linux-kernel - Re: copy_*

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Message-ID: <CAM_iQpU+xHww_JCYYA4kAKCXmOZpZeu=+rnCyRL-5dW5Uxk1hQ@mail.gmail.com>
Date:	Sun, 23 Oct 2011 10:50:08 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Xin Tong <xerox.time.tech@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: copy_*_user

On Sun, Oct 23, 2011 at 1:06 AM, Xin Tong <xerox.time.tech@...il.com> wrote:
> I am investigating copy_from_user  and copy_to_user in linux under
> i386. These two function both take a pointer with virtual address and
> a pointer with physical address.
>
> copy_from_user calls __copy_from_user_ll and copy_to_user  calls
> __copy_to_user_ll. It make sense to me that __copy_to_user_ll converts
> the virtual address to physical address using the current process's
> page table.
>
[...]
> But it seems to be that __copy_from_user_ll  is not converted the
> address at all before attempting to copy. Can someone help explain to
> me why ?
>

You missed that __copy_to_user_ll() only does that when CONFIG_X86_WP_WORKS_OK
is not defined. And there is a comment right inside __copy_to_user_ll() said:


               /*
                 * CPU does not honor the WP bit when writing
                 * from supervisory mode, and due to preemption or SMP,
                 * the page tables can change at any time.
                 * Do it manually.      Manfred <manfred@...orfullife.com>
                 */

this is why it uses kmap_atomic()+memcpy() to copy the data.

Also, all the addresses are virtual address.
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