|From:||Andi Kleen <ak-AT-suse.de>|
|Subject:||unregister_ioctl32_conversion and modules. ioctl32 revisited.|
|Date:||Wed, 15 Dec 2004 07:56:50 +0100|
|Cc:||pavel-AT-suse.cz, discuss-AT-x86-64.org, gordon.jin-AT-intel.com|
Hallo, There seems to be an unfixable module unload race in the current register_ioctl32_conversion support. The problem is that there is no way to wait for a conversion handler is blocked in a sleeping *_user access before module unloading. The module count is also not increase in this case. I previously thought it could be avoided by always putting the compat wrapper into the same module and using the reference counting that is done by VFS on ->module of the char device or file system converted. But that doesn't work because the ioctl32 lookup is independent from the file descriptor and a conversion handler can be entered even after ->release when the right number is passed. One way to fix it would be to put a pointer to the module into the compat ioctl hash table and increase the module count atomically from the compat code. But that would bloat the hash table a lot and I don't like it very much. Also you would either break the register_ioctl32_conversion prototype or make it a macro and assume all calls are from the same module, which is also quite ugly. A better solution would be to switch the few users of register_ioctl32_conversion() over to a new ->ioctl32 method in file_operations and do the conversion from there. This would avoid the race because the VFS will take care of the module count in open/release. Michael did a patch for this some time ago for a different motivation - he had some benchmarks where the hash table lookup hurt and it was noticeable faster to use a O(1) ->ioctl32 lookup from the file_operations for his application. An useful side effect would be also to the ability to support a per device ioctl name space. While the core kernel doesn't have much (any?) ioctls with duplicated numbers this mistake seems to be quite common in out of tree drivers and it is hard to fix without breaking compatibility. And it would be faster for this case of course too, so even performance critical in kernel ioctls could be slowly converted to ioctl32 I wouldn't do it for all, because the current central tables work reasonably well for them and most ioctls are not speed critical anyways. As for in kernel code it won't affect much code because near all conversion handlers in the main tree are not modular (alsa is one exception, there are a few others e.g. in some raid drivers). I expect it will be a bigger problem in the future though as ioctl emulation becomes more widespread and is done more in individual drivers. averell:lsrc/v2.6/linux% gid register_ioctl32_conversion | wc -l 75 averell:lsrc/v2.6/linux% In tree users are alsa, aaraid, fusion, some s390 stuff, sisfb, alsa My proposal would be to dust off Michael's patch and convert all users in tree over to ioctl32 and then deprecate and later remove (un)register_ioctl32_conversion Comments? -Andi
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