) { complete(dm_get_completion_from_kobject(kobj)); } EXPORT_SYMBOL(dm_kobject_release);