On Sat, Jun 27, 2020 at 10:17:26AM -0700, Eric Dumazet wrote:
[ ... ]

It seems strange that we want to add code in TCP stack only to cover a
limited use case (kind 254 and 0xEB9F magic)

For something like the work Petar Penkov did (to be able to generate
SYNCOOKIES from XDP), we do not go through tcp_parse_options() and BPF
program
would have to implement its own parsing (without having an SKB at
hand), probably calling a helper function, with no
TCP_SKB_CB(skb)->bpf_hdr_opt_off.

This patch is hard coding a specific option and will prevent anyone
using private option(s) from using this infrastructure in the future,
yet paying the extra overhead.

There is a discussion in patch 4 about not limiting this patch set
to option kind 254.  That will affect the usefulness of bpf_hdr_opt_off.

TCP_SKB_CB(skb) is tight, I would prefer keeping the space in it for
standard TCP stack features.

If an optional BPF program needs to re-parse the TCP options to find a
specific option, maybe the extra cost is noise (especially if this is
only for SYN & SYNACK packets) ?

Thanks