[PATCH net-next 06/10] cipso_ipv4: use iph_set_totlen in skbuff_setattr

[Paul Moore]

On Tue, Jan 17, 2023 at 9:47 PM David Ahern <dsahern at gmail.com> wrote:
> On 1/17/23 3:46 PM, Paul Moore wrote:
> >>
> >> In the BIG TCP case, when is the IPv4 header zero'd out?  Currently
> >> cipso_v4_skbuff_setattr() is called in the NF_INET_LOCAL_OUT and
> >> NF_INET_FORWARD chains, is there an easy way to distinguish between a
> >> traditional segmentation offload mechanism, e.g. GSO, and BIG TCP?  If
> >> BIG TCP allows for arbitrarily large packets we can just grow the
> >> skb->len value as needed and leave the total length field in the IPv4
> >> header untouched/zero, but we would need to be able to distinguish
> >> between a segmentation offload and BIG TCP.
> >
> > Keeping the above questions as they still apply, rather I could still
> > use some help understanding what a BIG TCP packet would look like
> > during LOCAL_OUT and FORWARD.
>
> skb->len > 64kb. you don't typically look at the IP / IPv6 header and
> its total length field and I thought the first patch in the series added
> a handler for doing that.

Thanks, I was just curious if there was some other mechanism but that works.

As of this moment, the patchset I'm working on is still independent of
the BIG TCP patches, and I want to make sure I'm not doing anything
that will make the BIG TCP patches any more challenging.

> >>>> In the GRO case, is it safe to grow the packet such that skb->len is
> >>>> greater than 64k?  I presume that the device/driver is going to split
> >>>> the packet anyway and populate the IPv4 total length fields in the
> >>>> header anyway, right?  If we can't grow the packet beyond 64k, is
> >>>> there some way to signal to the driver/device at runtime that the
> >>>> largest packet we can process is 64k minus 40 bytes (for the IPv4
> >>>> options)?
> >>>
> >>> at runtime, not as far as I know.
> >>> It's a field of the network device that can be modified by:
> >>> # ip link set dev eth0 gro_max_size $MAX_SIZE gso_max_size $MAX_SIZE
> >>
> >> I need to look at the OVS case above, but one possibility would be to
> >> have the kernel adjust the GSO size down by 40 bytes when
> >> CONFIG_NETLABEL is enabled, but that isn't a great option, and not
> >> something I consider a first (or second) choice.
> >
> > Looking more at the GSO related code, this isn't likely to work.
>
> icsk_ext_hdr_len is adjusted by cipso for its options. Does that not
> cover what is needed?

Adjusting the icsk_ext_hdr_len only applies to CIPSO labels that are
attached via the associated local sock, traffic that is labeled by
cipso_v4_skbuff_setattr() in the LOCAL_OUT or FORWARD netfilter hooks
does not have the icsk_ext_hdr_len adjustment.

Although as I mentioned earlier, I am adding a patch which would pad
out the IPv4 option header in the LOCAL_OUT labeling scenario so
icsk_ext_hdr_len will be adjusted for all locally generated
TCP/connected/is_icsk traffic.  Forwarded traffic still remains an
issue; but I think the only thing we can do is drop it and send an
icmp message back to the sender with an adjusted MTU value.

--
paul-moore.com