On Tue, Jan 17, 2023 at 9:47 PM David Ahern <dsahern@xxxxxxxxx> 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
