Date: Thu, 21 Aug 2008 22:35:19 +0200 From: Luigi Rizzo <rizzo@iet.unipi.it> To: gnn@freebsd.org Cc: net@freebsd.org Subject: Re: Small patch to multicast code... Message-ID: <20080821203519.GA51534@onelab2.iet.unipi.it> In-Reply-To: <m27iaa6v43.wl%gnn@neville-neil.com> References: <m27iaa6v43.wl%gnn@neville-neil.com>
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On Thu, Aug 21, 2008 at 03:11:56PM -0400, gnn@freebsd.org wrote:
> Hi,
>
> Turns out there is a bug in the code that loops back multicast
> packets. If the underlying device driver supports checksum offloading
> then the packet that is looped back, when it is transmitted on the
> wire, is incorrect, due to the fact that the packet is not fully
> copied.
>
> Here is a patch. Comments welcome.
>
> Best,
> George
>
> Index: ip_output.c
> ===================================================================
> --- ip_output.c (revision 181731)
> +++ ip_output.c (working copy)
> @@ -1135,7 +1135,7 @@
> register struct ip *ip;
> struct mbuf *copym;
>
> - copym = m_copy(m, 0, M_COPYALL);
> + copym = m_dup(m, M_DONTWAIT);
> if (copym != NULL && (copym->m_flags & M_EXT || copym->m_len < hlen))
> copym = m_pullup(copym, hlen);
> if (copym != NULL) {
I am slightly puzzled -- what is exactly the problem, i.e. what part
of the packet on the wire is incorrect ? The IP header is within hlen so
the m_pullup() should be enough to leave the original content intact.
The only thing i can think of is that it's the UDP checksum,
residing beyond hlen, which is overwritten somewhere in the
call to if_simloop -- in which case perhaps a better fix is
to m_pullup() the udp header as well ?
(in any case, it is worthwhile to add a comment to explain
what should be done -- the code paths using m_*() have become
quite fragile with these hw support enhancements that now
require selective modifications on previously shared, readonly buffers).
cheers
luigi
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