Commit | Line | Data |
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2995fa78 MP |
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); |