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

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Date:   Tue, 17 Jan 2023 19:47:15 -0700
From:   David Ahern <dsahern@...il.com>
To:     Paul Moore <paul@...l-moore.com>, Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, davem@...emloft.net,
        kuba@...nel.org, Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Pravin B Shelar <pshelar@....org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Ilya Maximets <i.maximets@....org>,
        Aaron Conole <aconole@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Mahesh Bandewar <maheshb@...gle.com>,
        Guillaume Nault <gnault@...hat.com>,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org
Subject: Re: [PATCH net-next 06/10] cipso_ipv4: use iph_set_totlen in
 skbuff_setattr

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?