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