On Tue, 30 Jun 2020 17:51:41 +0200 Nicolas Dichtel wrote:

Le 30/06/2020 à 08:22, Jakub Kicinski a écrit :
[snip]

quoted

My understanding is that for a while now tunnels are not supposed to use
dev->hard_header_len to reserve skb space, and use dev->needed_headroom, 
instead. sit uses hard_header_len and doesn't even copy needed_headroom
of the lower device.  

I missed this. I was wondering why IPv6 tunnels uses hard_header_len, if there
was a "good" reason:

$ git grep "hard_header_len.*=" net/ipv6/
net/ipv6/ip6_tunnel.c:                  dev->hard_header_len =
tdev->hard_header_len + t_hlen;
net/ipv6/ip6_tunnel.c:  dev->hard_header_len = LL_MAX_HEADER + t_hlen;
net/ipv6/sit.c:         dev->hard_header_len = tdev->hard_header_len +
sizeof(struct iphdr);
net/ipv6/sit.c: dev->hard_header_len    = LL_MAX_HEADER + t_hlen;

A cleanup would be nice ;-)

I did some archaeological investigatin' yesterday, and I saw
c95b819ad75b ("gre: Use needed_headroom") which converted GRE.
Then I think Pravin used GRE as a base for better ip_tunnel infra 
and the no-hard_header_len-abuse gospel has spread to other IPv4
tunnels. AFAICT IPv6 tunnels were not as lucky, and SIT just got
missed in the IPV4 conversion..