From owner-freebsd-hackers  Mon Feb 19 11:26:41 2001
Delivered-To: freebsd-hackers@freebsd.org
Received: from smtp.nettoll.com (matrix.nettoll.net [212.155.143.61])
	by hub.freebsd.org (Postfix) with ESMTP id 2A94537B401
	for <hackers@freebsd.org>; Mon, 19 Feb 2001 11:26:34 -0800 (PST)
Received: by smtp.nettoll.com; Mon, 19 Feb 2001 20:25:02 +0100 (MET)
Message-Id: <4.3.0.20010219202101.05cf15a0@pop.free.fr>
X-Sender: usebsd@pop.free.fr
X-Mailer: QUALCOMM Windows Eudora Version 4.3
Date: Mon, 19 Feb 2001 20:26:56 +0100
To: idobarnea@NewMail.Net, hackers@freebsd.org
From: mouss <usebsd@free.fr>
Subject: Re: Bug in creating ICMP error messages in FreeBSD4.2
Cc: andrew@cnsec.co.za
In-Reply-To: <3a912cee.150.0@NewMail.Net>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"; format=flowed
Sender: owner-freebsd-hackers@FreeBSD.ORG
Precedence: bulk
X-Loop: FreeBSD.ORG

At 14:25 19/02/01 +0200, idobarnea@NewMail.Net wrote:
>Hi,
>   I encountered the following problem in the 4.2 version.
>In ip_forward, the following lines intend to save the mbuf in case we want to
>send ICMP error later:
>  mcopy = m_copy(m, 0, imin((int)ip->ip_len, 64));
>  if (mcopy && (mcopy->m_flags & M_EXT))
>      m_copydata(mcopy, 0, sizeof(struct ip), mtod(mcopy, caddr_t));
>
>Later on, before sending the ICMP packet we do:
>  if (mcopy->m_flags & M_EXT)
>      m_copyback(mcopy, 0, sizeof(struct ip), mtod(mcopy, caddr_t));
>
>The problem as I understand it is that the m_copydata and m_copyback, actually
>do nothing (It just
>copies from mcopy to itself).

I'm speaking from memory, so don't take this for more than it is:)

As far as I understand:
m_copym copies the mbuf, but if there is external storage, only pointers
are copied. so you get two mbuf chains with a common external storage.
m_copydata will copy the external storage.
that's why there are both m_copym and m_copydata. so while
   m_copydata(mcopy, .... (mcopy...))
is surprising, it's not nothing. it copies the data pointed to in mcopy.

cheers,
mouss



To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message