linux-kernel - Re: copy_*

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Date:	Sun, 23 Oct 2011 05:23:44 -0400
From:	Xin Tong <xerox.time.tech@...il.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: copy_*_user

I found that __copy_from_user_ll(void *to, const void __user *from,
unsigned long n) eventually calls some asm which use mov and rep on
the passed-in to & from pointers. This is in kernel mode and to & from
are virtual addresses ? are not the kernel accessing physical RAM
directly ( without pagetable ) ?


Thanks

Xin



On Sat, Oct 22, 2011 at 10:50 PM, Américo Wang <xiyou.wangcong@...il.com> wrote:
> 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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