On Fri, Oct 9, 2020 at 1:38 PM Cong Wang <xiyou.wangc...@gmail.com> wrote: > > Interesting point. I think needed_headroom is 0 until we call > ipgre_changelink(), but needed_headroom is already being used > in multiple places for skb_cow_head() in the same file, I guess > they should be replaced with hard_head_len because for GRE tunnel > those are its link-layer header. What makes it more complicated > is that header_ops is actually set conditionally, so should be > hard_header_len/needed_head_room accordingly.
Looking a bit deeper, I doubt the ipgre_header_ops->create is necessary, because 1) all other tunnels devices do not have it (ip_tunnel_header_ops only contains ->parse_protocol); 2) GRE headers are pushed in xmit anyway, so at least SOCK_DGRAM does not need it; 3) ipgre_header() creates the GRE header, later ipgre_xmit() pulls it back, then __gre_xmit() builds GRE header again... Please correct me if I am wrong. Thanks.
