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

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Message-Id: <20061005133807.97827533.akpm@osdl.org>
Date:	Thu, 5 Oct 2006 13:38:07 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Andi Kleen <ak@...e.de>
Cc:	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

On 05 Oct 2006 21:57:04 +0200
Andi Kleen <ak@...e.de> wrote:

> Andrew Morton <akpm@...l.org> writes:
> 
> > 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.		
> 
> It is needed on lots of architectures that use separate address spaces
> like sparc64, m68k, s390 (and on x86 with 4:4 patches) 
> 

It needs set_fs(KERNEL_DS) if we're going to use __get_user() on both
callpaths.

I think we'll stick with the `is_user' version - less tricky, clearer.
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