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Date: Mon, 09 Jul 2007 23:15:09 +0200
From: Fabrice Bellard <fabrice@bellard.org>
Subject: Re: [Qemu-devel] [PATCH] linux-user EFAULT implementation
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To: qemu-devel@nongnu.org

Stuart Anderson wrote:
> On Fri, 6 Jul 2007, Stuart Anderson wrote:
> 
>> So, the question is:
>>
>>     Can I simplify this code to assume that guest and
>>     host addresses coexist and use the copy_*_user() or
>>     just the access_ok() interfaces?

No. Ideally you should use the same conventions as the Linux kernel and 
assume that you cannot access the user data directly. To summarise:

- access_ok() is needed before __copy_*_user and __[get|put]_user.

- access_ok() is not needed before copy_[to|from]_user and [get|put]_user.

- [get|put]_user do automatic endianness change.

- Use tswapx and the other related functions to change the endianness 
before or after copy_*_user.

- for specific cases requiring speed (e.g. buffers for read/write 
syscalls), specific tricks to avoid memory copies can be used with a 
lock/unlock style API.

> Attached is a diff [...]

Please send a unified diff.

For the time being, I would suggest to minimize the number of changes 
and just extend lock_user()/unlock_user() as you began to do to handle 
-EFAULT. The rest is mostly a question of cosmetics.

Regards,

Fabrice.