From: Arnd Bergmann <arnd@arndb.de>
To: monstr@monstr.eu
Subject: Re: [PATCH 20/30] microblaze_mmu_v1: uaccess MMU update
Date: Wed, 29 Apr 2009 12:58:40 +0200
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Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       john.williams@petalogix.com
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On Wednesday 29 April 2009, Michal Simek wrote:
> Here is that change which remove address space problem.
> 
> #define __clear_user(addr, n)???(memset((__force void *)(addr), 0, (n)), 0)
> 
> Am I right?
> 
> The same mishmash is for memset_fromio/memset_toio and maybe some others which I want to fix too.

This will work, but a better fix would be to define an inline
function that explicitly takes a __user pointer. This would give
you warnings when code accidentally calls __clear_user on a
kernel pointer (this also adds the might_sleep()):

static inline unsigned long __must_check 
__clear_user(void __user *to, unsigned long n)
{
	memset((__force void *)addr, 0, n);
	return 0;
}

static inline unsigned long __must_check 
clear_user(void __user *to, unsigned long n)
{
	might_sleep();
	if (unlikely(!access_ok(VERIFY_WRITE, to, n)))
		return n;

	return __clear_user(to, n);
}

The above is just the nommu variant. For mmu, you need to
have exception handling in __clear_user to take care of the
case where the address is part of the user mapping (access_ok)
but not currently mapped.

	Arnd <><
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