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

[David Ahern <dsahern@gmail.com>]

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.

> 
>>>> 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?