linux-kernel - Re: access

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Message-ID: <4A5DA96F.8000900@monstr.eu>
Date:	Wed, 15 Jul 2009 12:03:27 +0200
From:	Michal Simek <monstr@...str.eu>
To:	Paul Mundt <lethal@...ux-sh.org>, Arnd Bergmann <arnd@...db.de>,
	monstr@...str.eu, Linux Kernel list <linux-kernel@...r.kernel.org>,
	LTP <ltp-list@...ts.sourceforge.net>
Subject: Re: access_ok macor

Paul Mundt wrote:
> On Tue, Jul 14, 2009 at 07:13:20PM +0200, Arnd Bergmann wrote:
>> On Tuesday 14 July 2009, Michal Simek wrote:
>>> when the code tried to read/write from unaligned address (and in cpu
>>> is turn on unaligned exception) then is caused unaligned exception
>>> and asm code assemble/return value which is on that unaligned
>>> address. (Assemble it that read/write every byte separately). That
>>> will be harder to prevent all this cases because unaligned exception
>>> is in generic code.  What do you mean add __range_ok? Range checking
>>> is ok. The problem is when in case get_user kernel try to load
>>> unaligned addr - unaligned exception is perform and try to load that
>>> value separately.  If that page is not there, page fault handler is
>>> called and not find it, it is performed search from exception table
>>> and that address is not there of course - because address in pc is
>>> generic unaligned code. I think that handling this needs more code.
>>> Maybe if the address with from unaligned exception handler (there are
>>> some address which can caused it) and find out which aligned address
>>> is there and find out proper fixup for it.  I think that this could
>>> work.
>>>
>>> What do you think?
>> I think the key point is that the kernel should never try an unaligned
>> access. Other architectures already rely on this, so you can too.
> 
> No, other architectures used to rely on this, until it was no longer
> possible to do so. See for example, nfs. Unaligned accesses by the kernel
> must be handled by the architecture, unaligned accesses by userspace can
> be optionally fixed up.

Can you please look at John's email in this thread. How does SH handle this case?
I mean when for get/put user address is added unaligned address. How does your kernel handle it?

Michal


-- 
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
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