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

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Message-ID: <4FABA0F6.6090401@mev.co.uk>
Date:	Thu, 10 May 2012 12:05:26 +0100
From:	Ian Abbott <abbotti@....co.uk>
To:	Dan Carpenter <dan.carpenter@...cle.com>
CC:	Ian Abbott <ian.abbott@....co.uk>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	"fmhess@...rs.sourceforge.net" <fmhess@...rs.sourceforge.net>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	H Hartley Sweeten <hartleys@...ionengravers.com>
Subject: Re: [PATCH] staging: comedi: remove __user annotation inside of struct's

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.

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@....co.uk>        )=-
-=( Tel: +44 (0)161 477 1898   FAX: +44 (0)161 718 3587         )=-
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