dm-builtin.c 2.1 KB

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  1. // SPDX-License-Identifier: GPL-2.0
  2. #include "dm-core.h"
  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. void dm_kobject_release(struct kobject *kobj)
  43. {
  44. complete(dm_get_completion_from_kobject(kobj));
  45. }
  46. EXPORT_SYMBOL(dm_kobject_release);