Re: access_ok macor

[Michal Simek <monstr@monstr.eu>]

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)
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