From owner-freebsd-hackers@FreeBSD.ORG  Fri Apr 25 14:35:15 2008
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id D625810656B4
	for <freebsd-hackers@freebsd.org>; Fri, 25 Apr 2008 14:35:15 +0000 (UTC)
	(envelope-from adamw@signallake.com)
Received: from pro38.abac.com (pro38.abac.com [66.226.64.39])
	by mx1.freebsd.org (Postfix) with ESMTP id 4FA3F8FC20
	for <freebsd-hackers@freebsd.org>; Fri, 25 Apr 2008 14:35:15 +0000 (UTC)
	(envelope-from adamw@signallake.com)
Received: from [192.168.1.200] (c-24-218-111-132.hsd1.ma.comcast.net
	[24.218.111.132]) (authenticated bits=0)
	by pro38.abac.com (8.14.1/8.14.1) with ESMTP id m3PDxuRl013957
	(version=TLSv1/SSLv3 cipher=AES128-SHA bits=128 verify=NO)
	for <freebsd-hackers@freebsd.org>; Fri, 25 Apr 2008 06:59:57 -0700 (PDT)
	(envelope-from adamw@signallake.com)
Message-Id: <DAB770F2-C987-4AEB-BD02-BD1AEF099702@signallake.com>
From: Adam <adamw@signallake.com>
To: freebsd-hackers@freebsd.org
Mime-Version: 1.0 (Apple Message framework v919.2)
Date: Fri, 25 Apr 2008 09:59:55 -0400
X-Mailer: Apple Mail (2.919.2)
X-Spam-Score: 0.101 (HTML_MESSAGE)
X-Mailman-Approved-At: Fri, 25 Apr 2008 14:52:51 +0000
Content-Type: text/plain;
	charset=US-ASCII;
	format=flowed;
	delsp=yes
Content-Transfer-Encoding: 7bit
X-Content-Filtered-By: Mailman/MimeDel 2.1.5
Subject: CopyOut Size Limits
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>, 
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 25 Apr 2008 14:35:16 -0000

Hi, I am writing a custom system call that needs to transfer 16kb of  
data from the kernel to userspace. I am transferring the data out of  
the kernel by using copyout.  This seems to work for a small struct of  
data < 4k.

int my_system_call(struct thread *td, struct my_system_call_args *uap)
{
	my_structtype_t my_type;

	copyout(&my_type,uap->my_type,sizeof(my_type) ))!=0)
	{
		printf("\n copyout failed ret%d\n",error);
		return error;
	}

	printf("exiting kernel %d\n",error);
	return (0);
}

However once I expand my struct size beyond around 4k that I get a  
"Fatal Double Fault."  It seems like I am overrunning the kernel  
stack.  Does copyout use memory from the kernel stack?  What is the  
limit for copyout?  Is there some way to allocate additional space?   
Alternatively what is the appropriate method for transferring kbs of  
data from kernel to userspace?

Thanks,
Adam