LXR linux/drivers/md/dm-builtin.c

   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);
  50