linux-kernel - Re: [PATCH] bitmap: separate bitmap parsing for user buffer and kernel buffer

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Message-ID: <45246276.6000908@zytor.com>
Date:	Wed, 04 Oct 2006 18:40:06 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Morton <akpm@...l.org>
CC:	Reinette Chatre <reinette.chatre@...ux.intel.com>,
	Joe Korty <joe.korty@...r.com>,
	Inaky Perez-Gonzalez <inaky@...ux.intel.com>,
	Paul Jackson <pj@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bitmap: separate bitmap parsing for user buffer and kernel
 buffer

Andrew Morton wrote:
> On Wed, 4 Oct 2006 17:56:30 -0700
> Reinette Chatre <reinette.chatre@...ux.intel.com> wrote:
> 
>> +			if (is_user) {
>> +				if (__get_user(c, buf++))
>> +					return -EFAULT;
>> +			}
>> +			else
>> +				c = *buf++;
> 
> Is this actually needed?  __get_user(kernel_address) works OK and (believe
> it or not, given all the stuff it involves) boils down to a single instruction.		

On some architectures, kernel and user space are separate, overlapping 
address spaces.

If __bitmap_parse was an inline (and not exported), this would be okay; 
as it is, you end up doing the test dynamically under all circumstances, 
even though in most (if not all) cases the address space is know a priori.

	-hpa
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