Re: [PATCH] staging: comedi: remove __user annotation inside of struct's

Ian Abbott
2012-05-10T11:05:32Z

On 2012-05-09 15:19, Dan Carpenter wrote:
> On Wed, May 09, 2012 at 12:01:25PM +0100, Ian Abbott wrote:
>> Are there any handy macros for casting pointers to __user pointers,
>> something like
>>
>> #define _user(p) ((typeof(*(p)) __user *)(p))
>>
>> but preferably without the repeated expansion of 'p' in case of
>> side-effects?
>
> typeof() doesn't have side effects.
>
> #include<stdio.h>
>
> int main(void)
> {
> int x = 0;
> typeof(x++) y;
>
> printf("%d\n", x);
>
> return 0;
> }
>
> regards,
> dan carpenter

Yes, you're correct of course. I was unnecessarily worried about the
double-expansion of the macro parameter but it's safe in this case.

BTW, the intention of this macro is so you can write things like:

copy_from_user(data, _user(insn.data), insn.n * sizeof(insn.data[0]));

instead of:

copy_from_user(data, (unsigned int __user *)insn.data, insn.n *
sizeof(insn.data[0]));

where the member insn.data is a 'unsigned int *' but is used for both
__user pointers and kernel pointers.

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