Re: kernel thread and copy_to_user

[Robert Love <rml@tech9.net>]

On Fri, 2002-12-27 at 11:23, zw wrote:

> > But I thought while doing copy_to_user() the task should have 
> > a valid task->mm. no?
> 
> You mean the kernel thread, or the user task? I think you don't have 
> to do anything special on your kernel thread. You kernel thread, I 
> think, basically waiting there for a pointer from user space. Then 
> you call copy_to_user() in your kernel thread. That's it. I think.

Think about what devnetfs is asking.

The function is prototyped as copy_to_user(to, from, len) where to and
from are pointers.

If current->mm is invalid, then exactly whose "from" are you copying
to?  Remember, Linux is a virtual memory operating system: there can be
many mappings at the same address.

Likewise, with copy_from_user(), without a valid ->mm from where exactly
are you copying from?

The functions pull the to/from user addresses from the loaded user
address space.  Without a user address space, these functions do not
work.  Nor could they.

In short, to answer the original question, kernel threads cannot copy
from user-space since they have no user-space.  If you want to get data
into and out of a kernel thread, expose an interface (procfs, sysfs,
syscall, device file, etc.)

	Robert Love

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