Re: userspace vs. kernelspace address

From: Rock Gordon <rockgordon@yahoo.com>
To: Jan Hudec <bulb@ucw.cz>
Cc: linux-kernel@vger.kernel.org, kernelnewbies@nl.linux.org,
	Bernd Petrovitsch <bernd@firmix.at>
Subject: Re: userspace vs. kernelspace address
Date: Fri, 28 Jan 2005 13:40:51 -0800 (PST)	[thread overview]
Message-ID: <20050128214051.34768.qmail@web41411.mail.yahoo.com> (raw)
In-Reply-To: <20050128075209.GA14153@vagabond>

Hi everbody,

Thanks for your replies.

Lemme explain my problem a little bit more .... I have
a thread that does exactly similar things in
kernel-mode and user-mode (depending on how you
invoked it; of course, the kernel one is forked using
kernel_thread(), and the user one is from
pthread_create()). The architecture-dependant stuff is
taken care of by extensive use of __KERNEL__ macro
testing.

This particular thread gets a packet of data, the
header of which contains address to where it should be
copying the payload associated with that packet. The
kernel-mode thread will need to decide how to copy
data into another process' address space, so will the
user-mode thread.

However I think my copy_to_user and copy_from_user are
failing since the kernel-mode thread is copying data
into another process's address space, and I am not
sure how to do this. Do the get_fs() and set_fs()
combinations let you do that? If not, then how do I do
it?

Something like when you invoke the ->write or ->read
functions, you need to copy the requisite data into
the buffer the application provided you with.

Thanks and regards,
Rock

--- Jan Hudec <bulb@ucw.cz> wrote:

> On Fri, Jan 28, 2005 at 01:06:21 +0100, Bernd
> Petrovitsch wrote:
> > On Thu, 2005-01-27 at 09:14 -0800, Rock Gordon
> wrote:
> > > If I'm given a particular address, how do I test
> > > whether that address is from userspace or from
> kernel
> > > space?
> > 
> > You don't.
> > 
> > > I need to make these decisions from either
> inside a
> > > kernel module or a userspace program. The idea
> is I
> > > use memcpy() in the user-user version,
> > > copy_from/to_user in the kernel-kernel version,
> and
> > > prohibit the others.
> > 
> > You need to know where the address is from and use
> the correct function.
> 
> If the interface is defined as taking userland
> address, than kernel
> function passing a kernel address in is responsible
> for calling
> set_fs(KERNEL_DS) before and undoing it after. That
> way the
> copy_to/from_user does not complain.
> 
>
-------------------------------------------------------------------------------
> 						 Jan 'Bulb' Hudec <bulb@ucw.cz>
> 

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