netdev - Re: [PATCH net v2 3/4] ipv6: no CHECKSUM_PARTIAL on MSG

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Date:	Tue, 27 Oct 2015 11:37:21 -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>,
	Vladislav Yasevich <vyasevich@...il.com>,
	Benjamin Coddington <bcodding@...hat.com>
Subject: Re: [PATCH net v2 3/4] ipv6: no CHECKSUM_PARTIAL on MSG_MORE corked sockets

On Tue, Oct 27, 2015 at 11:29 AM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Tue, Oct 27, 2015, at 18:32, Tom Herbert wrote:
>> On Tue, Oct 27, 2015 at 9:44 AM, Hannes Frederic Sowa
>> <hannes@...essinduktion.org> wrote:
>> >
>> >
>> > On Tue, Oct 27, 2015, at 17:36, Tom Herbert wrote:> > -               if
>> > (cork->length + length > maxnonfragsize - headersize) {
>> >> > +       if (cork->length + length > maxnonfragsize - headersize) {
>> >> >  emsgsize:
>> >> > -                       ipv6_local_error(sk, EMSGSIZE, fl6,
>> >> > -                                        mtu - headersize +
>> >> > -                                        sizeof(struct ipv6hdr));
>> >> > -                       return -EMSGSIZE;
>> >> > -               }
>> >> > +               ipv6_local_error(sk, EMSGSIZE, fl6,
>> >> > +                                mtu - headersize +
>> >> > +                                sizeof(struct ipv6hdr));
>> >> > +               return -EMSGSIZE;
>> >> >         }
>> >> >
>> >> > +       /* CHECKSUM_PARTIAL only with no extension headers and when
>> >>
>> >> No, please don't do this. CHECKSUM_PARTIAL should work with extension
>> >> headers as defined, so this is just disabling otherwise valid and
>> >> useful functionality. If (some) drivers have problems with this they
>> >> need to be identified and fixed.
>> >
>> > I don't understand. The old code already didn't allow the use of
>> > opt_flen with CHECKSUM_PARTIAL.
>> >
>> Then that's a problem with the old code :-). Is there any other reason
>> that we can't use CHECKSUM_PARTIAL with extension headers other than
>> lack of correct driver support?
>
> The lack of correct driver support is a big bumper, but as I wrote, I
> don't see a reason to not lift this restriction in net-next. I proposed
> a new feature flag, or by looking at your series, we could probably use
> the extension header okay field for that.
>
Okay, but why bother doing this for net? This problem has obviously
existed for a while, and even if the restriction is maintained here
there are still other paths that don't go through ip_append_data that
could trip the bug. Also, drivers are welcome to fix their issues in
net I believe.

> I would be conservative in net though.
>
> Bye,
> Hannes
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