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Seeing as some of this is my code...

Guido, the ioctl docstring *does* mention mutate_arg.  I agree that the 
documentation should have been updated for 2.4 and 2.5... and the situation 
is a mess, yes, but one that I couldn't see a better way around when the 
feature was added (it was much discussed in the bug report at the time).

It certainly never occurred to me that you might need/want to pass a NULL 
terminated string to ioctl. 

I don't think this is a priority 9 report.