Re: [PATCH] staging: comedi: cast to (unsigned int *)

[Ian Abbott]

On 19/02/2021 09:03, David Laight wrote:
>> It's kind of moot anyway because the patch is outdated.  But the reason
>> for the ___force is that the same `struct comedi_cmd` is used in both
>> user and kernel contexts.  In user contexts, the `chanlist` member
>> points to user memory and in kernel contexts it points to kernel memory
>> (copied from userspace).
> 
> Can't you use a union of the user and kernel pointers?
> (Possibly even anonymous?)
> Although, ideally, keeping them in separate fields is better.
> 8 bytes for a pointer isn't going make a fat lot of difference.

This is for a UAPI header (eventually), so cannot add a new field.  For
an anonymous union, one tagged with __user and one not, the __user tag
would be removed during conversion from UAPI headers to
/usr/include/linux headers, leaving a union of two identically typed
members, which would look a bit odd.  The union also kind of hides the
problem.

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