netdev - Re: [PATCH] Revert "ip_gre: make ipgre_tunnel_xmit() not parse network header as IP unconditionally"

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Date:	Thu, 14 Mar 2013 11:53:47 +0900
From:	Isaku Yamahata <yamahata@...inux.co.jp>
To:	Timo Teras <timo.teras@....fi>
Cc:	netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] Revert "ip_gre: make ipgre_tunnel_xmit() not parse
	network header as IP unconditionally"

On Wed, Mar 13, 2013 at 05:56:43PM +0200, Timo Teras wrote:
> On Thu, 14 Mar 2013 00:05:10 +0900
> Isaku Yamahata <yamahata@...inux.co.jp> wrote:
> 
> > Hi.
> > 
> > On Wed, Mar 13, 2013 at 02:37:49PM +0200, Timo Teräs wrote:
> > > This reverts commit 412ed94744d16806fbec3bd250fd94e71cde5a1f.
> > > 
> > > The commit is wrong as tiph points to the outer IPv4 header which is
> > > installed at ipgre_header() and not the inner one which is protocol
> > > dependant.
> > > 
> > > This commit broke succesfully opennhrp which use PF_PACKET socket
> > > with ETH_P_NHRP protocol. Additionally ssl_addr is set to the
> > > link-layer IPv4 address. This address is written by ipgre_header()
> > > to the skb earlier, and this is the IPv4 header tiph should point
> > > to - regardless of the inner protocol payload.
> > 
> > Is this the case only for ETH_P_HNRP?
> > I wrote the patch having MPLS over GRE in mind.
> > Should it be something like this?
> >   if (protocol == htons(ETH_P_IP) || protocol == htons(ETH_P_NHRP))
> >     ....
> 
> No, the original code was correct for all protocols.
> 
> tiph refers to the IPv4 headers pushed by ip_gre driver in
> ipgre_header() function. This is _always_ present regardless of the
> inner protocol. Thus no checking for protocol should be done.
> 
> And for reference: NHRP does not have iphdr, it is completely different
> payload.
> 
> What happens is on xmit path is:
> 
>  1. ipgre_header() is always called first with NBMA destination
>     address and outer IPv4 + GRE headers are pushed here with
>     per-packet destination set
> 
>  2. ipgre_tunnel_xmit() is called, tiph points to the tunnel's
>     IPv4 header, gre_hlen is set to zero so that the header is not
>     pushed twice, but the existing header is modified later
> 
> Thus, it does not matter if the payload is MPLS, we will not refer to
> the mpls data - the skb has already tunnel's outer IPv4 and GRE header
> pushed, and tiph will refer to the tunnel's IPv4 header.

You're right. Thanks for explanation.
Originally I tried to copy TTL in MPLS label into outer IP header.
But dropped MPLS bits.
-- 
yamahata
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