dm-builtin.c (2128B)
1// SPDX-License-Identifier: GPL-2.0 2#include "dm-core.h" 3 4/* 5 * The kobject release method must not be placed in the module itself, 6 * otherwise we are subject to module unload races. 7 * 8 * The release method is called when the last reference to the kobject is 9 * dropped. It may be called by any other kernel code that drops the last 10 * reference. 11 * 12 * The release method suffers from module unload race. We may prevent the 13 * module from being unloaded at the start of the release method (using 14 * increased module reference count or synchronizing against the release 15 * method), however there is no way to prevent the module from being 16 * unloaded at the end of the release method. 17 * 18 * If this code were placed in the dm module, the following race may 19 * happen: 20 * 1. Some other process takes a reference to dm kobject 21 * 2. The user issues ioctl function to unload the dm device 22 * 3. dm_sysfs_exit calls kobject_put, however the object is not released 23 * because of the other reference taken at step 1 24 * 4. dm_sysfs_exit waits on the completion 25 * 5. The other process that took the reference in step 1 drops it, 26 * dm_kobject_release is called from this process 27 * 6. dm_kobject_release calls complete() 28 * 7. a reschedule happens before dm_kobject_release returns 29 * 8. dm_sysfs_exit continues, the dm device is unloaded, module reference 30 * count is decremented 31 * 9. The user unloads the dm module 32 * 10. The other process that was rescheduled in step 7 continues to run, 33 * it is now executing code in unloaded module, so it crashes 34 * 35 * Note that if the process that takes the foreign reference to dm kobject 36 * has a low priority and the system is sufficiently loaded with 37 * higher-priority processes that prevent the low-priority process from 38 * being scheduled long enough, this bug may really happen. 39 * 40 * In order to fix this module unload race, we place the release method 41 * into a helper code that is compiled directly into the kernel. 42 */ 43 44void dm_kobject_release(struct kobject *kobj) 45{ 46 complete(dm_get_completion_from_kobject(kobj)); 47} 48 49EXPORT_SYMBOL(dm_kobject_release);