[PATCH net v2 0/3] ipmr: forward GSO multicast packets

With fraglist GRO or UDP GRO forwarding on, a routed IPv4 multicast
stream arrives as GSO packets. ipmr_prepare_xmit() drops every DF
aggregate larger than the MTU, and a GSO packet sent to a VIFF_TUNNEL
vif is dropped during segmentation because ip_encap() sets no tunnel
GSO state. Once ip_encap() sets it, a fraglist aggregate reaches
skb_segment_list() with tunnel headers, which can push a frag_list
member below its head. The same panic happens today with a fraglist
aggregate routed into an ipip device.

Patch 1 makes room for the tunnel headers in skb_segment_list().
Patch 2 sets up IPXIP4 GSO state in ip_encap().
Patch 3 lets DF GSO packets pass when their segments fit the MTU,
leaving room for the outer header on tunnel vifs.

Tested in netns on veth, MFC entry to a plain vif and a tunnel vif,
software segmentation on egress, 4000 1316-byte datagrams per run, n=3
per arm, fraglist GRO, UDP GRO forwarding and UDP_SEGMENT, DF set and
clear:
- net (af32da41b032): DF aggregates dropped on both vifs (FragFails),
  non-DF aggregates dropped on the tunnel vif (tx_dropped); fraglist
  into an ipip device panics in skb_segment_list().
- v1: DF aggregates dropped on the tunnel vif (tx_dropped).
- this series: all 4000 delivered on both vifs, FragFails 0, no tx
  drops, no panic; ipip device 4000 of 4000.

Changes in v2:
- Add patches 1 and 2; v1 let DF GSO packets reach ip_encap() on tunnel
  vifs, where they were dropped during segmentation (Sashiko).
- Keep the additive MTU comparison and check mtu > encap before
  subtracting, so a route MTU below 20 cannot wrap (Sashiko).
- v1: https://lore.kernel.org/netdev/20261010131842.2157365-1-julius@bairaktaris.de/ (local)

Julius Bairaktaris (3):
  net: make room for the tunnel header in skb_segment_list()
  ipmr: set up GSO state for tunnel vifs
  ipmr: do not drop GSO packets that fit the MTU after segmentation

 net/core/skbuff.c |  4 ++++
 net/ipv4/ipmr.c   | 21 ++++++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

-- 
2.53.0