[lkml]   [2011]   [Oct]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: copy_*_user
On Sun, Oct 23, 2011 at 1:06 AM, Xin Tong <> 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 <>

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

Also, all the addresses are virtual address.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2011-10-23 04:53    [W:0.045 / U:7.952 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site