49 lines
2 KiB
C
49 lines
2 KiB
C
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#include "dm.h"
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/*
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* The kobject release method must not be placed in the module itself,
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* otherwise we are subject to module unload races.
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*
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* The release method is called when the last reference to the kobject is
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* dropped. It may be called by any other kernel code that drops the last
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* reference.
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*
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* The release method suffers from module unload race. We may prevent the
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* module from being unloaded at the start of the release method (using
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* increased module reference count or synchronizing against the release
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* method), however there is no way to prevent the module from being
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* unloaded at the end of the release method.
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*
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* If this code were placed in the dm module, the following race may
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* happen:
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* 1. Some other process takes a reference to dm kobject
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* 2. The user issues ioctl function to unload the dm device
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* 3. dm_sysfs_exit calls kobject_put, however the object is not released
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* because of the other reference taken at step 1
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* 4. dm_sysfs_exit waits on the completion
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* 5. The other process that took the reference in step 1 drops it,
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* dm_kobject_release is called from this process
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* 6. dm_kobject_release calls complete()
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* 7. a reschedule happens before dm_kobject_release returns
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* 8. dm_sysfs_exit continues, the dm device is unloaded, module reference
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* count is decremented
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* 9. The user unloads the dm module
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* 10. The other process that was rescheduled in step 7 continues to run,
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* it is now executing code in unloaded module, so it crashes
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*
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* Note that if the process that takes the foreign reference to dm kobject
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* has a low priority and the system is sufficiently loaded with
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* higher-priority processes that prevent the low-priority process from
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* being scheduled long enough, this bug may really happen.
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*
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* In order to fix this module unload race, we place the release method
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* into a helper code that is compiled directly into the kernel.
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*/
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void dm_kobject_release(struct kobject *kobj)
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{
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complete(dm_get_completion_from_kobject(kobj));
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}
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EXPORT_SYMBOL(dm_kobject_release);
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