netdev - Re: [PATCH net] ipv6: no CHECKSUM_PARTIAL on skbs with extension headers and recalc checksum during fragmentation

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Date:	Mon, 26 Oct 2015 22:25:58 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Tom Herbert <tom@...bertland.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Vlad Yasevich <vyasevich@...il.com>,
	Benjamin Coddington <bcodding@...hat.com>
Subject: Re: [PATCH net] ipv6: no CHECKSUM_PARTIAL on skbs with extension headers
 and recalc checksum during fragmentation

On Mon, Oct 26, 2015, at 20:39, Tom Herbert wrote:
> On Mon, Oct 26, 2015 at 11:44 AM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> wrote:
> >
> >
> > On Mon, Oct 26, 2015, at 15:19, Tom Herbert wrote:
> >> > We already concluded that drivers do have this problem and not the stack
> >> > above ip6_fragment. The places I am aware of I fixed in this patch. Also
> >> > IPv4 to me seems unaffected, albeit one can certainly clean up the logic
> >> > in net-next.
> >> >
> >> I don't understand why checksum for IP fragments is a driver problem.
> >> When fragments are sent to driver they should never have
> >> CHECKSUM_PARTIAL set (or maybe that is what you are seeing?).
> >
> > Because either the drivers or the hardware does not correctly iterate
> > over the extension headers to fetch the final nexthdr field which is
> > used to compute the checksum. This is different from IPv4.
> >
> > I can only guess e.g. from the e1000e driver:
> >
> >         case cpu_to_be16(ETH_P_IPV6):
> >                 /* XXX not handling all IPV6 headers */
> >                 if (ipv6_hdr(skb)->nexthdr == IPPROTO_TCP)
> >                         cmd_len |= E1000_TXD_CMD_TCP;
> >                 break;
> >
> Yes, but in the case of a fragment that code should never be hit since
> ip_summed shouldn't be CHECKSUM_PARTIAL for a fragment (maybe after
> the fix in ip_output). For other cases of extension headers the e1000e
> is broken since it apparently does call skb_checksum_help for
> protocols it doesn't understand (the /* XXX not handling all IPV6
> headers */ comment is worrisome!)

Agreed! I am testing with WARN_ON_ONCE in ip6_fragment if I can hit
another path where we would have to call skb_checksum_help. I need to
review IPv4 tomorrow if we need to do according changes there, probably.

Bye,
Hannes
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