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Re: [Qemu-devel] [PATCH] linux-user EFAULT implementation

From: Fabrice Bellard
Subject: Re: [Qemu-devel] [PATCH] linux-user EFAULT implementation
Date: Mon, 09 Jul 2007 23:15:09 +0200
User-agent: Thunderbird (X11/20070212)

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.



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