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       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                 :            : 
      44                 :          0 : void dm_kobject_release(struct kobject *kobj)
      45                 :            : {
      46                 :          0 :         complete(dm_get_completion_from_kobject(kobj));
      47                 :          0 : }
      48                 :            : 
      49                 :            : EXPORT_SYMBOL(dm_kobject_release);