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 12:39:10 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
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 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!)

Tom
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